Deciduous teeth, apart from the third premolars, degenerate before eruption. Broadly, marsupials may be divided into two groups on the basis of dentition: (1) the polyprotodonts (many incisors), including the dasyurids, numbat, opossums, marsupial moles, bandicoots, and bilby, the majority of which are carnivorous or omnivorous; and (2) the diprotodonts (two lower incisors), including the macropods, koala, wombats, possums, and gliders, the majority of which are herbivorous. The diprotodonts have a variably sized diastema between the incisors and molars. Wombats are the only diprotodont marsupials with one pair of upper incisors and the only marsupials with hypsodont teeth (rootles and ever growing).⁶⁷

Some species of macropods, particularly those in the genus Macropus, exhibit molar progression. Molars erupt caudally and migrate forward throughout life. Cheek teeth are shed from the front as they wear out, replaced functionally by the next tooth. Molar progression is unique to macropods, elephants, and manatees. In the koala, the occlusal surfaces of the molar teeth wear continuously throughout life, and the degree of tooth wear has implications for plane of nutrition.

Gastrointestinal System

Macropods are foregut fermenters. In a process known as merycism, macropods, particularly the browsers and grazers, occasionally regurgitate food into their mouths, which involves a rather violent heaving motion with vigorous movements of the forelimbs and the thorax. The bolus is not generally rechewed as in ruminants and is quickly reswallowed.¹⁴ Koalas, wombats, and most possums and gliders are monogastric, hindgut fermenters. Koalas possess the largest cecum relative to body size among all known mammals. The wombat small intestine is short; the colon is large and sacculated; and the cecum is vestigial. In possums and gliders, an expanded cecum is the main site for fermentation in most species. The intestines of the striped possum (Dactylopsila trivirgata) and honey possum (Tarsipes rostratus) are simple because of the lack of plant material in their diet. Coprophagy of cecal contents occurs in some species of ringtail possums. Dasyurids, bandicoots, and the bilby have simple gastrointestinal tracts.

In females, the urogenital sinus (into which the urethra discharges), together with the rectum, opens into the common vestibule (or cloaca). In males, the penis lies within the common vestibule. The urogenital openings are ventral to the rectum.

Skin

Many species of dasyurids, opossums, and the numbat lack a pouch. Some species develop a temporary pouch from folds of skin on either side of the abdominal teats during lactation. Male marsupials lack teats. Numbers of teats in females varies: 2 in koala, wombats, 4 in macropods, numbat, 4 to 12 in dasyurids, 2 to 6 in possums and gliders, 6 to 8 in bandicoots, bilby, and 4 to 13 in opossums. Marsupials are well endowed with cutaneous glands, which have various functions. Macropods generally have thin skin, particularly in the inner surfaces of the limbs, and is particularly pronounced in the forelimbs, where the vascularity of the subcutis is significantly increased. Wombat skin is considerably thicker than that in other marsupials and quite inelastic, and all three species have a characteristic “sacral plate” on the rump, almost as rigid as bone. Gliders are distinguished from possums by the presence of a gliding membrane extending from the hindlimb to the forelimb.

Sensory Systems

Macropods have a wide field of peripheral vision enabling them to see movement in almost every direction, and binocular vision enables them to have more precise close vision. They have good day and night vision. Despite this, they appear to have difficulty navigating obstacles and barriers that are not solid, especially when alarmed and fleeing. The iris is thick and uniformly brown, and the pupil is circular and may be dilated easily with tropicamide. The fundus is heavily pigmented (usually dark brown) ventrally, whereas dorsally it is usually lighter in color, and in some animals, the choroidal blood vessels may be seen through the pigment. The optic disk sits at the junction between these two zones and is well vascularized. Retinal vessels are not prominent. All macropods have persistent hyaloid vessels (seen as a tuft of vessels arising from the center of the optic disk and extending anteriorly toward the posterior lens capsule). In most macropods, they are fixed anteriorly and are not easily visualized. If not fixed, they may be seen moving within the vitreous. In many kangaroos, myelination of the nerve fiber layer may be seen extending from around the optic disk. This myelination is generally most obvious in the lateral and medial aspects.⁶²

The koala has relatively small, spherical, frontally placed eyes. The cornea occupies a relatively large segment of the globe. A tapetum extends across the retina above the optic disk, and, as in many other marsupials, the retina is essentially avascular except for a small area on the optic disc itself.⁵⁶ Mean intraocular pressure is 24.2 ± 6 mm Hg.²⁶ The iris is usually brown. The pupil is a vertically orientated slit. The lacrimal puncta are slitlike openings just inside the lid edges, 2 to 3 millimeters (mm) from the medial canthus. Dasyurids have a vascular retina. The arteries do not branch but form discrete capillary loops.

