Chapter 14 Mammals
Rabbits belong to the mammalian order Lagomorpha. They were originally classified with species such as rats, mice and hamsters as members of the order Rodentia because they all have chisel-shaped incisors that are open-rooted and continue to grow throughout the animal’s life. However, rabbits have two pairs of upper incisors while rodents have only one pair, leading to the reclassification of rabbits (and the related hares and pikas) as lagomorphs. Rabbits are burrowing herbivorous animals that live in large social groups. They are readily preyed upon by carnivores and much of their anatomy is adapted to sensing danger and making a rapid escape.
The wild European rabbit, Oryctolagus cuniculus, from which all our modern breeds are derived, weighs about 2.5 kg and is covered in brown ticked fur, often described as agouti-coloured. This colouration creates a dappled effect, which breaks up the outline and helps camouflage the individual. Selective breeding has led to the development of about 50 breeds of domestic rabbit ranging in weight from 1–8 kg, with a variety of fur textures and colours, many of which would be unsuitable for life in the wild.
The head is rounded, with long, black-tipped upright ear pinnae set high on either side. They are large, representing approximately 12% of the body surface, and very vascular, which enables them to be used as a means of thermoregulation. The pinnae are extremely mobile and are designed to pick up sounds of danger. The lop breeds of rabbit have been developed to have ears that hang downwards.
The eyes are protuberant and set on either side of the head, providing a wide range of monocular vision to detect predators. Rabbits are crepuscular, i.e. active at dawn and dusk, and their sight is adapted to lower light intensities. The lips are soft and covered in sensitive hairs. The upper lip is divided by a deep philtrum, which enables the rabbit to nibble grass very short.
Mature female rabbits develop a large fold of skin under the chin known as the dewlap from which they pull fur to line their nests before giving birth. The skin of the rabbit is well supplied with scent glands, which are used for territorial marking. They can be found under the chin, at the anus and on either side of the perineum.
The forelegs are relatively short and used for digging, while the hind legs are longer and provide the propulsive force for the characteristic hopping method of locomotion. They also kick the earth away when the rabbit is digging its burrow. There are five toes on each forepaw and four on each hind foot. Each toe ends in a sharp claw, those of the hind feet being long and straight. The feet are entirely covered in fur and there are no footpads.
When the rabbit is at rest, the entire plantar surface of the hind limb from toes to hock rests on the ground. When grazing, among a group of rabbits there will always be one or two standing upright on their hind legs, watching for predators. If danger threatens they will thump their hind legs on the ground to warn the others. Both of these behavioural patterns can be seen in pet rabbits. Rabbits have short fluffy tails with white undersides. As the rabbit runs the white colouration ‘flashes’ to warn the rest of the group of possible danger.
The skeleton of the rabbit (Fig. 14.1) is delicate and makes up only 7–8% of the body weight. This is in contrast to the skeleton of the cat, which makes up 12–13% of body weight. The cortex of the long bones is normally thinner than those of the cat and older caged rabbits may additionally develop osteoporosis from lack of exercise and low calcium intake.
Rabbits are herbivorous and have been likened to ‘little horses’ in that both the rabbit and the horse are hindgut fermenters, i.e. the main chamber for the breakdown of plant material is part of the large intestine. Unlike the horse, however, the digestive system of the rabbit allows for rapid passage of food through the tract and rapid elimination of fibre. This has enabled the body size and weight of the rabbit to remain small, which allows the animal to show the speed and agility necessary to escape predators. By contrast, in the horse, fibre remains in the gut for some time, necessitating the evolution of a large-volumed fermentation chamber and consequently a large body size (see Ch. 16). The digestive tract of the rabbit (Fig. 14.3) is relatively long and makes up 10–20% of body weight.
The opening of the mouth is small, the tongue is relatively large and the oral cavity is long and curved, making examination of the cheek teeth and intubation for anaesthesia difficult. All the teeth (Fig. 14.4) are open-rooted and grow continuously throughout life. They must be kept worn down by hard or fibrous food materials. The dental formula is:
If the teeth are misaligned or the diet does not include sufficient fibrous material, the teeth will not wear properly and the rabbit will suffer from a range of malocclusion problems (Fig. 14.5). Malocclusion is one of the most common reasons for rabbits being presented to vets but it can be prevented by including large quantities of good quality hay in the diet.
