The koala (Phascolarctos cinereus) is the only member of the Family Phascolarctidae and is included, with wombats, in the Suborder Vombatiformes. Three different ‘races’ or subspecies have been described, based on geographic range and morphologic characteristics: P. cinereus cinereus, found in New South Wales, P. c. adustus, found in Queensland, and P. c. victor, found in Victoria. Recent molecular genetic analysis has determined that morphologic variation between northern and southern populations is clinal, and might reflect adaptation to climate over an extensive range rather than a significant divergence at the DNA level (Houlden et al. 1999). The tentative conclusion was that P. cinereus should not be considered as having three subspecies, but should be considered as a single evolutionarily significant unit.
The koala is found in eastern Australian sclerophyll forests and woodlands, and there are populations in Queensland, New South Wales, the Australian Capital Territory, Victoria and South Australia (Melzer et al. 2000). It has a widespread distribution and is abundant in many areas. The distribution, however, is largely fragmented, with populations separated by unsuitable habitat or cleared land (Martin & Handasyde 1999).
Estimates of population size are few and inconsistent. The conservation status varies throughout its distribution, reflecting regional perceptions of current threats to koalas and to their habitat (Melzer et al. 2000). Populations in New South Wales and Queensland are subject to a number of key threatening processes, including habitat loss through clearing of native vegetation and urban development, and populations are declining in those states. Increasing urban expansion also contributes to mortality in local populations via increases in road-associated deaths and exposure to attack by domestic dogs.
In contrast, unsustainably high population levels have developed in six isolated patches across Victoria, and on Kangaroo Island (SA) (Menkhorst et al. 1998). Habitat destruction and hunting contributed to a marked decline in populations in Victoria and South Australia during the early 20th century. Koalas were reintroduced in some mainland habitats in these states, via translocation from flourishing populations that were established on off-shore islands during the late 19th and early 20th centuries. Unsustainable densities in isolated populations (including some of those established on these islands, and several mainland populations living in remnant habitat) have resulted in overbrowsing of forage trees, widespread tree death and, in some cases, starvation of koalas (Martin 1985a).
Koala populations in South Australia have largely been re-established using animals translocated from a single colony that flourished after being introduced to French Island (Vic.), during the 1880s. Three South Australian populations were found to have low heterozygosity compared with an undisturbed mainland Victorian population, and this low allelic diversity was significantly correlated with the occurrence of testicular abnormalities (Seymour et al. 2001).
Government wildlife agencies have been responding to overabundance in koala populations in south-eastern Australia for many years; management techniques have included translocating animals to less populous release sites, fertility control, and culling (Duka & Masters 2005). While culling allows immediate reduction of koala numbers to a sustainable level, and is believed by many biologists to be the most humane method for population reduction, it is not considered an acceptable management tool within the National Koala Conservation Strategy (ANZECC 1998). Translocation has become less viable as limited unoccupied habitat remains. It requires careful assessment of the health status of the animals to be moved and the habitat condition at their destination in order to avoid high mortality rates. In Victoria, development of cost-effective fertility control methods for use in large numbers of animals is considered a priority for effective management of overabundant populations (Menkhorst 2004).
Koalas in the southern part of their range (Vic. and SA) are larger and have longer darker fur than their northern (Qld) counterparts. Those in New South Wales are intermediate in size and appearance. Body weight ranges and averages given for Queensland and Victorian koalas are: Queensland females 4.1–7.3 kg (av. 5.1 kg), males 4.2–9.1 kg (av. 6.5 kg); Victorian females 7.0–11.0 kg (av. 8.5 kg), males 9.5–14.9 kg (av. 12.0 kg) (Martin & Handasyde 1995). Some large captive male koalas in south-east Queensland exceed 9.1 kg, and an exceptional individual had reached 11.1 kg when just under 7 yr old.
Free-ranging female koalas commonly live for 13–18 yr; males are capable of surviving to a similar age but tend to have a shorter life expectancy due to hazards encountered during the breeding season (Martin & Handasyde 1999). Longevity records for koalas in captivity appear to be 23 yr for a female that was supported with regular supplementary feeding (see 5.3.4d) in her later years, and 20 yr 5 mo for a female that was not supplemented (G Rawlinson pers. comm.).
Koalas are largely nocturnal but exhibit some activity during the day. Daily feeding typically occupies a total of about 1–4 hr, and is accomplished in several bouts of varying duration (averaging around 20 min, but ranging between 5 min and 1–2 hr). It is most commonly observed during a 4–5 hr period around dusk. About 19–20 hr of each day is spent sitting, resting or asleep, while social behaviour, grooming, and changing branches or trees occupy only a fraction of daily activity (Robbins & Russell 1978; Lee & Martin 1988; Mitchell 1990a; Martin & Handasyde 1999).
2 ANATOMY AND PHYSIOLOGY
2.1 General body form
The koala has relatively long limbs adapted for climbing. On the fore limb, digits one and two oppose digits three, four and five, effecting a forcipate grip. On the hind limb, digit one is opposed to the other digits; digits two and three are syndactylous (fused) and used for grooming. All digits other than digit one of the hind foot have sharp claws. As in most marsupials, the koala lacks a patella. Epipubic bones are present. The tail is reduced to a stump consisting of 6–7 caudal vertebrae (Cork 1987; Lee & Martin 1988).
Mature males have a sternal scent gland which produces an oily secretion with a distinctive smell, and stains the surrounding fur on the chest. Females have a pouch which opens on the ventral midline in the caudal half of the abdomen. In non-breeding females the pouch is merely a shallow triangular skin fold enveloping two tiny teats. In mated females that are nearing the time of birth, the pouch expands but the teats remain small. Pouch young (PY) do not cause a noticeable bulge in the pouch until they are about Most females resent any attempt to stretch the pouch sphincter to look inside. Scattered flecks of the young’s faeces are normally evident on or around unfurred PY, and the interior of the pouch looks moist and glistening. Well-furred PY should look dry and clean. As young continue to be suckled for some months after they leave the pouch, one enlarged mammary gland with an elongated teat may be evident in an otherwise empty pouch.
The koala has relatively small, almost spherical, frontally placed eyes. The cornea occupies a relatively large segment of the globe, with the corneal diameter only slightly smaller than the equatorial diameter. A tapetum extends across the retina above the optic disc, and as in many other marsupials the retina is essentially avascular except for a small area on the optic disc itself (Schmid et al. 1992). Examination of 28 koalas determined that mean intra-ocular pressure was 24.2 ± 6 mmHg (Hirst et al. 1992a). The iris is usually brown, but very occasionally it may be blue (e.g. in heterochromia) or, in albino koalas, unpigmented. The pupil is a vertically oriented slit. Relative size and position of the nictitating membrane is similar to that in a dog or cat, and it follows the same nasal-to-temporal sweep across the eye. The lacrimal puncta are slit-like openings just inside the lid edges, 2–3 mm from the medial canthus. The 0.5–1 mm diameter opening of the nasolacrimal duct is located on the ventrolateral aspect of the nasal floor at the mucocutaneous junction (Blanshard 1994).
2.2 Digestive system
The dental formula of the koala is: I 3/1, C 1/0, PM 1/1, M 4/4 (Eberhard 1972). Incisors are used to manipulate branches and leaves, but are utilised to tear off leaves only by very young animals. The first pair of incisors in either arcade is well-developed, while the other upper pairs are smaller and fairly slender. Canines are present only in the upper jaw and are small peg-like structures which probably serve no real purpose. There is a single pair of premolars in each arcade bearing a relatively sharp longitudinal ridge, and it is these teeth which slice through the stems of individual leaves (Lee & Martin 1988). Each of the four pairs of upper and lower molars has four pyramidal cusps, which interdigitate with those on the opposing arcade. This creates a series of cutting edges, between which the leaf is chewed using a cutting–shearing action (Lanyon & Sanson 1986a). There is no crushing component in the occlusal action, thus the efficiency of mastication depends upon the maintenance of cutting edges on the molars (Lanyon & Sanson 1986a).
Throughout a koala’s life there is continual wear of the occlusal surfaces of the cheek teeth. The longitudinal cutting ridge of each premolar and the pyramidal cusps of each molar are gradually worn flat. Wear is more advanced on the anterior cheek teeth (Eberhard 1972; Martin 1981). The degree of tooth wear has significant implications for the koala’s plane of nutrition. As the cusps of the molar teeth become worn and the total length of their available cutting edges decreases, the proportion of larger leaf fragments in the koala’s stomach increases. Particle sizes in the caecum and proximal colon, however, remain similar to those in younger animals. These findings indicate that older koalas not only must excrete large particles more quickly, but also must increase their total leaf intake in order to maintain the same amount of fine digesta in the large intestine (Lanyon & Sanson 1986b). Increasing leaf intake causes an accelerated rate of tooth wear (see 7.1.12).