Immune System

Significant differences exist in the development of the immune systems of marsupials and eutherian mammals. No lymphoid tissue is present at birth, and the neonate is not immunocompetent. Most species reach immune system maturation at the time of first release from the maternal teat, approximately half way through pouch life. Passive transfer of maternal antibodies occurs throughout lactation, and antibodies are absorbed unchanged across the gut epithelium of the pouch young (PY). This passively acquired immunity is short lived, and most maternal antibody will be lost by about 4 weeks after separation from the mother, with the young only protected by an underdeveloped active immune system. Secretions from the pouch epithelium and macrophages found in marsupial milk may play roles in immunologic protection of the PY.⁵²

In healthy macropod PY, the thymi are firm, bulging structures on the ventral side of the neck and may be visualized in unfurred or early furred PY. The thymi regress fully by sexual maturity.⁵⁹ Superficial lymph nodes are not palpable in healthy macropods. The superficial inguinal, superficial axillary, rostral mandibular, mandibular, and facial lymph nodes are palpable in healthy koalas. Popliteal and subiliac lymph nodes are absent.²⁵

Reproductive System

Marsupials have a diffuse, epitheliochorial choriovitelline placenta. Bandicoots have a diffuse placenta, with both choriovitelline and chorioallantoic contributions.⁶⁶ Female reproductive anatomy is similar in all marsupials. Each of the paired lateral uteri opens into a vaginal cul-de-sac through its own cervix. The paired lateral vaginae both open separately into the urogenital sinus at the level of the urethral opening and are separated along their length by a soft tissue median septum (Figure 33-1). During parturition, the fetus tunnels through this septal tissue to form a median vagina. In most marsupials, the median vagina closes rapidly after passage of the fetus and then re-forms with each subsequent parturition. In most macropods, opossums, and the honey possum the median vagina becomes epithelialized after the first parturition and remains patent as a permanent median vagina.⁶⁶

The Macropodidae are the only marsupials to produce semen, which coagulates to form a seminal plug in the female’s urogenital sinus and vaginae after ejaculation¹⁴ and may be seen protruding from the female’s common vestibule.

Urinary System

The urinary system is similar in all marsupials. Unique to marsupials is the path of the ureters from the kidney to the bladder. The ureters pass between the median and lateral vaginae on each side. This anatomy has consequences for surgical procedures of the female reproductive tract. In macropods, the koala, and the wombats, the bladder is intrapelvic when not distended.

The urethra of male macropods has paired valvelike cusps approximately 2 to 3 centimeters (cm) proximal to the external orifice. Each cusp is approximately 5 mm long, with its free margin directed toward the external urethral orifice. A similar but larger pair of cusps lies immediately distal to the urinary bladder sphincter. These cusps may make urethral catheterization difficult. In koalas, two regions of the male urethra may potentially impede catheterization. The first is a narrowing of the lumen and some mucosal irregularity in the vicinity of the openings of the bulbourethral glands. The second is the prostatic sinus, a groove on either side of the urethral crest just distal to the neck of the bladder.

Metabolism and Thermoregulation

Brown adipose tissue, which is used by eutherians to generate heat, has not been found in marsupials.²² Some marsupials will enter one of two types of torpor. Shallow daily torpor in which the body temperature drops to 11° C to 28° C and lasts 2 to 20 hours occurs in opossums, dasyurids, small possums, and possibly in the numbat and marsupial moles. Deep, prolonged torpor (hibernation), in which body temperature drops to 1° C to 6° C and lasts 1 to 3 weeks occurs in pygmy possums (Burramyidae), the feathertail glider (Acrobates pygmaeus), and the Monito del Monte.²¹

Marsupials cannot thermoregulate until about half way through pouch life. Marsupial metabolism is lower than that of eutherians, with macropods at 25% and the koala and hairy-nosed wombats (Lasiorhinus spp.) very low at 44%. Basal metabolic rates are highest in the small insectivorous dasyurids (similar to eutherians), whereas rates for other dasyurids are less than half those in eutherian carnivores of similar size.³⁰ All wombats, in particular the hairy-nosed species, have extremely low requirements of energy, protein, and water. Plasma concentrations of thyroid hormones in wombats are the lowest recorded for any mammal.⁴ Koalas and hairy-nosed wombats rarely drink fluids.

Thermoregulation is by evaporative mechanisms (panting, sweating, and licking). A unique feature of kangaroos is that sweating stops as soon as exercise stops, even if body temperature is still elevated and the animal is still panting rapidly.^14,67 A method of evaporative heat loss by spreading saliva on the forearms is common in macropods that have undergone a period of exertion or anxiety. Panting is the major means of evaporative cooling in the koala. Wombats are more tolerant of cold than heat and cannot sweat, using postural thermoregulation or salivating on their forelegs and chest to aid cooling. Dasyurids do not sweat and cope with excessive heat by licking and panting to increase evaporative cooling.

