1.1 Biological characteristics of the domestic rabbit (Oryctolagus cuniculus)
1.1.1 Origins of the domestic rabbit
Domestic rabbits are descended from the European rabbit, Oryctolagus cuniculus. The ancestral form probably evolved in the Iberian Peninsula and spread to other parts of the Mediterranean (Fox, 1974). Fossil records show that the European rabbit was confined to the Iberian peninsula and southern France following the Pleistocene era. While rabbits have been associated with man since Roman times, they have only been truly domesticated for around 200 years. The geographical range of the rabbit has been altered significantly by man, who from Roman times onwards placed rabbits on islands on various shipping routes, to allow them to breed and form a ready source of food. Thus their European range significantly altered, and rabbits proved very successful where the climate and geographical conditions were suitable. More recently rabbits were introduced in Victoria, Australia, where their success, due to rapid breeding and a lack of suitable predators, rapidly became a plague that cost the Australian Government millions of dollars and led to the implementation of myxomatosis virus for biological control. Although European rabbits have been released in North America, the presence of suitable predators, an unsuitable climate and other species filling their ecological niche proved insurmountable. In North America the native wild rabbit is either Sylvilagus floridanus (cottontail) or Sylvilagus bachmani (brush rabbit). The North American jackrabbit, Lepus californicus, is from the hare genus.
It is not clear when the European rabbit was introduced into Great Britain. The Romans brought many food animals with them, such as pheasant and quail, and it is believed that they not only introduced rabbits but also kept them in cages, thereby starting the process of domestication. The modern pet rabbit still retains many of the characteristics of its wild counterparts despite changes in size, colour, coat texture and temperament.
Rabbits belong to the order Lagomorpha, which are characterized by the presence of a second small pair of upper incisors or peg teeth situated behind the central incisors. Lagomorphs were once considered to be a suborder of the Rodentia, which is divided into Sciuromorpha (squirrel-like rodents), Myomorpha (mouse-like rodents) and Hystricomorpha (porcupine-like rodents) that includes guinea pigs and chinchillas. Current opinion suggests that Rodentia and Lagomorpha have no fundamental similarities and on the basis of structural features and serological data, Lagomorpha show more affinity to Artiodactyla (hoofed mammals) (Nowak, 1999). Other lagomorphs include hares and pikas. All members of the Lagomorpha order are terrestrial and eat only vegetation.
1.1.2 Wild rabbits
The behavioural characteristics of lagomorphs differ between species. For example, cottontails (Sylvilagus spp.) do not dig burrows, although they may use burrows made by other animals. Females dig holes to make nests and sit over the hole to suckle the young. Vegetation is used to cover the fur-lined nest between feeds. Cottontails are solitary animals, in contrast with Oryctolagus cuniculus, which live in groups with a defined social hierarchy (Nowak, 1999).
The European rabbit, Oryctolagus cuniculus, prefers a sandy, hilly terrain with shrubs and woody plants and is not found at altitudes above 600 m. It often digs complex burrows or warrens that can be 3 m deep and 45 m long. The tunnels are about 15 cm in diameter and the living chambers 30–60 cm high. The main surface entrances are usually indicated by mounds of earth but there are numerous other small openings that lack these mounds. Oryctolagus cuniculus is essentially nocturnal, leaving the burrow in the early evening and returning in the morning, although it can be seen grazing or basking during the day. The home range is rarely larger than 20 hectares (Nowak, 1999).
Wild rabbits live in groups of two to eight adults plus juveniles with a defined social hierarchy (McBride, 1988). The group’s territory is defended by the males, while the females dig out deep burrows to nest in. Male rabbits within the group will establish a dominance hierarchy, with the older heavier males at the top. Aged males that have been usurped by younger, fitter rabbits are driven from the group to become solitary satellite males (Lockley, 1978). Young male rabbits are also often driven from the group when they reach puberty either to join another warren or to lead solitary lives in the hedgerows. The females tend to remain within the original group. Female rabbits are less aggressive towards each other than males, but will defend a chosen nesting site ferociously. Territories are scent marked with pheromones from the scent glands on the chin and genital area or by urine marking. Dominant males will continually scent mark their territory by rubbing their chins on branches and bushes and leaving piles of strategically placed faeces. They also mark territory by spraying urine, sometimes on to other individuals.
When wild rabbits emerge from their burrows at dusk, they begin to feed. Initially, they graze grass and vegetation, raising their heads at intervals to survey the surroundings, perhaps chewing through a long stalk or blade of grass at the same time. After half an hour or so, they will start to look around for other palatable plants to nibble. They are constantly on the lookout for danger and will readily bolt back to their burrow. Hard faecal pellets are always voided above ground, never in the burrow and soft caecotrophs are usually consumed during periods of rest underground, although occasionally rabbits exhibit this behaviour above ground (Lockley, 1978). The only vocal sounds that are made are a loud high-pitched scream of terror or a range of growls and hums that denote pleasure or defence. Apprehensive or frightened rabbits will thump the ground with their hind feet. The loud thumping sounds acts as an alarm signal to other rabbits in the vicinity.