2.2.2 Gastrointestinal tract
Koalas are monogastric hindgut fermenters. The serosa of the stomach is off-white and the organ is usually rounded by a moderate amount of firm digesta. Part of the colon has a ligamentous attachment to the greater curvature of the stomach. The gastric mucosa normally is covered in a diphtheritic rubbery white coating. The stomach contents should be fairly dry, densely packed, finely chewed leaf material. In older animals, relatively large leaf and stem fragments, strands of fibre and cords of mucoid saliva may be evident. A large circular fleshy pink gastric gland, evident from the serosal surface, takes up much of the lesser curvature between the cardiac and pyloric sphincters, and discharges thick off-white to light brown fluid through a number of large openings on the mucosal surface.
Externally, the duodenum is greyish red and fairly thin-walled. The mucosa normally appears slightly hyperaemic, and its contents dull greyish or reddishgrey The jejunum is a collapsed translucent thin-walled tube about 15 mm across, with a fine reticular surface pattern, and the contents are usually translucent or slightly turbid greenish-yellow. Specimens of the cestode Bertiella obesa may be visible through the thin wall; occasionally these migrate into the duodenum or even through the pyloric sphincter after death (Spratt 1978). Thicker-walled than the jejunum, the ileum is pinkish, opaque, and about 10 mm across. Its contents are generally pale greyish-brown.
Koalas possess the largest caecum (up to 2 m long) relative to body size for any known mammal (MacKenzie 1918). Greyish- to brownish-olive loops of the caecum and proximal colon (which are superficially similar in appearance) occupy the entire ventral portion of the abdomen. The caecum and proximal colon should be heavy with moist thick brownish-olive digesta containing few recognisable leaf fragments. Both are oval in cross-section, i.e. not distended by their contents. The mucosal surface area of the caecum and similarly capacious proximal colon is augmented by a series of longitudinal folds. Digesta evenly coats the mucosal surface and longitudinal folds, and a thin layer is retained on the mucosa after gentle rinsing under running water (McKenzie 1978). Obvious dehydration of digesta closest to the mucosal surface is abnormal. The proximal colon gradually narrows and its longitudinal mucosal folds disappear. The character of the digesta also changes, with larger leaf fragments predominating and the consistency becoming thicker and drier. Cylindrical masses of digesta are divided into pellets of increasing dryness and firmness in the distal colon, which is yellowish, tapering and relatively thick-walled.
A consistent feature in histological sections of the caecum and proximal colon is the presence of a tall fringe of mixed bacteria tenaciously adhering to the mucosa (McKenzie 1978). Culture of some of these bacteria requires very specific and selective agar media (Osawa 1992). The biochemical and metabolic characteristics of the caecal microflora and their full significance in digestion or detoxification of the koala’s diet still are being investigated (Osawa et al. 1993a).
2.2.3 Liver, gall bladder and pancreas
In young koalas the liver is dull red, as in other species. With increasing age, dark pigment is accumulated and the liver becomes a uniform very deep purplish- or bluish-red. All lobe edges (except on a few of the small processes in the centre of the visceral surface) are normally very sharp. The gall bladder is a finger-like sac about 10–15 mm diameter and 5–6 cm long. Bile may be translucent and greenish, or whitish-gold with an iridescent character (Dickens 1975). The pancreas is pale or dull pink, and the distal end of the pancreatic duct is dilated to form a small flat chamber of clear fluid adjacent to where the duct enters the duodenum.
The spleen is usually contracted and dull pink, with tapered edges. Posteriorly it is narrow and strap-like, with a slightly broader and thicker anterior end, and normally is no more than about 10 mm thick.
2.4 Urogenital system
The kidneys are simple (not lobulated). When empty, the urinary bladder is contracted, small and rather thick-walled, and lies within the pelvic cavity. The mucosa is gently undulating and usually off-white to very pale pink in colour.
The female reproductive tract is typical of other marsupials, with paired uteri and cervices. Each ovary, together with the fimbriated proximal extremity of the otherwise short convoluted oviduct, is hidden away inside a peritoneal pocket or bursa which has a small aperture (the peritoneal ostium) located in its posterior wall. The vaginal system consists of a median vaginal canal (through which the foetus passes) divided by a septum, an anterior cul de sac region into which each cervix protrudes, and two narrow lateral vaginal canals (through which sperm travel) connecting the culs de sac and the urogenital sinus. The urogenital sinus, into which the very short urethra also discharges, is approximately 40–45 mm long and opens into the cloaca, anterior to the rectum and immediately caudal to the bifid clitoris, which is a distinctly reddish triangular structure (MacKenzie 1919; Obendorf 1988).
Like other marsupials, male koalas have a prominent scrotum which is anterior to the cloacal opening. The penis has a bifid glans, each half of which is further grooved to give a four-pronged appearance. Slightly ret-roverted cornified spines are located on the penile shaft. The unlobed prostate encircles the urethra immediately caudal to the urinary bladder, and there are three pairs of bulbourethral glands (Young 1879; Temple-Smith & Taggart 1990).
2.5 Lymphatic system
The superficial inguinal, superficial axillary, rostral mandibular, mandibular and facial lymph nodes are palpable in healthy koalas (Hanger & Heath 1991; Blanshard 1994). The mandibular nodes are quite small and difficult to discern when not enlarged. The superficial inguinal nodes are like flat lima beans approximately 10–20 mm in length, lying in the groin near the origin of the epipubic bones. Several small axillary nodes form a tapered cord-like chain 15–20 mm long, deep in the axilla. The rostral mandibular lymph node is approximately 5–8 mm long, positioned caudal to and slightly ventral to the commissure of the lips, over the mandible at the anterior border of the masseter muscles. Facial nodes sometimes may be present over the maxilla, 10–20 mm below the eye. Popliteal and subiliac lymph nodes are absent. The ileocaecal junction is flanked by two circular caecolic lymphoid patches each about 20–30 mm diameter, and there is a cluster of small mesenteric lymph nodes, two to four of which may be 7–12 mm long (Hanger & Heath 1994).
2.6 Miscellaneous anatomical features
The linea alba is broad and almost translucent, and there is no curtain-like omentum. Koalas have an intact mediastinum, so some pathological conditions can be confined to one side of the thoracic cavity. The surface of the cerebral cortex lacks conspicuous sulci.
2.7 Fat depots
Even in healthy well-nourished captive koalas, fat depots are meagre in comparison to those of most eutherian mammals. Localised subcutaneous fat may be present in the groin or axilla, and intra-abdominal depots may be observed in animals in good body condition around the edges of the kidneys, in the splenic mesentery and, in exceptional cases, as a veneer over the lumbar muscles.
2.8 Food intake
Mean food ingested (wet matter) has been estimated in feeding trials with Eucalyptus punctata to be within the range 64.2–102 g/kg0.75/d (Harrop & Degabriele 1976). Based on quoted average body weights, we could therefore expect Queensland koalas to consume approximately 220–415 g of this leaf per day, and Victorian koalas to consume approximately 320–660 g of leaf per day.
2.9 Water turnover
Free-ranging koalas rarely drink, deriving water from that preformed in eucalypt leaves and any moisture on the surface of the leaves (Lee & Martin 1988). Water balance in captive and free-ranging koalas has been studied (Degabriele et al. 1978). Five captive koalas showed mean water intakes from preformed water in eucalypt leaves of approximately 25–30 g/kg/d. Mean total water uptakes for captive animals from preformed water in leaves, metabolic water and drinking water (not always made available) were about 45–70 g/kg/d (63.3–92.3 g/kg08/d). When drinking water was withdrawn, the most important contribution to maintaining water balance was a reduction in faecal water content, which decreased from 52–54% to 43%. The measured water turnover of these captive animals was about half or less than that calculated for nine free-ranging koalas, at 122 g/kg/d (179 g/kg0.8/d). Mean total body water (5.35 L) represented 77.4% of body weight. Restrictions in activity imposed on the captive koalas by confinement in small enclosures were considered the most likely cause of the observed difference in water turnover rates.
