Rabbit Anatomy

Rabbits are such endearing animals. Those big eyes, huge ears, wiggly nose, those strong back legs and that unusual tail. Let's learn what makes the rabbit the creature that it is.

Rabbits are unique creatures, members of the group of animals known as lagomorphs. They are not members of the rodent family, as is often thought. The scientific name of the domestic rabbit is Oryctolagus cuniculus.

The nose of the rabbit is relatively small, but the sense of smell is quite good. Because they are animals subject to predation, their senses are well-developed. So, in addition to an excellent sense of smell, they also have well-developed eyesight. Since their eyes are positioned more laterally than most other mammals, this provides them with a panoramic field of vision (approximately 190 degrees) in order for them to better view predators. They have a light sensitivity that is eight times greater than that of humans. Rabbits possess a double retinal system of rods and cones in the eye. Interestingly, their eyes readily detect motion and apparently are sensitive to the blues and greens present at twilight (when they are often out feeding). While they can see well to the sides, they are unable to view the small area below the mouth, so they rely on the sensitive lips and whiskers to aid them in finding food.

Rabbits possess a third eyelid, called the nictitating membrane that moves across the cornea (surface of the eye) and offers protection and lubrication.

The most common problems associated with the eyes and nose are infections caused by the bacterium, Pasteurella multocida. This bacterium can be responsible for a myriad of problems in the rabbit, and in the eyes, it can manifest as conjunctivitis and a greenish discharge from the eyes. Pasteurellosis can also cause a nasal discharge, usually a greenish color, as well. Other bacteria can cause eye and upper respiratory infections as well. The cornea can also become ulcerated from trauma, abrasions and aggressive bacterial infections.

Interestingly, if for some reason, the rabbit is restrained tightly around the neck for any length of time (as for ear cleaning, grooming, etc.), it can cause the eyeball to swell and protrude, due to occlusion (cutting off) the blood supply to the external jugular vein, which is the primary venous drainage of the eye and head (unlike other animals, where the major drainage is via the internal jugular vein, which is harder to compress due to its deeper location).

Those huge rabbit ears do serve several purposes other than making a rabbit look like a rabbit! The ears represent a large portion on the total body surface of the rabbit, approximately 12%. The ears are fragile and sensitive and should NEVER be used to lift or restrain a rabbit. The ears have quite an amazing blood supply that helps in regulation of body temperature, and they have the largest shunts between arteries and veins of the ears, for that purpose. Of course, those big ears are also wonderful for gathering sound. Rabbits do have very acute hearing.

Most ear problems are related to a rabbit that has contracted ear mites. Rabbits with ear mites usually have a black/brown waxy exudate in the ear canals, and this often has a bad odor. Another ear problem involves the middle or inner ear, and can result in a head tilt, abnormal eye movement called nystagmus and balance problems.

Rabbit teeth are open-rooted, meaning that they grow continuously. The upper and lower incisors are modified to act as chisel-like cutting instruments. Incisors can grow 10-12 centimeters per year, and if the teeth are not properly aligned, this can result in overgrowth of the teeth, especially the incisors, although the molars and premolars (cheek teeth) can develop sharp points that can irritate the tissues of the mouth. Evaluation of the cheek teeth requires the use of an otoscope, fiberoptics endoscope or other device to visualize them properly, since rabbits have a very small, narrow mouth that only opens a few millimeters. Overgrown incisors may need to be trimmed frequently to prevent problems with eating. If the teeth are severely maloccluded, it might be best to have a veterinarian remove them surgically, to provide a permanent solution.

The skin and fur of rabbits have some unique properties. The fur is very dense and fine, and the skin is thin, and can tear easily. Grooming is necessary on a regular basis to remove excess shed hair to help prevent the ingestion of hair resulting in hairballs (trichobezoars) in the gastrointestinal tract. If a rabbit requires surgery, care must be taken to pull the skin taut to prevent cutting the skin when using electric clippers.

