Oral cavity

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:

Fig. 14.4 Normal dentition of the rabbit.

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.

Fig. 14.5 Incisor malocclusion in a rabbit.

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.

Stomach

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.

Small intestine

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.

Large intestine

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.

Digestion

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:

In this way, nutrients produced by microbial fermentation are made available to the rabbit. Food material passes through the digestive system twice in 24 hours.

Urine

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.

Male

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.

Fig. 14.6 Urogenital system of the male rabbit.

Female

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.

Fig. 14.7 Urogenital system of the female rabbit.

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.

Sexual differentiation

This can be difficult in young rabbits and is easier after the testes of the male descend into the scrotum (Fig. 14.8).

Fig. 14.8 Sexing a rabbit.

(With permission from O’Malley B 2005 Clinical anatomy and physiology of exotic species. Elsevier Saunders, Edinburgh.)