2.10 Thermoregulation and metabolic rate
Under laboratory conditions, koalas subjected to various air temperatures from about 4.5–30°C generally maintained their body temperature between 35.5–36.5°C. When air temperature was 40°C the koalas’ body temperature rose to 37.0°C. The dorsal fur of the koala is the most insulative of any marsupial examined to date. The ventral surface has half the hair density of the dorsal surface, and is more reflective of solar radiation. Postural adjustments (particularly in relation to wind direction) confer insulative flexibility (Degabriele & Dawson 1979).
In order to conserve energy, the koala leads a sedentary lifestyle and has a very low metabolic rate—only 74% of the predicted value for a marsupial (Degabriele & Dawson 1979). Panting is the major means of evaporative cooling at high ambient temperatures. Sweat glands are found on the palmar and plantar surfaces of the paws, but these areas do not have any specialised vascular adaptations for effective evaporative cooling to occur (Degabriele & Dawson 1979). Continuous panting and profuse salivation, which may wet the chest and fore limbs, have been recorded occasionally at extremes of temperature. On occasions, the palmar and plantar surfaces of the paws reportedly become encrusted with salt (Eberhard 1972). The lower critical temperature for koalas (i.e. the temperature above which their body’s heat production is minimal) is about 10°C in winter and 15°C in summer, the difference due to seasonal changes in fur length and possibly tissue insulation. Basal metabolic rate was recorded at 25°C (Degabriele & Dawson 1979). Thus, if koalas are held in ambient temperatures somewhere between 10–15° and 25°C, they will be expending minimal energy on thermoregulation.
2.11 Adrenocortical function
The koala has a low adrenal weight: body weight ratio (approximately 45 mg/kg) compared to eutherian mammals. Otherwise, the adrenals have a typical mammalian structure, and all gross and (in one study) histological changes evaluated in association with disease were considered a normal mammalian response (Booth et al. 1990). Cortisol is the major glucocorticoid present in the blood of the koala. Its concentration fluctuates in irregular slow pulses throughout each 24 hr period (within the range <1–46 nmol/L), so that measurement in a single blood sample would be of little use in interpreting the effect of a presumed stressor (McDonald et al. 1990).
2.12 Immune response
The apparent susceptibility of koalas to infectious and neoplastic disease has resulted in a widespread belief that koalas are inherently immunocompromised, or ‘immunologically lazy’ (Wilkinson 1989). The first studies specifically designed to characterise the koala’s immune response initially focused on the establishment of an in vitro cellular assay to examine koala lymphocyte proliferative responses to a range of mitogens. Antibody (Ab) responses and cell-mediated immunity were then investigated in vivo and in vitro following challenge with a variety of pathogenic and non-pathogenic antigens (Ags) (Wilkinson 1989, 1996). The results indicated reduced immune responses to the range of Ags classically utilised to monitor immune capability in eutherian mammals. Analysis of the kinetics and dynamics of Ab production to a wide range of protein Ags demonstrated that humoral responses were slower than those of other species (see 10.5.1e). These studies also suggested that the cell-mediated immune responses were less well-developed than in other species (Wilkinson 1989, 1996). In contrast, another major study investigated the in vivo immune response of koalas to innocuous Ags and determined that the koala was capable of producing a range of strong immune reactions to these Ags, including Ab and T-cell mediated responses (Emmins 1996). At least two classes of immunoglobulin (IgG and IgM) have been recognised in the koala (Wilkinson 1989, 1996; Emmins 1996).
Recent and ongoing studies aim to provide more information on the response of the koala immune system to infectious diseases (Hemsley & Canfield 1997; Higgins et al. 2005a). An understanding of the impact of retroviral infection on the immune system of koalas has only recently begun to develop (see 10.11.1). Koala retrovirus (KoRV) is believed to cause immunosuppression; increased levels of KoRV have been seen in animals with clinical chlamydiosis, and high levels of KoRV RNA in plasma have been causally associated with leukaemia/lymphoma (Tarlinton et al. 2005).
3.1 Breeding season
Koalas breed seasonally, and the majority of births occur during summer. In Victoria the mating period begins in September (Lee & Martin 1988). Young are born between October and May, with most births occurring between the beginning of November and the end of January (Martin & Handasyde 1990). The breeding season is slightly earlier for northern populations than for the southern populations. At an institution in south-east Queensland holding captive koalas, the first matings for each breeding season generally occur in early August, and the last in late April (O’Callaghan & Blanshard 1992). In a captive colony in the northern hemisphere the peak breeding period is March–May (Thompson 1987).
3.2 Reproductive activity in males
Free-ranging male koalas are sexually mature at approximately 2 yr of age, however, competition from larger males restricts opportunities for mating until they are 4–5 yr old (Martin & Handasyde 1990). While male koalas are capable of mating at any time of year, reproductive activity is increased during the breeding season. Studies of males on French Island (Vic.) determined that there is a marked increase in mean plasma testosterone concentration in late July–August, with the mean concentration remaining high until January (Handasyde et al. 1990a). This increase in mean plasma testosterone concentration coincides with increased frequency of male reproductive behaviours, including ‘bellowing’ vocalisations, scent marking of trees with the sternal gland, and aggressive male–male interactions, prior to the commencement of reproductive activity in October (Mitchell 1990b). Adult male koalas move frequently and have larger home ranges during the breeding season (Mitchell 1990a).
3.3 Reproductive activity in females
Free-ranging females generally commence breeding late in their second or early in their third year (Martin 1981); they may produce one offspring per year and continue to breed until they are 14 yr old (Martin & Handasyde 1990). Two captive females have been recorded giving birth before 13 mo of age and at 16.5 yr old, respectively (O’Callaghan & Blanshard 1992), with successful weaning of the young in both cases.
Handasyde (1986) determined that oestradiol and progesterone are the major reproductive steroids in the peripheral plasma of the female koala. Studies of females on French Island (Vic.) demonstrated that the plasma concentration of oestradiol was highest at the beginning of the breeding period in October, and remained high until the following January. Between March and October, plasma oestradiol concentration was relatively low. The concentration of progesterone also exhibited a seasonal pattern, being low during the period of reproductive quiescence and intermittently rising markedly during November–March.
3.3.1 Oestrous cycle
If female koalas are prevented from mating, they are seasonally polyoestrous (Handasyde et al. 1990a; Johnston et al. 2000a). Peaks in plasma oestradiol occur during oestrus and immediately before birth (Johnston et al. 2000a). A number of studies have indicated that the luteal phase of the oestrous cycle is induced by the physical act of mating (Handasyde et al. 1990a; Johnston et al. 2000a, 2000b; Johnston et al. 2003). Plasma progesterone remains low during non-mated oestrous cycles, suggesting that these cycles are non-luteal and anovulatory. After mating, oestrous behaviours cease and plasma progesterone concentrations rise markedly (Johnston et al. 2000a).
Behavioural observations, morphology of the pouch and plasma concentrations of oestradiol and progesterone have been used to characterise the oestrous cycle in mated and non-mated captive koalas (Johnston et al. 2000a). The mean duration of 22 non-mated oestrous cycles (with no luteal phase) was 32.9 ± 1.1 d. The mean duration of oestrus for 24 non-mated cycles was 10.3 ± 0.9 d. The inter-oestrus interval for six mated non-parturient oestrous cycles (with a luteal phase) was 52.5 ± 0.8 d. Behavioural oestrus in captive female koalas is characterised by weight loss, vocalisation, restlessness, pseudo-male mounting of cage mates, and episodes of convulsive jerking or ‘hiccoughing’ behaviour, particularly in response to the call or presence of a male (Smith 1980; Thompson 1987; O’Callaghan & Blanshard 1992).
Johnston et al. (2000a) suggested that physical stimulation of the urogenital sinus is required to initiate the luteal phase, including ovulation. However, subsequent research determined that the luteinising hormone surge which occurs 24–32 hr post-insemination can be induced by deposition of semen without physical stimulation, and it was proposed that a factor (or factors) present in koala semen promotes induction of the luteal phase (Johnston et al. 2004).
3.3.2 Gestation and parturition
Smith (1979) reported a gestation period of 34–36 d. Visual confirmation of the presence of young during twice-daily pouch checking of mated captive koalas in south-east Queensland between 1989–1995, however, determined that for 103 pregnancies with known errors of 0.00–0.87 d, values for maximum possible duration of gestation were mean 34.09 ± 0.53 d, median 33.97 d, observed range 32.98–35.64 d (W Blanshard & P O’Callaghan unpub.).
