Chapter 1
Basic Small Mammal Anatomyand Physiology

Classification of small mammals

The commonly seen species of small mammals in veterinary practice are classified in Table 1.1.

RABBIT

Biological average values for the domestic rabbit

Table 1.2 gives the biological parameters for domestic rabbits.

Musculoskeletal system

The skeletal system of rabbits is light. As a percentage of body weight, the rabbit's skeleton is 7-8%, whereas the domestic cat's skeleton is 12-13%. This makes rabbits prone to fractures, especially of the spine and the hindlimbs.

Skull

Unlike many rodents the mandible is narrower than the maxilla, and the temporomandibular joint has a wide surface area, allowing lateral movement of the mandible in relation to the maxilla. The hemimandibles are fused rostrally with a fibrocartilaginous ligament which is also unusual for many rodents.

Axial skeleton

The spinal formula is generally C7, T12, L7, S4, Ca16 (where C represents cervical vertebrae; T, thoracic vertebrae; L, lumbar vertebrae; S, sacral vertebrae; Ca, coccygeal vertebrae). However, many rabbits have 13 thoracic and 6 lumbar while some have 13 thoracic and 7 lumbar vertebrae so there is considerable breed and individual variation. The cervical vertebrae are box-like and small and give mobility. The thoracic vertebrae possess attachments to the 12 (usually) paired ribs, which are flattened in comparison to cat's ribs. The first seven pairs of ribs articulate directly with the sternum. The last five pairs do not, with the most caudal three pairs being unconnected to the rest and so are free floating. The pelvis is narrow and positioned vertically. The iliac wings meet the ischium and pubis at the acetabulum, where an accessory bone unique to rabbits, called the os acetabuli, lies. The pubis forms the floor of the pelvis and borders the obturator foramen which is oval in rabbits.

Appendicular skeleton

The scapula is slender and there is a hooked supra-hamate process projecting caudally from the hamate process. The scapula articulates with the humerus which in turn articulates with the radius and ulna. In rabbits, the ulna fuses to the radius in older animals and the two bones are deeply bowed. The radius and ulna articulate with the carpal bones, which in turn articulate with the metacarpals and the five digits.

The femur is flatter than a cat's ventrodorsally, and the tibia and fibula are fused in the rabbit. The tibia articulates distally with the tarsal bones where there is a prominent calcaneus bone. The tarsals articulate with the metatarsals which articulate with the four hindlimb digits.

The hindlimbs are well muscled and powerful.

Respiratory anatomy

Upper respiratory tract

Rabbits, like horses, are nasal breathers, with the nasopharynx permanently locked around the epiglottis. For this reason, upper respiratory disease or evidence of mouth breathing is particularly problematic. The nasolacrimal ducts open onto the rostral floor of the nasal passage. The epiglottis is not visible easily from the oral cavity, making direct intubation difficult. It is narrow and elongated and leads into the larynx which has limited vocal fold development. The larynx leads into the trachea which has incomplete C-shaped cartilage rings for support.

Lower respiratory tract

The trachea bifurcates into two primary bronchi. There are two lungs, which are relatively small in proportion to the overall rabbit's body size. This means that even minor lung disease may cause serious problems. Each lung has three lobes (although the right lung caudal lobe is subdivided into medial and lateral segments), with the cranial ones being the smallest (see Figure 1.1). Rabbits do not have respiratory bronchioles leading to alveoli rather they have so-called vestibules that contain the alveoli (Cruise and Brewer, 1994).

Respiratory physiology

The main impetus for inspiration derives from the muscular contraction and flattening of the diaphragm. The lung parenchyma possesses a cellular population that is well supplied with anaphylactic mediating chemicals. These are strong enough to cause fluid extravasation and blood pooling, as well as spasms within the walls of the main pulmonary arterial supply, leading to rapid right-sided heart failure when their release is triggered.

Digestive system

Oral cavity

The dental formula is:

Table 1.1 Classification of commonly seen small mammals.

Order Lagomorpha Rodentia Eulipotyphyla Didelphimorphia Diprotodontia Carnivora Suborder Myomorpha Hystricomorpha Sciuromorpha Caniformia Family Leporidae Muridae Cricetidae Caviidae Chinchillidae Octodontidae Sciuridae Erinaceidae Didelphidae Petauridae Mustelidae Species Domestic rabbit (Oryctolagus cuniculus) Rat (Rattus norvegicus) Mouse (Mus musculus) Gerbil (Meriones unguiculatus) Syrian hamster (Mesocricetus auratus) Russian hamster (Phodopus sungorus) Chinese hamster (Cricetulus griseus) Guinea pig (Cavia porcellus) Chinchilla (Chinchilla lanigera) Degu (Octodon degus) Siberian chipmunk (Eutamias sibiricus) Eastern chipmunk (Tamias striatus) Black-tailed prairie dog (Cynomys ludovicianus) African pygmy hedgehog (Atelerix albiventris) Virginia opossum (Didelphis virginiana) Sugar glider (Petaurus breviceps) Domestic ferret (Mustela putorius furo)

Table 1.2 Biological parameters for the domestic rabbit.

Biological parameter Domestic rabbit Weight (kg) 1.5 (Netherland dwarf) to 10 (New Zealand whites and Belgian hares) Rectal body temperature (°C) 38.5-40 Respiratory rate at rest (breaths per minute) 30-60 Heart rate at rest (beats per minute) 130 (New Zealand whites) to 325 (Netherland dwarf) Gestation length (days) 29-35 (average 31) Litter size Typically 5-8 Birth weight (g) 30-100 Weaning age (weeks) 4-6 Age at sexual maturity (months) Male 5-8 Female 4-7 Lifespan (years) 6-13

The premolars and molars look physically similar and are often referred to simply as 'cheek teeth'. It should be noted that some breeds of rabbit, particularly the lop breeds, may have fewer cheek teeth in the maxilla (five instead of six on each side).

All rabbit teeth are elodont, growing continuously throughout life and the root apices are open (aradicular) with germinal tissue located at the apices producing the new tooth enamel and dentine. The premolar/molar enamel is folded providing an uneven occlusal surface with the ipsilateral jaw which allows interlocking (so-called interlocking lophs). Wear is kept even by the lateral movement of the mandible across the maxilla, allowing independent left and right arcades to engage in mastication. The incisors help differentiate Lagomorpha from Rodentia as rabbits, pikas and hares have two smaller incisors, or 'peg teeth', behind the maxillary incisors, whereas rodents have only two upper incisors. The larger (rostral) incisors only have enamel on the labial surface, whereas the smaller (caudal) maxillary peg teeth have enamel on the labial and lingual sides. This creates a wedge-shaped bite-plane where the lower incisors close immediately behind the upper large incisors and fit into a groove made by the peg teeth. The permanent rostral incisors are present at birth, although the peg teeth are replaced by permanent peg teeth at around the second week of life. The deciduous premolars present at birth are replaced and joined by permanent molars by the fourth week of life. There are no canines; instead, there is a gap, or diastema, between the incisors and premolars (see...