Rabbit Biology
The Wild Rabbit
The domestic rabbit (Oryctolagus cuniculus) originated from the European wild rabbit. There are many other species of rabbit, and these, along with cottontails, pikas and hares make up the Order Lagomorpha. Man’s relationship with the European or ‘true’ rabbit was first recorded by the Phoenicians over 1,000 years BC, when they termed the Iberian peninsula ‘i-shephan-im’ (literally, ‘the land of the rabbit’), which the Romans converted to the Latin form, Hispania, and hence the modern word Spain. The wild rabbit has long been hunted, but it is unclear exactly when domestication first took place. The Romans kept rabbits in walled enclosures (‘leporaria’) and there is evidence that they brought them to Britain, but they did not survive at this time. In Europe, and especially France, the domestication process was well under way by the fifth century, and in the twelfth century the Normans brought them to Britain, where they became established and remain as both a domestic and wild animal. Man also transported the rabbit throughout the world, often with devastating effect; absence of predators in Australia and New Zealand has led them to become a pest. However, the rabbit has not become established in the wild in North America.
Domestication
With domestication came the development of different breeds and varieties (colours). All domestic rabbits are the same species as the wild European rabbit (Oryctolagus cuniculus). In 1995, 61 breeds and 531 varieties of rabbit where recognised in the United Kingdom, and more are constantly evolving by selective breeding and mutation. Many pet rabbits are cross-breeds.
Over the centuries, man has used the rabbit for food, sport, clothing, as a scientific model and a hobby (the rabbit ‘fancy’). The keeping of rabbits as pets developed in Victorian times, and their popularity has grown enormously, to the current situation where rabbits are the third most popular mammalian pet in the UK. Although traditionally kept as and thought of as a children’s pet, increasing numbers are kept by adults and as house pets, becoming as much a part of the family as the more traditional cat or dog.
Rabbit Anatomy And Biology
Rabbits are social, burrowing herbivores that are natural prey for a large number of carnivores. As a prey species they have evolved to be constantly vigilant, lightweight and fast-moving, with a highly efficient digestive system that enables them to spend the minimum time possible above ground and in danger of capture. For the same reason of not attracting predator attention, rabbit behavior is not florid and overt and relies heavily on scent.
Life expectancy of the rabbit is 5-8 years, but some individuals live to 10 years or more.
Musculoskeletal system
The rabbit skeleton is light, making up only 7-8% of body weight. The front limbs are short and fine, in contrast to the long and powerful hind limbs. The plantar surface of the hind limb from the tarsus distally is in contact with the ground at rest. The spine is naturally curved. Body conformation varies greatly depending on the breed, from 1kg to 10kg, and from the squat or "cobby" shape of the dwarf breeds to the lithe and lean ("racy") Belgian Hare. Skull morphology can lead to disease, especially dental problems, in some breeds. For example some dwarf breeds have a mandibular prognathism which causes incisor malocclusion, and breeds with a foreshortened skull seem predisposed to nasolacrimal duct and dental problems.
Great care must be taken when handling rabbits. Osteoperosis is often present due to lack of exercise and low calcium intake, and a kick from the powerful hind legs can result in lumbar vertebral fractures (usually L6/L7).
The forelimb has five digits and the hind limb four.
The vertebral formula of the rabbit is C7T12L7S4C16. Thirteen thoracic vertebrae are seen in some animals.
Rabbit muscle is light pink in colour.
Dentition and Oral Cavity
The upper lip of the rabbit is cleft. The dental formula is 2/1 0/0 3/2 3/3. The incisors are used for grazing, and food is then passed to the back of the mouth for grinding. Incisors have enamel layer only on the anterior surface, which wears more slowly than the posterior surface, thereby maintaining a chisel shape for cutting herbage. The vestigial second pair of upper incisors are located directly behind the first pair and are known as "peg teeth". All teeth are open rooted, long-crowned and grow continuously. The cheek teeth are wider apart on the maxilla than on the mandible, and the lower teeth grow faster than the upper.