Females have been observed to assume a slightly hunched position during parturition (Thompson 1987; O’Callaghan & Blanshard 1992). Parturition is rapid (approximately 30–90 sec elapses between the neonate’s emergence from the cloaca and its entry into the pouch), and appears to cause some discomfort for the female (O’Callaghan & Blanshard 1992). Blood, fluid or membranes may be visible on the fur around the cloaca or pouch following parturition, but this is not always the case. Neither the degree of apparent resentment of pouch inspection nor the tightness of the pouch sphincter should be assumed to provide unequivocal evidence of parturition or the timing thereof (W Blanshard & P O’Callaghan unpub.; Johnston et al. 2000a).
Koalas usually have only a single young (Fig. 8.1). Although twins have been recorded, their survival may be compromised by the limited capacity of the pouch, which may result in premature emergence (Martin & Handasyde 1990).
Structures used to house captive koalas vary considerably in design from extensive open free-range areas containing mature trees, to semi-enclosed or totally enclosed pens furnished with a network of poles and branches (Drake et al. 1990). A koala enclosure must effectively and safely contain its occupants, cater to their arboreal existence by providing freedom of movement both vertically and horizontally, offer shelter against extremes of climate, allow the animals ready access to appropriate quantities of palatable uncontaminated food, and be easy to maintain in a clean and hygienic condition.
Unless fully enclosed, walls or fences lined with material such as sheet metal are required to prevent escape from the enclosure. Walls should be a minimum of 1.2 m high, preferably with a rounded top edge, and be smooth or fully lined on the inside surface with sheet metal. Higher fences which would otherwise allow climbing (e.g. wire mesh) can be koala-proofed by affixing a strip of sheet metal at least 600 mm in height to the inside of the fence, beginning at least 1 m above ground level (P O’Callaghan pers. comm.). As koalas can jump horizontally from trees or poles, any climbable structure within the enclosure should be at least 1.8 m from the top of the perimeter wall or fence. The trunks of mature trees can be encircled with sheet metal guards to prevent koalas climbing to areas where they would be irretrievable (Drake et al. 1990).
4.2 Enclosure fixtures and fittings
Typically enclosures are furnished with upright natural timber poles bearing single or multiple resting forks. These may be linked by horizontal natural timber poles. It is usual to provide at least two resting forks per animal. The most common method of presenting browse to the koalas is to have tubular water containers (‘water buckets’ or ‘leaf pots’), into which the cut ends of branches can be inserted, attached to the upright poles at an appropriate height to allow the leaves to be within reach of koalas sitting in the resting forks (Drake et al. 1990; Blanshard 1994; Jackson et al. 2003). Six millimetre thickness, 100 mm diameter PVC plumbing pipe (with an end cap) is ideal for practical and extremely durable water buckets (P O’Callaghan pers. comm.). Two different designs are illustrated in Blanshard (1994) and Jackson et al. (2003).
Fresh drinking water should always be available in a separate receptacle on the floor of the enclosure. Koalas have been known to eat soil occasionally. The caecal contents of six dead koalas examined specifically for such material revealed varying quantities of sand grains and small stones in each, with the total weight of gravel ranging from less than 1 g up to 7 g (Bolliger 1962). If the enclosure does not have a soil substrate, some captive facilities elect to provide a bowl of soil. Overhead sprinklers and/or shadecloth shelter sails can be used to keep animals cool during hot weather.
4.3 Enclosure maintenance
Faecal pellets and fallen leaves should be raked up and removed from under the cage furnishings every day. Urine should be hosed off concrete floors daily or every second day, and soil or sand substrate should be replaced if heavily soiled with urine. Koala urine leaves sticky residues on the enclosure furniture, and faecal pellets become trapped or trampled into crevices in the surface of poles and resting forks. Thorough physical removal of faecal material followed by scrubbing of all poles once per week is recommended. A detergent with or without a mild disinfectant (quaternary ammonium compound or benzalkonium chloride) (G Rawlinson pers. comm.) can be used. Poles should be rinsed before re-use. All browse should be removed and protected from contamination during cleaning procedures. Water containers should be removed, rinsed and refilled before the leaves are replaced.
Water level in the leaf containers should be checked and topped up twice daily (more frequently in very hot weather), as freshly cut eucalypt foliage takes up significant amounts of water. At least some new browse should be made available every day. Branches from the preceding day can be inspected to see if it is worthwhile leaving any of them in the enclosure, but if no young leaves (‘tip’ or ‘tips’) remain the branches should be replaced.
4.4 Hospital housing
Ideally, all but the most acutely ill or injured koalas should be housed individually in large cages or enclosures which allow them to climb freely, both vertically and horizontally. These cages should be well away from the sight/sound/smell of domestic animals. Each koala should be provided with at least two comfortable resting forks. These can be on the same large pole, or on separate poles with a horizontal pole connecting the two. A near-vertical upright pole with a side branch (>50 mm in diameter) at an angle of 45–60°, no closer than 30–40 cm from the top of the upright, makes a suitable resting fork. Poles cut from stringybark or ironbark eucalypts are easier for debilitated or injured koalas to climb than smooth-barked species, but are more difficult to clean effectively and disinfect. A gently-inclined, rough-barked pole from the ground to a raised fork may enable a weaker koala to reach an elevated position.
Water containers for branches of leaves should be placed at the base of, or attached to, the sitting pole(s). The water container should not be so wide that the koala may inadvertently descend into it when climbing down the tree. Branches of leaves need to be securely tied or held against the upright poles, as the koala may try to sit among the branches of browse. Drinking water should be made available in a stable shallow bowl on the floor of the enclosure.
If the koala is very weak or unable to climb, a stable fork or other structure for support at ground level will be required. The use of plastic floor matting which allows urine to drain away underneath will greatly reduce soiling of the fur in koalas confined to the ground. Intensive care patients (those severely debilitated through illness or injury) can be set up in standard small companion animal hospital cages. Very thick soft padding such as a padded dog bed covered with a towel or blanket should be provided, on which the animal can lie or be propped up. It is common for wildlife carers to manage a debilitated koala indoors at home using an oval plastic laundry basket containing a roll of padding that the animal can lean against (J Starr pers. comm.). A second laundry basket may be inverted and tied on top of the first if extra security is required. Leaf in an appropriate water container should be secured close to the animal. Additional warmth may be provided using a heating pad, hot water bottle or heat lamp.
4.5 Moving animals and transport
Most koalas adapt quickly to routines, and can be lifted and moved short distances without much fuss if this is done on a regular basis. A sturdy bucket may facilitate moving individuals within a hospital facility. The koala is lifted by the wrists and lowered into the bucket, in which it can sit securely, holding the rim of the bucket for support. Bucket and koala can be lifted, carried or weighed without unduly disturbing the animal. Urine samples are often left behind in the bucket and may be useful if a specimen is required.
Koalas can be transported safely in hessian sacks (cushioned to prevent untoward movement if in a motor vehicle) for short periods. For longer trips a well-ventilated wooden crate, fitted with either a vertical centre timber pole to grip or a timber fork, may be used. Timber-framed, mesh-sided or slatted transport crates with the outside covered by hessian or similar material are more satisfactory, and are commonly used to transport koalas over long distances and in planes. A fork and leaf container are fitted to the centre of the floor of the crate. Australian Government specifications apply to crates used for overseas air transport (DEH 2004).
Ideally, koalas should not be transported in temperatures outside their thermoneutral range (below 10–15°C or above 25°C). Be particularly careful of higher than ambient temperatures in the interior of a motor vehicle, as both air flow and the koala’s capacity to make thermoregulatory postural adjustments are restricted.
4.6 Individual marking and identification techniques
The permanent and positive identification of captive koalas is essential for effective animal management. Free-ranging koalas that are part of field studies, and contracepted or translocated animals, are also marked using various techniques. Passive integrated transponder (PIT) tags (microchips) and/or eartags are used most commonly. Tattooing the inside of the pinna has also worked well (P O’Callaghan pers. comm.; Jackson et al. 2003). Eartags have the advantage in field situations that animals may be identified from a distance (particularly if large coloured tags are used). A disadvantage is that tags may fall or be pulled out, so they frequently are used in conjunction with microchips.
It is essential that PIT tags be inserted correctly to prevent loss or infection. The standard site for insertion in koalas is SC on the dorsal midline between the scapulae. Directing the tip of the insertion needle caudally and closing the skin wound with tissue glue reduce the likelihood of the microchip falling out post-insertion (Vogelnest 1995).