The incisors have enamel only on the outer surface, which wears more slowly than the inner surface, creating the characteristic chisel shape needed for nibbling plant material. In the upper jaw the second pair of incisors are vestigial pegs and lie behind the first pair. There are no canine teeth and the space between the incisors and the cheek teeth is known as the diastema. The premolars and molars – cheek teeth – are flattened table teeth for grinding food. The jaw moves in a circular fashion to force the food against their roughened surfaces. The lower teeth grow at a faster rate than the upper teeth.
This is a simple, thin-walled chamber which acts as a reservoir for food and which is never truly empty. There are well developed cardiac and pyloric sphincters. Rabbits are unable to vomit because of the arrangement of the cardia in relation to the stomach.
The duodenum and ileum are long and have a relatively small lumen. The ileum terminates at the caecum, where there is a rounded structure, the sacculus rotundus. Inside this the mucosa is arranged in a network of lymph follicles and this area is often known as the ileocaecal tonsil.
The caecum is the largest organ in the abdominal cavity and lies on the right side. It is blind-ending, thin-walled and sacculated and coils around the other organs, folding in on itself three times. It terminates in a vermiform appendix, which contains abundant lymphoid tissue. Food passes on into the colon, which is also sacculated but shorter, with a smaller-diameter lumen.
Rabbits are herbivorous, monogastric, hindgut fermenters. The ingested plant material passes down the tract by peristaltic contractions and undergoes enzymic digestion in the stomach and small intestine. The partially digested material enters the caecum, where it mixes with colonies of microorganisms responsible for the fermentation and breakdown of cellulose found within plant cell walls.
The now semi-liquid material passes into the colon. Contractions here pass fluid back into the caecum for re-use in the fermentation process and also separate fibrous from non-fibrous material, resulting in the production of two types of faeces:
The rabbit is an obligate nose breather, i.e. it must breathe through its nose. (Mouth breathing is often a poor prognostic sign.) The nose twitches 20–120 times per minute but ceases under general anaesthesia. The glottis is small and difficult to see as the view is impaired by the relatively large tongue. This can make intubation difficult and precautions must be taken to avoid reflex laryngospasm.
The thymus gland remains a considerable size into adult life. It lies ventral to the heart and runs cranially to the thoracic inlet. The thoracic cavity is quite small and breathing mainly involves the diaphragm. The lungs have three lobes on each side; the cranial lobes are small.
The kidneys are unipapillate, i.e. a single medullary pyramid drains into the renal pelvis and ureter. In the doe the ureters drain into the bladder, which is tough but thin-walled, and the urethra empties into the ventral wall of the vagina; in the buck the ureters drain low down on the neck of the bladder.
In the normal healthy rabbit this can vary in colour from deep red to yellow or white. It may also vary in turbidity from clear to cloudy (this is due to the presence of calcium). The kidneys are the main route for the excretion of calcium, and serum levels depend on dietary intake – excessive intake may cause calcification of the aorta and kidneys.
The male rabbit is known as a buck. There are two testes which in the adult male lie in two almost hairless scrotal sacs cranial to the penis (Fig. 14.6). The testes descend at about 12 weeks of age but the inguinal canal remains open; there is no os penis. The buck has no nipples.
The female rabbit is known as a doe. The reproductive tract of the doe is bicornuate, i.e. it has two separate uterine horns designed to hold litters of young (Fig. 14.7). There is no uterine body and each horn has its own cervix opening into the vagina. The mesometrium is a major fat storage organ. The doe has four or five pairs of nipples.
The female rabbit is an induced ovulator and does not have a well-defined oestrous cycle. There are periods of sexual receptivity every 4–6 days. Ovulation occurs within 10 hours of coitus. The age of sexual maturity varies with the breed: small breeds are mature at about 5 months while larger ones mature as late as 8 months. The young are altricial, i.e. they are born hairless, deaf and blind and are totally dependent on their mother for the first few weeks of life. They are protected from the environment and predators by a warm, fur-lined nest but if disturbed the dam may eat them.
Fig. 14.8 Sexing a rabbit.
(With permission from O’Malley B 2005 Clinical anatomy and physiology of exotic species. Elsevier Saunders, Edinburgh.)
This group includes the small animals that are among the most popular children’s pets: rats, mice, gerbils, hamsters, guinea pigs, chinchillas and chipmunks. They all belong to the order Rodentia – derived from the Latin rodere, meaning to gnaw – the common characteristic of which is that they have prominent, yellow-coloured incisor teeth with an open pulp cavity. This means that the teeth continue to grow throughout the animal’s life and to maintain them at a normal length the animal must gnaw on hard or fibrous food material.