The oral commissure is small, and the oral cavity long and curved. Cheek folds across the diastema make visualisation of the cheek teeth difficult in the conscious animal. The tongue is large and has a mobile rostral portion and a relatively fixed thicker caudal portion (torus).
There are four pairs of salivary glands: parotid, submaxillary, sublingual and zygomatic.
Skin
Female rabbits possess a large fold of skin under the chin known as the dewlap, from which they pull hair to line the nest before kindling. The toes and metatarsal areas are completely covered with hair, and there are no footpads. Scent glands are located on the underside of the chin, either side of the perineum (inguinal glands) and at the anus (anal glands). The inguinal glands are large and pouch-like and often contain a yellow/brown oily deposit.
Fur
Hair coat depends on breed. The normal coat consists of a short soft undercoat protected by longer guard hairs. The only hairless areas are the tip of the nose, part of the scrotum and the inguinal folds. In the Rex breed the guard hairs are shortened so do not protrude above the level of the undercoat. Satin breeds have an altered hair fibre structure that gives the coat a characteristic sheen. Angora rabbits have very long undercoat and guard hairs that are harvested for spinning into wool, and needs regular grooming to prevent matting. The guard hairs are the first to emerge in new-born kits, followed by the undercoat. By a few days this soft baby coat is well-developed, and it persists until about five or six weeks of age. An intermediate or pre-adult coat then replaces this, followed by the adult coat by about six to eight months of age. Thereafter most rabbits molt approximately twice a year, (spring and autumn) but this can vary. Molting starts at the head and proceeds caudally. Pregnant or pseudo-pregnant does undergo a loosening of the hairs on the belly, thighs and chest, which are then easily plucked to line the nest and expose nipples.
Tactile vibrissae are present on the muzzle, which are used to help locate food and when underground.
Does possess four to five pairs of nipples on the ventrum. Nipples are absent in the male.
Eyes and Ears
The large eyes are located laterally (prey species) and rabbits have a blind spot in the area beneath the mouth, so food is detected by the sensitive lips and vibrissae. The lens is large and almost spherical, and the ciliary body is poorly developed, so accommodation is limited. The retina is merangiotic, with the optic disc lying above the midline of the eye and retinal vessels spreading horizontally out from it. The optic disc has a natural depression or cup. There is no tapetum lucidum.
A third eyelid is present and the Harderian gland is located just behind it. This gland has two lobes, the upper being white and the lower larger and pink in colour. The gland is larger in males, especially during the breeding season.
The nasolacrimal duct has a single lacrimal punctum in the medial aspect of the lower eyelid. From here there is a short (approx 2 mm) canaliculus coursing medially and ventrally into a funnel-shaped lacrimal sac, supported medially by the lacrimal bone. The duct then enters the maxilla through a semicircular foramen in the lacrimal bone. The duct has two sharp bends as it courses towards the nose, proximally in the maxillary bone and at the base of the incisor teeth. The duct narrows at these points, and this, plus the fact that the epithelium of the duct is undulating, and the opening into the nasal meatus is very small, mean that the nasolacrimal duct is very prone to blockage in the rabbit.
The ears are highly vascular and are involved in heat regulation, containing with large arterio-venous shunts. Ear size varies between breed, and those that hang down are referred to as "lops".
Digestive Tract
Rabbits are hind-gut fermenters, adapted to digest a low quality, high fiber diet consisting mainly of grass. However, unlike other hind-gut fermenters, for example the horse, the rabbit has a very rapid gut transit time and eliminates fiber from the digestive tract as soon as possible. This permits body size and weight to remain low, which is advantageous in a prey species. In the wild feeding takes place mainly in the early morning, evening and at night.