5.1 Browse selection and dietary preference
Koalas are strictly folivorous and exist almost exclusively on the leaves of certain Eucalyptus spp. The natural distribution of eucalypt species varies tremendously between and within different Australian states, and so does the koalas’ opportunity to utilise them as food trees. There are several hundred different eucalypts, and koalas are known to feed on at least 40–50 species throughout eastern Australia (Southwell 1978; Warneke 1978; Drake et al. 1990; Pahl & Hume 1990). Koalas have occasionally been observed eating the foliage of non-eucalypt species, but these do not seem to be used significantly as food trees (Martin & Handasyde 1999).
While utilisation of different eucalypts may vary from one locality to another (and may change within localities from one season to another), a relatively small number of eucalypt species tend to be eaten regularly by the majority of koalas that have access to them. This tendency is recognised consistently in many localities throughout eastern Australia where the distribution of these eucalypts and koalas coincide, and they are regarded as preferred food trees or primary browse species (Fleay 1937; Robbins & Russell 1978; Warneke 1978; Hindell & Lee 1990; Martin & Handasyde 1999).
Koalas have been reported to show a degree of preference for young rather than mature foliage, but a preference for young leaves is not consistent. Animals in a socially stable population may show preferential use of some trees within their individual home ranges. While koalas may generally utilise the foliage of a particular eucalypt species, foliage from some individual trees of that species may not be eaten. Individual animals may differ in their utilisation of different eucalypt species and in their utilisation of individual trees within each species (Fleay 1937; Robbins & Russell 1978; Warneke 1978; Hindell & Lee 1990; Martin & Handasyde 1999). There have been a number of studies investigating the feeding patterns of koalas, to determine the characteristics of preferred foliage. Leaf selection has been found to correlate positively with leaf nitrogen content and water content, and correlate negatively with fibre content and total sugar content (Pahl & Hume 1990; Osawa 1993). The role of Eucalyptus spp. essential oils in determining koala leaf selection has also been investigated. Phenolic compounds known as formylated phloroglucinol compounds (FPC) have a strongly negative effect on feeding by marsupial arboreal folivores, regardless of other features of nutritional composition of leaves (Lawler et al. 1998). Koalas have been found to have a higher tolerance for FPC than two other arboreal folivores, the common ringtail (Pseudocheirus peregrinus) and the common brushtail (Trichosurus vulpecula), giving them a greater ability to utilise eucalypt foliage as a sole source of nutrients. While FPC may influence whether a plant is consumed, the amount eaten is probably determined by a range of factors including nutrient composition (Hume 1999; Moore & Foley 2005).
5.2 Digestion of eucalypt leaf
Eucalypt foliage is low in available carbohydrate, protein and ash but is high in lipid and phenolic compounds (including lignin). The lipid fraction includes essential oils and other plant secondary metabolites or antinutrients (Hume 1999). The fibre in eucalypt leaves makes up 30–50% of dry matter (DM) in the leaf, and tends to be highly lignified and therefore difficult to digest. Fats comprise around 17% of the DM, and much of this fraction consists of essential oils, which are toxic, and waxes, which are not digested to any great extent (Cork 1987). Proteins make up only 5–15% of the DM. Phenolic compounds may reach 25–30% of the DM and have the potential to form complexes with dietary proteins, making the latter difficult to digest (Cork 1987; Hume 1999).
Fluid and fine particles (which are potentially more digestible because of their greater surface area to volume ratio) are selectively retained in the caecum and proximal colon, while less digestible coarser particles are eliminated more rapidly. Mean retention times for digesta are 99 hr (4.1 d) for particles and 213 hr (8.9 d) for fluid (Cork & Warner 1983). Despite the size of the hindgut, studies show that the small intestine is the major site of absorption of the products of digestion, and that bacterial fermentation seems to provide only about 9% of the total energy derived from eucalypt leaves (Cork & Sanson 1990).
5.3 Feeding koalas in captivity
5.3.1 Eucalypts fed to captive koalas
Foliage from preferred food trees forms the staple diet of captive koalas. Diets are varied by providing foliage from other known food trees. The number of eucalypt plantations established by zoos and wildlife sanctuaries is testament to the fact that the same trees can be cut many times to provide browse for koalas. Regrowth, whether plantation-grown or on naturally growing Eucalyptus spp., is the basic food resource for most captive koala colonies (Drake et al. 1990; Blanshard 1994).
Preferred food trees utilised to feed captive koalas include:
- E. camaldulensis—river red gum;
- E. globulus spp.—Tasmanian & southern blue gum;
- E. microcorys—tallowwood;
- E. ovata—swamp gum;
- E. punctata spp.—grey gum;
- E. tereticornis—forest red gum;
- E. viminalis—manna gum.
Some other food trees commonly utilised to feed captive koalas include:
- E. goniocalyx—long-leaved box;
- E. grandis—fooded gum;
- E. haemastoma—scribbly gum;
- E. maculata—spotted gum;
- E. moluccana—gum-topped box;
- E. nichollii—narrow-leaved black peppermint;
- E. obliqua—messmate;
- E. resinifera—red stringybark;
- E. robusta—swamp mahogany;
- E. saligna—Sydney blue gum;
- E. sideroxylon—red ironbark.
Koalas also are maintained in captivity outside their natural range (in Tasmania, Western Australia, parts of South Australia, and overseas). These animals are provided with foliage grown in plantations as well as apparently palatable local eucalypt species. For comprehensive state-based lists of eucalypt species utilised to feed captive koalas, refer to Drake et al. (1990), Jackson et al. (2003) and DEH (2004).
Individual koalas vary in their preference for different eucalypts. Just because a species appears on a list of koala food trees doesn’t mean that all individuals will eat it. A minimum of two or three branches of palatable browse approximately 1 m in height, bearing as much young tip as possible, are required daily to feed a healthy koala (Booth 1991). It is important to offer a choice of species each day, and to vary the species offered from one day to the next.
5.3.2 Collection and presentation of browse
Cut browse should be kept off the ground as much as practicable to prevent contamination of the leaves. Branches can be sprinkled with water to help preserve them in transit, and they should be transported in a closed vehicle or under a tarpaulin to prevent drying by the wind and exposure to exhaust fumes.
Branches should be placed upright with their cut ends in water as soon as possible after harvesting. If not destined for use on the same day they should be stored in a cool location in the shade and out of the wind, or in a coolroom.
Foliage from at least two or three known food tree species should be made available each day, although full loads of the most preferred species can be offered occasionally.
The choice of species should be changed from one day to the next. Foliage of some eucalypt species may be eaten quite enthusiastically on the first day it is offered, but may be rejected on subsequent days when its novelty value has waned.
Given the opportunity, captive koalas will selectively tend to eat tips and younger leaves before mature leaves, for the majority of eucalypt species. If large branches bearing tip are cut, some mature leaf is always available at the base of the branch anyway.
New foliage should be provided every day, and the cut ends of the branches should always be standing in water to keep the leaves as fresh as possible.
Branches on which the tip has been incompletely browsed can be rotated or repositioned within the enclosure to facilitate access to all the leaves.
Identify whether browse has been offered in excess of requirements, or whether foliage has been rejected by the animals in favour of other branches of the same or different species. This is easiest if those who put up the branches also take them down again.
5.3.3 Artificial diet
Providing fresh browse for captive koalas is labour-intensive and expensive. This prompted investigation into development of an artificial diet. Pahl and Hume (1990) reported some success feeding a combination of a thick paste (made of low-lactose milk powder and ground Eucalyptus spp. foliage) and a thin biscuit. The biscuit was manufactured to contain moisture, nitrogen and fibre in concentrations similar to those found in preferred leaf. While the artificial diet provided a valuable supplement to the natural diet, it did not completely replace the need for fresh foliage.
5.3.4 Feeding hospitalised koalas
a Determining food preferences
The appetite of sick koalas usually becomes more capricious than normal, and even captive animals may decline to eat species which they consume avidly when in good health.
The growth stage of leaves also becomes more critical when animals are sick. Sometimes magnificent tip will be passed over in favour of withered mature leaves at the base of a branch. Stripping and eating bark but not leaves may occur. Alternatively, the koala may bite through woody stems and eat these, allowing leaves to drop to the ground. Remember that an alternative to such bizarre eating behaviour is to eat nothing at all, so the carer should try to cater to the koala’s whims however strange they seem.
b Exploiting feeding opportunities
Koalas often start eating after being disturbed from sleep or rest. Paradoxically, this can have been an unpleasant disturbance (such as receiving an injection), or something as simple as having their cage opened and fresh leaves put in or their existing leaves rustled or rearranged. Many sick koalas eat quite enthusiastically for a short time after fresh leaves are placed in their cage, then lapse into a trance-like state or fall asleep before a significant amount has been consumed. In these cases, the time spent eating and the amount of leaf eaten can sometimes be increased by offering different species of eucalypts in turn, in increasing order of preference, making a substitution each time the koala stops eating.