Mites and lice are uncommon in the fur, however, occasionally the rabbit fur mite, Cheyletiella parasitovorax is diagnosed. It may appear as dandruff or dry skin, and the hair may be thin over the rump, back and back of the neck. In some cases, the rabbit with fur mites may exhibit pruritis (itching). Fleas can also infest rabbits, especially those rabbits that are housed outdoors or those that live in a multiple-pet household with dogs or cats. As with the treatment for fur mites, a vet knowledgeable about rabbit medicine should be consulted, and the environment should also be treated for parasite infestation.

Rabbits on a low fiber diet may barber their own fur. Those that are subordinate may be barbered by the more dominant rabbit. Does in heat may barber their own fur and pregnant does that are close to giving birth may pull hair from the forelegs, the dewlap, lower part of the chest and hips to build a nest.

Rabbits with dental disease may drool excessively, with the saliva causing a moist dermatitis. Female rabbits with a large dewlap (a fold of skin over the throat and neck area) may develop a moist dermatitis in that area if they drink out of a water bowl instead of a water bottle. Correction of the underlying problem is imperative so that the skin can heal.

The skin around the urinary opening, perineum and ventral abdomen may become scalded from chronic exposure to urine. This may occur as a result of a rabbit that has urinary tract problems. Urine scald may also result from a rabbit with spinal problems (having problems holding urine) and may also occur from mobility problems associated with obesity. Treating the underlying cause is important in correcting the skin problem and preventing future dermatitis. Salves and ointments usually are detrimental, and not helpful. Clipping the fur away is helpful, and the application of an astringent will help the skin heal. However, always consult your rabbit vet for medical problems and only use medications prescribed for your rabbit.

Rabbits have three pairs of scent glands used in scent-marking behavior. Since rabbits are so territorial, they use these glands to define their territories. There are chin glands, anal glands and inguinal glands. The size of the glands and the amount of marking behavior are related to the level of sexual activity of each intact rabbit. Male rabbits, bucks, mark more than females, does, and dominant rabbits mark more frequently, and most often, in the presence of subordinates. Does also use the scent glands to mark their kits.

The skin of the female rabbit has four or five pairs of mammary glands and nipples, and unlike other mammal species, bucks do not have any nipples. Females may develop an infectious mastitis or a non-infectious cystic mastitis of the mammary tissue. Cystic mammary glands may eventually progress to malignancy and the cancer may spread through the lymph nodes, lungs or other organs.

Rabbits tolerate the cold much better than high heat and humidity. The heavy fur, combined with obesity, in older rabbits, may result in overheating, heat stress, infertility or death. Temperatures lower than 40 degrees F can be dangerous, and rabbits should be provided with protection from the cold and wind to prevent illness or injury. Temperatures above 85 degrees F are not well-tolerated.

The skin of a rabbit infected with rabbit syphilis or vent disease caused by the spirochete bacterial organism (Treponema paraluis-cuniculi), may show lesions suggestive of the disease, such as ulcers, scabs, white lesions or swelling. This usually occurs on the skin of the perineum and genitalia, however, lesions around the chin, lips, nostrils and eyelids may also occur and present as white, firm projections from the skin, almost appearing as white, horny structures.

Cats, dogs and rodents have footpads; however rabbits have, instead, compressed coarse hair that covers under the toes and along the hock areas. Rabbits housed improperly, and those that are overweight, may develop ulcerated hock and metatarsal areas. If you notice that the feet or hocks appear ulcerated or infected, veterinary care should be sought. Rabbits have very sharp toenails that should be periodically trimmed during routine grooming.

The skeleton of the rabbit is quite delicate when compared to that of other animals. Rabbits should always have their hind end supported when they are lifted, as they are likely to kick, which may result in a fractured back and resultant spinal cord damage. Tibial fractures also are commonly encountered. Other bones may become broken due to kicking, improper handling or other traumatic incidents.