Miscellaneous

Macropods have hearts one-third larger than those of comparably sized eutherian mammals and may move more air through their lungs with each breath.¹⁴ Hand-reared, adult macropods that have a nervous temperament may display a wide-eyed expression and trembling of the head, neck, and upper body when approached. This is termed tintibulation and is an apparent expression of anxiety or excitement. In the koala, the linea alba is broad and almost translucent, and no curtainlike omentum exists. Even in healthy, well-nourished captive koalas, fat depots are meager in comparison with those in most eutherian mammals.

Hand Rearing

Successful hand rearing of marsupial PY requires a detailed understanding of neonatal development and physiology, the unique pouch environment, maternal care, passive immunity, development of the immune system, and milk composition.⁴³ The major challenges in hand rearing marsupials are concerned with simulating the functions of the pouch, which provides a stable thermal environment until the PY becomes endothermic, and providing an appropriate milk substitute that may meet the changing needs of the developing PY until weaning and minimize exposure to pathogens.⁹

The milk of marsupials must support the PY from its embryonic state at birth until independence. The composition, therefore, changes profoundly during the course of lactation. The net effect of these changes is a gradual increase in energy content throughout lactation, meeting the increasing energetic demands of the growing PY as it becomes more active and develops endothermy. Calcium, phosphorus, sodium, and potassium concentrations also change during lactation.^32,43,67

Marsupial milk has very low levels of lactose, oligosaccharides being the predominant carbohydrates for most of lactation. The transport of sugars into the intestinal mucosal cells is a relatively slow process. Mother-reared PY drink small volumes frequently. Hand-reared PY are fed larger volumes less frequently. When hand-reared PY are fed milk containing lactose, the capacity of the mechanism for transporting sugars into the cells is exceeded; hence unabsorbed lactose accumulates within the intestinal lumen, increasing the osmolality of the gut contents, resulting in watery diarrhea, dehydration, malabsorption of other nutrients, malnutrition, and retarded growth. Oligosaccharides are significantly larger than lactose and thus exert a lower osmotic effect.^43,49

Numerous milk formulas have been used. Critical factors in the choice of a formula are low lactose content, and the formula should mirror the changing composition of natural milk from early to late lactation. A number of formulae that satisfy these requirements are available.⁴³ Nevertheless, hand-reared PY of a wide range of species have thrived on a wide range of formulae, and the formula chosen is not the only key to success. Hygiene, husbandry, stress management, consistency, carer experience, and a maternalistic, nurturing instinct are equally as important.

The weaning process needs to be carefully managed for many species. Herbivorous species require inoculation of the gut with appropriate bacteria at this time. Hand-reared koalas that have not consumed pap must be fed natural pap or a substitute. Macropod and wombat young should be given access to fresh feces from healthy adults or mixed with water or formula and fed as a slurry.

Chemical Restraint^27,70

Injectable chemical restraint agents may be administered by hand, intravenously (IV) or intramuscularly (IM), or remotely by using darts in larger species. In wombats, the sacral plate over the rump makes it impossible to give injections or dart in this area. Inhalational agents may be administered via a face mask or induction chamber (small species) or via an endotracheal tube after induction. Intubation of macropods, koalas, and wombats may be difficult. They have a narrow gape and dental arcade, and the distance from the oral opening to the larynx is substantial. Koalas have a long soft palate. In macropods and wombats, lateral recumbency, with the head extended, is preferable, whereas in koalas, intubation is possible in lateral, sternal, or dorsal recumbency. A long, narrow-bladed laryngoscope, a small rigid endoscope, or a long, curved transilluminator may be used to visualize the glottis. Long endotracheal tubes are required. A guide stylet may be inserted into the trachea and the endotracheal tube passed over it. Blind intubation may also be achieved by grasping the larynx between the thumb and forefinger of one hand and passing the endotracheal tube over the tongue with the other until the tube touches the glottis. As the animal takes a breath and air is heard rushing through the tube, the tube is inserted into the trachea. Larger dasyurids, possums, gliders, bandicoots, bilbies, and opossums are easily intubated with the aid of a small, narrow-bladed laryngoscope. A stylet may be required. For some possums and gliders, ventral recumbency, with the neck extended and head held vertically (similar to intubating a rabbit), may be used.