Koalas generally will not eat individual leaves that have been removed from branches and offered by hand. Nevertheless, animals which are not distracted from eating by close human proximity may have their leaf intake selectively modified. Small branchlets of leaves can be held near the koala so that the leaves it might derive the most benefit from (e.g. younger, higher water content, less fibrous, easier to digest) are always the most accessible. Some koalas even stop trying to manipulate the branchlet themselves, and just expect the next leaf to be ready as soon as they are. Although very time-consuming, this process increases the number of leaves eaten in a given period and maximises consumption of younger leaves. These may be important factors in the fluid and energy balance of koalas which are spending less time eating than they should.
d Supplementary feeding
The nutrient intake of koalas can be boosted by supplementation with low-lactose high-energy milk powders mixed to a semi-liquid paste with water and administered orally via a syringe (Wood 1986, 1987; Handasyde et al. 1988; Pahl & Hume 1990; Osawa & Carrick 1990). Powders commonly used for this purpose have included (but are not limited to):
- Di-Vetelact® low-lactose animal supplement (Sharpe Laboratories);
- Infasoy® (S26 Soy) Infant Formula (Wyeth Nutritionals);
- Karicare™ Soya Formula (Nutricia Australia);
- PediaSure® Complete (Abbott Australasia);
- Portagen® (Mead Johnson Nutritionals);
- Prosobee® (Mead Johnson Nutritionals).
There is a diverse range of sources of protein, fat and carbohydrate in these products, and despite their specialised normal diet koalas seem remarkably tolerant of all these ingredients. It appears to matter little which product is used (Booth 1991; Blanshard 1994; Booth & Blanshard 1999; C Flanagan pers. comm.; J Hanger & G Gipp pers. comm.; A McKinnon pers. comm.; G Rawlinson pers. comm.).
Koalas vary in their response to attempted supplementary feeding. A proportion of animals find the taste to their liking, learn to recognise the syringe as containing something they would like to eat and actively take the tip of the syringe into their mouths. Others may be indifferent to supplementation and not voluntarily accept the syringe tip, but will swallow most of the administered paste without undue protest or evasive action. A small proportion of koalas appear to despise either the paste or the physical process involved with being supplementary fed (or both); in these few, the energy they expend trying to avoid ingesting the paste initially probably negates the beneficial effects of the minimal amount swallowed. With gentle persistence, however, very small but increasing amounts of paste may be taken during successive days until a significant quantity of extra nutrients is being accepted.
Cathetertipped syringes are ideal for paste administration to co-operative koalas, but may deliver too large a mouthful of paste to animals still being accustomed to the supplementation process. It is common practice to give a small volume (e.g. 10 mL) of water PO by syringe following paste administration (as the paste may be sticky in the animal’s mouth), and after every feed it is important to clean any residue off the koala’s fur using a damp cloth or paper tissue.
Five to 8 kg koalas eating little or no leaf, when provided approximately 60 mL of paste per day (usually in 2–3 divided doses), are reported to maintain their body condition (Blanshard 1994; Booth & Blanshard 1999). Smaller quantities of paste (e.g. 20–40 mL/d) are given to animals which are eating some leaf, or those which have regained their appetite but still have to make up considerable deficits in body condition after illness. When koalas are eating nothing but paste, finely-blended eucalypt leaf can be added to the paste mixture to provide some fibre (see 10.2.6) and to ensure the specialised bacteria in the caecum and proximal colon are not totally deprived of their usual medium.
6 RESTRAINT AND ANAESTHESIA
6.1 Physical restraint techniques
6.1.1 Retrieval or capture from trees
A koala may be encouraged to back down a tree by a hand or other object placed on top of its head. Koalas which are out of reach can sometimes be ‘flagged’ down, using a rod or pole with a cloth on the end persistently waved over or touched against its head (Fig. 8.2b). Placid koalas can have their fore feet unhooked from the tree and slowly be pulled downwards and away from the tree to disengage their hind feet, before additional physical restraint (see 6.1.2) or placement into a sack or transport box, as appropriate. Sick or badly injured koalas are usually fairly docile and relatively easy to handle. Less tractable koalas (e.g. healthy free-ranging adults) can be very aggressive and may bite and/or scratch in attempts to escape. As these animals descend the tree they can be enveloped in or backed into a hessian sack held open against the trunk.
During field studies, koalas are sometimes captured by researchers who climb as high up the tree as fleasible, then use a long pole to drape a rope noose (in which a knot has been incorporated to prevent overtightening) around the animal’s neck. Traction on the rope is used to prevent the koala climbing any higher, and it is then flagged down within reach, secured in a sack and lowered to assistants waiting below. Occasionally, free-ranging koalas will leap between branches to avoid capture, or simply launch themselves into open space. It is therefore advisable to have two or more assistants, holding the corners of a tarpaulin or a blanket, under the tree to catch the koala should it fall (Fig. 8.2a).
Successful capture of free-ranging koalas from trees by remote injection of anaesthetic agents via a small dart has been reported (Lynch & Martin 2003). The method was trialled as a means of reducing the need for climbing and to shorten the duration of capture procedures, therefore decreasing stress for the animals. Darted koalas were caught using cargo nets as they fell or were dislodged from trees.
6.1.2 Physical restraint
The majority of koalas respond to gentle and adept handling, so the degree of physical restraint applied should be proportional to the task and can be scaled up according to the animal’s reactions. Some useful techniques for handling and manually restraining koalas are shown in Figure 8.3.
Docile koalas can be picked up from behind, by grasping around the wrists. Lift the koala off the ground fairly slowly so that it can feel its weight is being supported. Even placid animals will only dangle patiently for a short time (Fig. 8.3a). Lateral traction on one or both arms will be necessary if it attempts to bite.
Alternatively, first the wrists then the ankles can be encircled in an initial handgrip so that the koala is restrained in a sitting position, facing away from the handler (Fig. 8.3b). After lifting, rest the animal’s back against your body (Fig. 8.3c), and/or support its rump on some other object (e.g. table or lap). Pull the koala in closer to your body if it tries to struggle, for greater control. In co-operative animals, this is an ideal restraint position for collecting blood samples, as you can modify your grip to hold up a cephalic vein (Fig. 8.3d). Greater dexterity is required to avoid attempts to bite, so if the koala is persistent its head should be secured by another person (Fig. 8.3e).
Like all arboreal animals, koalas tend to panic when unable to grip anything with unrestrained paws. If necessary, a koala can safely be held against the ground, or picked up and carried, by grasping it with one hand around the back of the neck and the other over the pelvis, so that the claws and teeth are directed away from the handler (Fig. 8.3f). In relatively shaggy southern koalas the fur provides useful handholds. The fur of northern koalas may be too short to grasp, but their smaller size generally enables the pelvic bones and the back of the neck to be secured within each hand span. Towels, blankets and hessian sacks can facilitate the handling and restraint of aggressive animals (Fig. 8.3g).
A retrospective review of mammal bite injuries treated at emergency departments of Victorian public hospitals 1998–2004 revealed that 0.05% were inflicted by koalas (MacBean et al. 2007). Koalas should never be lifted around their chest (like a human baby). Holding or carrying an unhabituated koala clinging to the handler’s body leaves the handler vulnerable to bites and scratches, especially on the face and neck. Unless habituated, only dependent juveniles (which may become more distressed if unable to cling to something for security) should be held this way.
6.2 Chemical restraint
Neither premedication with atropine nor fasting is considered essential for koalas (Blanshard 1994). Excessive salivation under anaesthesia is not usually observed, and the consistency of normal stomach contents does not lend itself to regurgitation. Some clinicians recommend pre-anaesthetic fasting for 6 hr to reduce stomach fill (Shima 1999). Anaesthesia in koalas is generally straightforward, but periods of apnoea and, in un-intubated animals, respiratory stridor due to the long soft palate are relatively common. Changing the animal’s position and intubation may alleviate some of these problems.
6.2.1 Injectable agents
Propofol 2.5–3.0 mg/kg as a bolus IV will result in recumbency and light anaesthesia sufficient for intubation in placid captive animals. Larger doses (titrated to effect) may be required when animals are less accustomed to being handled (Blanshard 1993). Standard domestic animal dose rates (6–8 mg/kg IV, administered to effect, and 4–5 mg boluses given as required for maintenance of anaesthesia) have also been used (Vogelnest 1999). Propofol is ideal for very short procedures where good relaxation and fast recovery are desirable, and can be topped up if necessary. It is also useful for induction of anaesthesia prior to maintenance with a gaseous agent. Both induction and recovery are rapid and extremely smooth.