Rabbits have a relatively small chest cavity. When it comes to breathing, rabbits must normally breather through their nose. It is therefore a grave sign if a rabbit must resort to mouth-breathing. Because rabbits have such a small oral cavity, it is difficult to place an endotracheal tube inside the trachea (windpipe) during anesthetic procedures. If a rabbit has significant inflammation of the upper respiratory tract due to infection with Pasteurella or another bacterium, without being intubated during anesthesia, there is a higher risk of mortality during the procedure.

If you ever should notice a nasal or ocular discharge, audible breathing, mouth breathing or other respiratory signs, you should have your rabbit examined by a veterinarian familiar with rabbits.

The rabbit heart is relatively small when compared with that of other species, and smaller rabbits usually have faster heart rates than larger ones, and rates range from 180 to 250 beats per minute. In the rabbit, the right atrioventricular valve of the heart has only two valve leaflets (cusps) instead of the usual three in other species (bicuspid vs. tricuspid). Rabbit veins are very thin-walled and are prone to hematoma formation (a blood-blister under the skin) after having blood drawn, so it is important for vets to take special precautions during and after venipuncture to prevent tears and hematoma formation.

Inside the chest cavity, in addition to the heart and lungs, the rabbit has a thymus, which is a lymphoid gland found in the mediastinal area (inside the chest cavity, on the midline, and ventral to the heart) of the chest and the lower part of the neck. The thymus gland is essential to the normal development of the immune system in juvenile animals, and in most species of animal, the thymus begins to involute, or shrink down, at around the time of puberty and the lymphoid tissue is then replaced with fat. In the rabbit, the thymus is functional and present throughout the life of the rabbit.

The abdomen of the rabbit is large in comparison to the chest cavity. The gastrointestinal tract is long, and the rabbit has a large stomach and cecum. The large stomach usually contains food (hay, pelleted feed, vegetation and fecal pellets that have been ingested) at all times. Due to the unique anatomy of the rabbit's stomach, it is unable to vomit. If the stomach is distended by food, gas, foreign bodies or hair, or if the liver is very enlarged, this can prevent the stomach from emptying into the intestines. The last portion of the small intestines connects to the cecum and is an expanded area of the intestine called the sacculus rotundus, which has a honeycomb external appearance, contains a large number of lymph follicles, and is sometimes referred to as the ileocecal tonsil. This is also a common site for foreign body impaction.

The cecum, a portion of the intestines, is by far, the largest internal organ in the abdomen. While the human cecum is also called the appendix and appears to serve no useful function (other than to provide a diagnostic challenge to human surgeons), the cecum of the rabbit folds on itself three times and contains semifluid ingesta. Normally, the intestines contract sequentially, to propel ingesta through the gastrointestinal tract from north to south, so to speak, and this is called peristalsis. But in the rabbit, there are also antiperistaltic contractions that move fluid and ingesta retrograde back up through the colon and into the cecum. In the cecum, fermentation of the intestinal contents occurs, and periodically, the cecum contracts and the fermented ingesta is propelled into the colon and then out the anus, where the fecal pellets are directly ingested by the rabbit. This is called coprophagy or cecotrophy, meaning the ingestion of feces. The soft feces from the cecum are called night feces or cecotropes, and they are clusters of small pellets and not single, hard fecal pellets, as are normally produced during the daytime. The night cecotropes are coated with a type of mucus that acts as a barrier to the acidic pH of the stomach, ensuring that the contents will be absorbed from the small intestine.

Other abdominal organs include the pancreas, which has two functions: one to provide enzymes for digestion and the other to control sugar levels in the tissues and bloodstream. The liver is located under the diaphragm and performs various functions regarding metabolism, detoxification and digestion. The rabbit does possess a gallbladder and the main secretion from the gallbladder is bile, containing biliverdin, rather than bilirubin, which most mammals produce.