The gastrointestinal tract makes up 10-20% of body weight. The stomach is thin-walled, and poorly distensible with a well-developed cardia and pylorus. Vomiting is not possible in the rabbit. Food and caecal pellets are always present in the stomach. The duodenum and jejunum are narrow, and at the end of the ileum there is the sacculus rotundus, rich in lymphoid follicles, and also known as the ampulla ilei or ileocaecal tonsil. The caecum is very large, thin-walled and coiled, and has many sacculations (or haustrae) . It terminates in the vermiform appendix, which is also rich in lymphatic tissue. The caecum lies on the right side of the abdomen. Caecal contents are normally semifluid. The colon is sacculated and banded. Colonic contractions separate fibrous from non-fibrous particles, and fibre moves rapidly through for excretion as hard faecal pellets. Antiperistaltic waves move fluid and non-fibrous particles back into the caecum for fermentation. Three to eight hours after eating, and thus mainly at night, soft, mucus-covered caecal pellets are expelled and eaten directly from the anus (a process known as caecotrophy, coprophagy, refection, or pseudorumination). Arrival of the caecotrophs at the anus triggers a reflex licking of the anus and ingestion of the caecotrophs, which are swallowed whole and not chewed. A muscular band of richly innervated tissue with a thickened mucosa, the fusis coli, lies at the end of the transverse colon and acts to regulate colonic contractions and controls production of the two types of pellets.
The most prevalent caecal bacteria are of the anaerobic gram-negative genus Bacteroides, Proprionobacteria and Butyrivibrio bacteria. Gram negative ovals, fusiform rods, large ciliated protozoa (Isotrichia) and yeasts (Cyniclomyces guttulatus) are also present. Coliforms are not present in normal animals.
The mucus covering protects the caecal pellet bacteria from the low pH of the stomach. Caecotrophs remain in the stomach for up to six hours with continued bacterial synthesis, and eventually the mucus layer dissolves and the bacteria are killed. This process of caecotrophy allows absorption of nutrients and bacterial fermentation products (amino acids, volatile fatty acids and vitamins B and K), and the redigestion of previously undigested food. A food item can thus pass twice through the digestive tract in 24 hours.
The pancreas is diffuse and located in a pocket formed by the transverse colon, stomach and duodenum.. A gall bladder is present, and rabbits secrete mainly biliverdin in the bile rather than bilirubin. The pancreatic duct and the bile duct are separate.
The liver has four lobes.
Spleen
The spleen is flat and elongated and lies on the dorsolateral surface of the greater curvature of the stomach.
Respiratory System
Rabbits are nose-breathers (mouth breathing is a very poor prognostic sign). The nose moves up and down in a normal rabbit ("twitching") 20-120 times a minute, but this will stop when the rabbit is very relaxed or anaesthetised. The glottis is small and visually obscured by the back of the tongue. Reflex laryngospasm is common in the rabbit, which can complicate endotracheal intubation. The thoracic cavity is small, and breathing is mainly diaphragmatic. The lungs have three lobes, and the cranial lung lobes are small (left smaller than right). Large amounts of intrathoracic fat are often present. The thymus remains large in the adult rabbit and lies ventral to the heart, extending in to the thoracic inlet.
Cardiovascular System
The heart is relatively small and lies cranially in the thoracic cavity. The right atrioventricular valve has only two cusps. The rabbit aorta has neurogenic rhythmic contractions.
Urinary System
Rabbit kidneys are unipapillate. Urine is the major route of excretion for calcium. Serum calcium levels in rabbits are not maintained within a narrow range, but are dependent largely on dietary intake, with excess excreted via the kidney. Rabbit urine is often thick and creamy due to the presence of calcium carbonate crystals. It can also vary in colour from pale creamy yellow through to dark red (often mistaken for haematuria by owners), due to the presence of porphyrin pigments thought to be derived from the diet.
Reproductive System
Does have no uterine body, two separate uterine horns and two cervices opening into the vagina. The vagina is large and flaccid. The mesometrium is a major site of fat deposition.
The placenta is haemochorial. Bucks have two hairless scrotal sacs either side and cranial to the penis. There is no os penis. The inguinal canals remain open throughout life (see Breeding below)
Rabbit Physiological Data
Heart rate 180-300/minute
Respiratory rate 30 – 60 /minute
Body temperature 38.5 – 40.0C
Daily food consumption (pellets) 50g/kg
Daily water consumption 50-15-ml/kg
Daily urine production 10-35 ml/kg
Body weight adult (breed dependant) 1-6kg
Body weight new-born kit 30-80g
Life expectancy 5-8 years