Figure 8.3 Physical restraint options for koalas vary according to task as well as demeanour, temperament and habituation to handling. a) Docile koalas may be moved short distances while dangling by their wrists. b) The hind legs can be secured in the handgrip. c) Animals can be lifted or carried in a sitting position. d) Once body weight is supported the handler may be able to occlude the cephalic vein. e) A second person may be required to restrain the head. f) Unassisted grip for otherwise-uncontrollable struggling and biting animals. g) A towel, blanket or hessian sack can facilitate restraint, e.g. for gaseous anaesthesia. Photos: a-f) K Bodley and Lone Pine Koala Sanctuary and g) Melbourne Zoo.
Alfaxalone at 3.0 mg/kg IM or 1.5 mg/kg IV provides sufficient anaesthesia to allow examination and/or intubation prior to supplementation with inhalant agents (J Hanger pers. comm.). IV administration provides approximately 10 min anaesthesia. A transient period of apnoea may occur at the time of induction when using an IV bolus.
Tiletamine/zolazepam at 5–10 mg/kg IM or 2.5 mg/kg IV is reported to give good restraint for short procedures (Blyde 1990; Shima 1999). There is some retention of muscle tone and spontaneous limb movements. At the low end of the IM dose range it produces sedation, and at the higher end of the IM dose range surgical anaesthesia. Moderate salivation has been reported with tiletamine/zolazepam anaesthesia, but caused no problems as the swallowing reflexes remained intact (Bush et al. 1990). A dose of 7 mg/kg IM has provided an appropriate level of anaesthesia for minor surgical procedures (e.g. insertion of subdermal contraceptive implants using a needle applicator) in free-ranging Victorian koalas (K Handasyde pers. comm.).
Dose rates in the order of 3.5 mg/kg tiletamine/zolazepam plus 55 μg/kg medetomidine given via darts fitted with 12 mm needles provided light to surgical anaesthesia in 9/19 animals, and heavy (9/19) or moderate (1/19) sedation in animals captured after being darted (Lynch & Martin 2003). Variability in response was believed to be due to variability in dart placement and drug deposition in tissues. Another three animals that were caught manually and hand-injected IM with 3.0–3.6 (mean 3.3) mg/kg tiletamine/zolazepam plus 46–59 (mean 52) μg/kg medetomidine directly into the gluteal musculature demonstrated reliable induction to light or surgical anaesthesia. Careful monitoring of anaesthesia was advocated, as medetomidine may cause alterations in heart rate, respiration and blood pressure, and may interfere with thermoregulation. Ready availability of the specific medetomidine antagonist, atipamezole, was recommended. Two of the 19 animals captured after darting with a tiletamine/zolazepam plus medetomidine combination by Lynch and Martin (2003) showed slow respiratory rates and periods of apnoea, and were given 0.5 mg atipamezole IM to restore a more regular respiratory pattern. A third animal received the same dose because its rapid induction time indicated that the dart may have delivered the drugs IV. At the end of a brief (only 5–10 min) period of handling, all animals in the study were given 2 mg atipamezole IV, and recovered to a state of light sedation within 2 min. Most of the koalas still appeared to be very slightly affected by the drugs when released 30–60 min after reversal, although all were able to climb and the releases were uneventful.
Ketamine (16 mg/kg IM) was reported to produce light restraint with muscular hypertonicity (Robinson 1981). Acetylpromazine (0.4 mg/kg IM) or xylazine (0.6 mg/kg IM) was given simultaneously with ketamine to reduce this effect (Robinson 1978, 1981). Koalas took up to 2 hr to achieve full recovery when xylazine was used. Ketamine (5 mg/kg) plus xylazine (5 mg/kg) as a single IM injection reportedly provides deep sedation. This can be reversed by giving yohimbine (0.2 mg/kg IV) (Blyde 1990). Care was advised if animals showed signs of shock or poor health. Up to 15 mg/kg ketamine plus 5 mg/kg xylazine will produce surgical anaesthesia when given IM.
6.2.2 Inhalant agents
Halothane (Dickens 1978a) or isoflurane (Wilkinson 1989; Booth 1991) in oxygen are commonly used for induction and/or maintenance of anaesthesia. Mask induction using 3.5–5.0% isoflurane in oxygen results in smooth rapid induction and rapid anaesthetic recovery. Gradually increasing the isoflurane concentration from 0.5% until anaesthesia is achieved provides a smoother and more stable anaesthetic (D Higgins pers. comm.).
Mask induction of hospitalised koalas can usually be achieved by an unassisted operator, without restraining the koala in a sack. Sit the koala on the table, facing away from you. Place the edge of a large towel gently around the koala’s neck and secure the towel behind the neck with one hand so that it is snug but not tight. Lean forward against the koala as you apply the mask with your other hand, to prevent it backing away. The koala will probably attempt to scratch the mask off using a front leg, from which you should be protected by the folds of towel. If greater restraint is required, lean over the koala’s back and use both elbows to restrict the animal’s lateral movement (and to tighten the bottom edges of the towel down onto the table).
Intubation is readily achieved given a little practice and appropriate equipment. A 3.0–5.5 mm endotracheal (ET) tube is suitable for most adult Queensland koalas and Victorian animals under 10 kg body weight. The most awkward aspect of intubation, particularly in smaller individuals, is that the narrowness of the dental arcades at the oropharyngeal opening can impede introduction of a standard laryngoscope blade that is long enough to visualise the epiglottis. Use of a narrow laryngoscope blade, a perspex-tipped pen-type light source, or an angled elongated light stem from a diagnostic kit may be helpful in these cases. The long soft palate usually obscures the epiglottis from direct view until pushed dorsally using the ET tube. Intubation is possible with the animal in standard lateral recumbency, however, some clinicians prefer the koala in sternal recumbency (J Hanger pers. comm.), or may try dorsal recumbency so that the soft palate does not obstruct visualisation of the epiglottis (Shima 1999). It is appropriate for practitioners to begin with the patient positioning technique they prefer in more familiar species, e.g. domestic cats.
Using a stylet can assist when directing the ET tube. A standard ET tube stylet can be pre-threaded through the tube so it is protruding from the end by a few centimetres. The stylet tip is used to push the soft palate and epiglottis out of the way so the larynx can be visualised, and is then passed over the epiglottis into the larynx. The ET tube is threaded into the larynx over the stylet, and the stylet withdrawn (J Hanger pers. comm.). Another technique for intubating marsupials describes inserting the end of a long polypropylene catheter (e.g. a canine urinary catheter) into the larynx before passing an ET tube over the catheter and into place, then withdrawing the guide (Shima 1999). Alternatively, the ET tube can be pre-threaded over a fine rigid endoscope, which then serves both as a stylet and the means of visualising and entering the glottis (Carlisle et al. 1989; V Nicolson pers. comm.). Lubricating the endoscope with silicone spray before placement of the ET tube is recommended if the tube fits snugly over it.
Blind intubation can also be achieved without additional equipment (L Vogelnest pers. comm.). With the koala in left lateral recumbency the animal’s neck is extended and the tongue pulled out of the mouth by an assistant. The larynx is held externally between the thumb and forefinger of the left hand while the ET tube is passed over the base of the tongue with the right hand, until the tip is resting on the glottis. As the animal takes a breath the tube is passed into the larynx and trachea. A deep plane of anaesthesia makes blind intubation easier.
6.2.3 Anaesthetic recovery
Koalas should be denied the opportunity to climb until fully recovered, but will be less agitated during recovery if they can grip onto something (e.g. the side of a mesh cage, a rolled-up towel or a heavy branch placed on the ground) while their co-ordination returns to normal. Either move the koala to a quiet recovery cage/area soon after extubation, or leave it undisturbed as long as possible and only transfer it to the recovery cage/area after it has spontaneously tried to sit up.
7 CLINICAL ASSESSMENT
Initial assessment of the koala should be performed with the least possible disturbance to the animal. Most koalas will adapt to the routines of captivity fairly readily, given a little time and non-threatening interaction with their carers.