The kidneys, ureters, urinary bladder and urethra are all parts of the urinary tract of the rabbit. Interestingly, a two kilogram rabbit will drink about as much water as a 10 kilogram dog in a normal day, so when compared to other species, rabbits have a high water intake normally. The most common problems related to the urinary tract involve high calcium levels in the blood that create stones, sludge or sand that may affect the kidneys, ureters, bladder or urethra. Most mammals have a fractional urinary excretion of calcium of less than two percent, but in the rabbit, the range for calcium excretion is around 45-60 %. This calcium that is excreted through the kidneys can result in the mineral causing a variety of problems as it precipitates out, resulting in depression, decreased appetite, weight loss, lethargy, blood in the urine, decreased or no urinary output, straining to urinate, grinding the teeth, a hunched posture and urine scalding of the skin around the hind end. The urine may appear cloudy, creamy or turbid. If radiographs are taken, it may be possible to visualize calcium sand or stones in the urinary tract. To lessen the calcium excreted through the kidneys, rabbits should be offered grass and timothy hay and vegetables. Alfalfa-based pellets should not be offered, as alfalfa is high in calcium, and if any pellets are offered at all, timothy/grass hay based pellets should be given.

Renal failure may occur due to a number of reasons. Several compounds and medications are toxic to the kidneys in rabbits. Cysts may occasionally be found in the kidneys as an inherited problem. The urine of rabbits may occur in a variety of colors, including red, due to porphyrins from plants being excreted in the urine.

A microsporidian protozoal organism, called Encephalitozoon cuniculi has an affinity for kidney and nervous system tissues. Signs of infection in the kidneys are usually sub-clinical and not apparent, although microscopic lesions may occur.

The female reproductive tract of the rabbit is unique in several ways. There are two separate uterine horns, but no uterine body, as dogs and cats possess. Each uterine horn opens into the vagina separately. Rabbits are induced ovulators, like cats and ferrets, and do not have an estrus cycle. The external orifice is the vulva, and it usually swells and becomes reddish-purple when the doe is receptive to the buck for breeding.

The male rabbit has two hairless scrotal sacs forward from the penis, and in most mammals the scrotal sacs are caudal (behind) the penis. The testicles descend into the scrotal sacs by about 12 weeks of age, however, the inguinal canals do not close, as they do in many other species, preventing the testicles from retracting back up into the body cavity. The penis overlies the anus, and it can be protruded by gentle pressure on either side of the genital opening. There are two inguinal pouches, one on either side of the urogenital opening in both sexes, and these pouches may be filled with a strong-smelling, dark-colored glandular secretion.

Any discharge from the vulva should be evaluated by a rabbit veterinarian. Does may develop cancer (adenocarcinoma) of the uterus. Polyps and endometrial hyperplasia of the lining of the uterus can occur, and this may eventually progress to cancerous lesions. Endometritis, metritis and pyometra also occur in the uterus. Infection may be low-grade or very serious, with the uterine horns filling with bacteria, white blood cells, debris and fluid. Abdominal palpation, radiographs and/or ultrasound results may show enlarged uterine horns. Rarely, abscesses of the ovaries may occur, usually secondarily to pyometra.

The testicles of a buck may become infected with bacteria, resulting in orchitis. Usually, the rabbit with orchitis may have intermittent fever, decreased appetite and weight loss. The testicles may be swollen, hot or red, or the abscesses may be internal, with minimal swelling. The epididymis may become infected, as well. The epididymis is an elongated structure connected to the far end of the testis. Infection may affect both the epididymis and testicle at the same time, or each structure may be infected independently.

Castration of the buck and ovariohysterectomy (spay) of the doe is recommended at an early age to prevent reproductive system problems later in life. These procedures are commonly performed by the experienced rabbit veterinarian, however, the spay procedure is a more serious surgery, as this is a major abdominal operation, as it is in most female animals of any species.

The tail is composed of the coccygeal vertebrae, tendons, ligaments, nerves, arteries, veins, muscles and skin. The last bone in the tail is usually somewhat pointed. The tail is moveable, and can be used in communication between rabbits. Tail flagging may occur during the breeding ritual.

We have now covered the anatomy of the rabbit from the tip of the nose to the tip of the tail. Being in the family of lagomorphs, they have many unique anatomical differences than other commonly kept exotic pets.

Cadeusus
Copyright 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.
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