Many of the behavioural characteristics of their wild ancestors are still present in the modern-day pet rabbit. Domestication has resulted in rabbits that are far tamer than their wild counterparts and easy to handle. Although some domestic rabbits still retain the tendency to dig holes and live underground, many do not, with the result that domestic rabbits that escape or are released do not survive for long in the wild. Conversely, wild rabbits seldom become tame in captivity, although the occasional individual will overcome its fear of humans. Hand-reared orphans usually grow into fearful adults. Even rabbits that are born as a result of egg transfer from a wild rabbit to a domesticated tame host retain their shy nature (Adams, 1987).
1.1.3 Breeds of rabbits
Domestication has resulted in a wide range of breeds with different attributes. They can be roughly divided into two groups: fancy breeds and fur breeds (Sandford, 1996). The fur breeds include rex, Angoras and satin rabbits with their beautiful coat textures. Fancy breeds include the Belgian hare, and lop and dwarf rabbits with their varying physical characteristics. Most pet rabbits belong to the smaller breeds such as dwarf lops, Dutch or English. Pedigree rabbit breeders often sell surplus stock to the pet trade and occasionally one of the more obscure breeds may turn up as a pet. Pedigree stock is identified by aluminium rings slipped over the hock when the rabbit is 8–10 weeks old. The rings are supplied by the British Rabbit Council in a range of sizes. Each ring has the year of birth and a unique number from which the rabbit can be identified. Many pet rabbits are the result of interbreeding between pets and are cross-breeds. As with other domestic animals, there are breed predispositions to disease. For example, dwarf rabbits are prone to congenital incisor malocclusion (Fox and Crary, 1971). Dutch, Havana and tan rabbits have a high incidence of uterine neoplasia (Greene, 1941).
1.1.4 Angoras
Angoras have been bred for wool production for hundreds of years. The wool is plucked or sheared and either spun on its own or mixed with sheep’s wool. Plucked wool is superior to shorn wool. Commercial Angoras are kept in a specialized manner to prevent staining and matting of the fur. After defleecing, woollen jackets can be worn for 2–3 weeks to reduce heat loss or a strip of fleece can be left along the back (Lebas et al., 1998). Commercial Angoras are not provided with bedding but are kept on wire mesh floors and hay is provided in a rack. The long fine coat is a definite disadvantage for the pet animal as it difficult to keep the rabbit free from knots and mats. It is not surprising that a high number of Angoras arrive at rescue shelters for rehoming. The breed is prone to intestinal obstruction by felts of ingested hair.
1.1.5 Diurnal rhythms
Many behavioural and physiological processes of rabbits show a marked diurnal rhythm. In the late afternoon wild rabbits emerge from their burrows to feed, explore, socialize and mate. Grazing resumes during the early morning before the rabbit returns to the warren. Hard faecal pellets are voided during these periods above ground. During the day, caecotrophy (see Section 1.3.1) takes place while the rabbit is resting in the burrow, typically between 08.00 and 17.00 h. Female rabbits give birth in the morning and feed their young at night (McBride, 1988). Domesticated rabbits also follow a natural daily rhythm. Laboratory rabbits that are fed ad lib consume little food between 06.00 h and midday and increase their intake between 17.00 h and midnight, eating most food during the night. Caecotrophs are expelled during periods of minimal feed intake in the morning and sometimes during the evening. If food is restricted, caecotrophs are excreted approximately 5 h after a meal. If a collar is fitted to prevent the ingestion of caecotrophs, feeding still ceases during caecotroph excretion, suggesting that cessation of food intake is not associated with gastric filling (Hörnicke et al., 1984).
The diurnal feeding pattern affects digestive processes and caecal motility, which also follow a circadian rhythm. Ingestion of food is associated with increased caecal motility and the excretion of hard faeces. Caecotrophy is associated with a decline in caecal contractions, so caecal contractions are at a maximum when the animal is feeding. If food is withheld completely, the circadian rhythm of caecal contractions is maintained, but at a lower frequency that does not correlate with soft or hard faeces production (Hörnicke et al., 1984). Absorption of volatile fatty acids and their metabolism in the liver shows a circadian rhythm parallel to the activity of the adrenal gland. Volatile fatty acid absorption into the portal circulation is greatest during the hard faeces phase of digestion, although arterial levels remain remarkably constant (Vernay, 1987). Bile acid production shows a circadia...