Preanesthetic fasting is not generally necessary for macropods, koalas, wombats, bandicoots, and bilbies; however, regurgitation may occasionally occur (particularly in macropods), and intubation is recommended for longer procedures. Preanesthetic fasting for 6 to 8 hours is recommended for dasyurids and 1 to 2 hours for larger possums, gliders, and opossums. In koalas, periods of apnea and, in unintubated animals, respiratory stridor caused by the long soft palate is relatively common. Profound and prolonged tranquillization has been seen in wombats with the use of long-acting neuroleptic agents. Ptyalism, poor relaxation, constant limb and jaw movements, and prolonged recoveries may be seen with tiletamine/ zolazepam in some species. In addition tachycardia, respiratory depression, apnea, muscle rigidity and deaths have been seen in possums and gliders.

Box 33-3 provides a list of chemical restraint agents, regimens, and doses used in marsupials.

Laparotomy

Consideration must be given to the voluminous GI tract in some species, the presence of a pouch (with or without PY) in females, and the epipubic bones. With the animal in dorsal recumbency, a ventral midline approach is used. If access to the caudal abdomen is required, tilt the body head down to about 30 degrees. In both sexes, when access to the cranial abdomen is required, the approach is straight forward. In females with a pouch, the approach will depend on the size of the pouch and the presence of PY. If the pouch is empty or the PY may be temporarily removed, a midline incision may be made inside the pouch, midway between the teats and the cranial border of the pouch. A useful technique is to use stay sutures to hold the pouch open. If a PY is present and cannot be removed, an incision may be made lateral to the pouch opening at the cranial end of the epipubic bone. The skin is then reflected toward the midline to expose the linea alba.

In the koala, extreme care is required when opening the abdominal cavity through a ventral midline incision. Subcutaneous fat is absent; the linea alba is thin; the abdominal muscles are 2 to 3 mm thick over the ventrolateral abdominal wall; and the cecum and proximal colon lie immediately below the incision site.⁸

Ovariohysterectomy is technically more difficult and risky because of limited access and the unique anatomy of the reproductive tract. The only indication for the surgery may be therapeutic reasons. The approach is as for ovariectomy. A hole is made in the broad ligament just lateral but close to the ovaries to avoid catching the ureters (which should be visualized). The ovaries are exteriorized with gentle traction on the suspensory ligament. The vessels are clamped and ligated. Once both ovaries are ligated and detached, the broad ligament is torn off, staying as close to the lateral margins of the blood vessels as possible to avoid damaging the ureters. The uteri are ligated at the immediate proximal end of the cervices (see Figure 33-1). They are first ligated separately, and then a single ligature may be placed around both uteri. Transfixation ligatures may be required. Once the uteri are removed, a continuous suture to close the stump may be necessary.

Sterilization of koalas and macropods may also be performed via laparoscopic tubal ligation or ovariectomy. The animal is positioned as above. Following routine surgical preparation, the abdomen is insufflated with carbon dioxide (CO₂). Three small incisions are made to allow insertion of the grasping forceps, laparoscope, and cautery scissors into the caudal abdomen. For tubal ligation, each oviduct is visualized and manipulated with the forceps so that transection and cautery may be achieved. For ovariectomy, the ovary is manipulated and elevated with the grasping forceps and removed from the ovarian bursa. Electrocautery (with or without endoclips for large animals) is used to ligate the ovarian vessels. Laparoscopic long-blade scissors are then used to free the ovary from its abdominal attachments. Once free, the cannula is retracted through the skin incision to allow direct removal of the ovary from the abdominal cavity through the incision.

Orthopedics

All standard orthopedic principles and techniques may be readily applied to fracture repair in marsupials. In macropods, the unique anatomy of the hindlimbs and the massive mechanical stresses that are placed on the bones of the hindlimbs and pelvic girdle during locomotion make adequate stabilization of fractures of these bones difficult. The prognosis for successful repair in adult macropods is generally poor. Hand-reared PY that may be confined to a pouch during convalescence are better candidates for fracture repair. External fixateurs have been used with success to repair fractured tibias in older hand-reared macropods. In small species and perhaps the tamest of larger species, fractures of the metatarsi may be repairable with either internal or external fixation.

Miscellaneous Procedures

Tail amputation has successfully been performed in macropods, with the animals adapting well to the loss of the tail, despite the obvious reliance on the tail for a pentapedal gait.^65,68 Standing up from lateral recumbency may occasionally be a problem. A standard technique for tail amputation is used.

Cataract surgery in macropods is technically easy; however, many cases develop postoperative glaucoma, probably secondary to chronic uveitis. The prognosis even with treatment is poor, and many cases require globe evisceration or enucleation or euthanasia. In PY with galactose-induced cataracts, surgical removal of the lens often reveals extensive opacification of the vitreous, making the prognosis for return to normal vision poor. Because of these complications it is difficult to justify cataract surgery in macropods.⁶²

Conjunctival ablation in koalas may be indicated (as an adjunct to topical treatment, systemic treatment, or both) in chronic cases of chlamydial disease, where proliferative conjunctivae obscure vision.⁸