7.1 Physical examination
7.1.1 Body temperature
A thermometer can be inserted several centimetres into the rectum, which opens into the cloaca dorsal to the urogenital opening. Normal body temperature is 35.5–36.5°C (Degabriele & Dawson 1979). Hypothermia is relatively common in ill or debilitated koalas, and the body temperature may fail to register on a clinical thermometer (below 32°C). Hyperthermia is relatively uncommon. Febrile states have been observed occasionally, e.g. 37.9°C in a case of septicaemia and associated inflammation, 38.7°C in a septicaemic koala which was convulsing (Blanshard 1994), and 39.4°C in a case of acute disseminated toxoplasmosis (Dubey et al. 1991).
7.1.2 Pulse rate
7.1.3 Respiratory rate
Normal respiratory rate at rest is around 10–15 breaths/min (Dickens 1978a). Free-ranging koalas are unlikely to be ‘at rest’ when being examined. Panting (rapid shallow breathing with the mouth closed) is normal at higher ambient temperatures, or in agitated animals.
Thoracic auscultation is hindered by the koala’s dense fur and by the relatively small area occupied by the lungs. The chest is short and wide, and much of the lateral rib cage protects cranial abdominal organs rather than the heart and lungs. The ventral and ventrolateral aspects of the thorax usually yield the clearest sounds. The stethoscope should be slid right up into the axillary region. Abdominal auscultation should reveal occasional peristaltic sounds. Percussion with the fingertips should generate only dull sounds, as the normal abdomen contains no accumulations of gas.
7.1.5 Lymph nodes
Numerous superficial lymph nodes are palpable in healthy koalas (see 2.5). These lymph nodes should be systematically palpated to detect enlargement and asymmetry. Unilateral or bilateral enlargement of the rostral mandibular or facial lymph nodes may cause subtle alteration in the shape of the koala’s face.
7.1.6 Body weight
Regular weighing should be incorporated into the routine of any koala in care. The koala can be placed directly on the weighing platform of large scales, or transferred into a small cage, sack or strong bucket (see 4.5) in which it can be carried to and from the scales and weighed. In some zoological parks captive koalas are accustomed to being transferred directly onto a small tree fork suspended from a spring balance, in which they sit quietly while being weighed. Changes in the body condition, hydration status and gut fill (as well as the presence of PY in females) should all be considered when interpreting changes in body weight.
7.1.7 Body condition
Body condition can be assessed objectively by palpation of muscle mass over the scapulae (Dickens 1978a; Wood 1978). Although changes in muscle mass are apparent on palpation of other areas (lateral to the tibial crest, in the lower part of the arm and upper part of the forearm, and in the temporal fossa), it is more difficult to quantify progressive changes in these areas than over the scapulae. When present, subcutaneous fat depots are relatively small and confined to the axillary and inguinal areas (and around the pouch in females), so a koala’s ribs will always feel prominent irrespective of whether the animal is robust or emaciated.
Place a hand across the koala’s shoulders and rub your thumb over the supraspinatus and infraspinatus muscles. As body condition declines, the spine and edges of the scapula become much more prominent. Convex muscle surfaces may become flat or concave, and the infraspinatus seems to contract in overall size until muscle can only be felt clearly against the humeral end of the scapular spine. Assessment of body condition should be done on a daily basis in sick animals, as noticeable changes in body condition may precede significant changes in body weight. If the koala is dehydrated and fluid therapy is given, baseline body condition should be re-evaluated after an equilibration period, as there will be a noticeable improvement in apparent muscle mass. An example of a numerical body condition scoring system is provided in Table 8.1.
7.1.8 Hydration status
Changes in skin tone give the best guide to hydration status. Skin over the scapular area should slide smoothly backwards and forwards under your thumb. As dehydration progresses the movement of the skin becomes retarded and feels tacky. Small folds of skin pinched up over the top of the head (between the ears) usually slide briskly back into place. Koalas seem to have rather inelastic skin over much of the body, so that folds of skin twisted up over the shoulders, for example, may remain in this position for several seconds, even in healthy animals. A history which includes anorexia, rapid weight loss, polydipsia (atypically drinking from a water bowl), or the production of small dry faecal pellets should always alert the clinician that a koala may be dehydrated.
7.1.9 Gut fill
The enormous capacity and prolonged retention time of digesta in the caecum and proximal colon ensures that healthy koalas have a well-rounded ovoid body shape. In healthy juveniles and in frail but otherwise healthy aged animals, the abdomen can sometimes seem disproportionately large. After a few days of reduced food intake the abdominal girth reduces, and the animal becomes straight-sided or noticeably concave behind the costal arch.
7.1.10 Abdominal palpation
The stomach and both kidneys can be palpated in a relaxed koala. When full, the stomach occupies much of the left anterior abdomen and extends beyond the costal arch. The position and consistency of the caecum and proximal colon make identification of other organs difficult, and the epipubic bones impede palpation of the urinary bladder.
7.1.11 Gait and demeanour
Most koalas are not overly inquisitive, but when placed on the ground they usually walk or bound towards a nearby vertical structure (e.g. table leg, door jamb or even a vertical shadow or contrasting stripe of paint on a wall) in anticipation of climbing to safety. When walking slowly on the ground koalas have a rather high-stepping gait with both fore and hind legs. When agitated or alarmed they may break into a bounding gallop, or sit bolt upright with the ears erect and eyes wide open. If threatened they may raise one or both arms in the air, ready to swipe an attacker. When unwell, koalas take little or no interest in their surroundings, and adopt a ball-like resting posture with the head lowered and ears drooping, even in an unfamiliar location like the floor of a veterinary surgery.
7.1.12 Tooth wear class
Advanced tooth wear can be a limiting factor in an older koala’s recovery from illness or injury, and in otherwise healthy aged koalas it may be the primary reason for debility (Eberhard 1972; Robinson 1978; Lanyon & Sanson 1986b). The cheek teeth of every adult koala coming into care should be checked as part of the initial examination. In anaesthetised animals the premolars and all four molars can be visualised. In a conscious koala you may be able to slide your index finger or a pen into the diastema between the lower incisors and premolars to pull the mouth open. It is helpful to have an assistant restrain the koala’s forearms to reduce the possibility of getting scratched. Concentrate on the upper dental arcade, especially the premolars and first molars.
In very old animals, the occlusal surfaces of the upper premolars are worn down level with the hard palate and may have formed a shallow C or deep horseshoe shape. Ultimately, the two roots of each premolar may become exposed as separate islands (Martin 1981; Martin & Handasyde 1999). Perimeter notches (or even fractures) may appear in the thinning occlusal surface of the upper first molar, which by now will also be almost level with the hard palate. Gingival recedence and/or formation of periodontal pockets along the labial aspect of the upper molars may occur. If the roots of the premolar tooth on either side are exposed, it is unlikely to be in the best interests of a free-ranging koala to proceed further with treatment. Similarly, if one or both premolars are horseshoe-shaped and the surface of the first molar is worn flat so that it is level with the hard palate, euthanasia should be considered if the animal is seriously injured or already in poor body condition. The degree of wear on the second, third and fourth molars should be assessed if you are unsure about whether or not to proceed.
Figure 8.4 illustrates different stages of wear on the upper premolar and first molar teeth of 12 captive koalas in south-east Queensland between 1–15+ yr of age, as well as the full rows of upper cheek teeth of another aged, otherwise healthy individual whose degree of overall tooth wear was incompatible with effective mastication and ongoing maintenance of body condition. As there is considerable individual variation in the rate of tooth wear (R Martin pers. comm.), these images are presented as a reference series to assist with quantitative recording of relative tooth wear, rather than any potential usefulness in determining the absolute age of individuals. Routine husbandry practices should have biased these captive koalas’ access to soft young foliage, and their rates of tooth wear may or may not reflect those typical of free-ranging animals from other localities.
7.1.13 Urine and faeces
The appearance of normal urine is described in section 8.5. Faecal pellets are khaki to olive in colour and tapered slightly at either end. Freshly passed pellets often have a glistening appearance due to a thin transparent coating of mucus. When broken open they reveal a densely-packed mass of fibre. In very aged animals with worn teeth, successive pellets are sometimes linked by strands of unchewed fibre. If food intake has been reduced for some time, shorter narrower pellets are produced. Occasionally, mauve faecal pellets are passed by healthy animals. The mauve colour seems to be confined to the mucous coating of the pellet. Segments of the cestode Bertiella obesa may adhere to faecal pellets (see 10.11.4a).
Many carers routinely record the number of faecal pellets produced per 24 hr period. Anecdotally, counts in the order of 90–200 pellets (or more) per 24 hr are expected in normal koalas. Pellet counts < 90 per 24 hr may reflect reduced food intake or compromised gastrointestinal function.