Introduction

Even in the neutered pet, rabbits draw very little distinction between sexual behavior and social behavior (Harriman 1995). Normal actions like territoriality, attention-seeking behaviors like honking and circling, and nesting behavior such as digging and chewing, are all intimately tied to the instinctive drive to reproduce.

If you are reading this article as part of the Basic Rabbit Care Teaching Module, please go to the sections on male rabbits, female rabbits, newborn rabbit, and rabbit milk. Then take the brief quiz to test your knowledge.

“Breed like rabbits”

The prolific nature of the rabbit has linked them with fertility and the cycle of life and death since ancient times. In fact the idea of the Easter bunny probably arose from the medieval belief that rabbits, as a creator of life, ushered in the dawn (Fig 1) (Mayer 2003).

Figure 1. Rabbits have been associated with fertility and the cycle of life since ancient times. Photo credit: Elliott Brown via Flickr Creative Commons. Click image to enlarge.

One female rabbit can potentially deliver up to 60 young per year. Because of this fecundity, early explorers carried rabbits as a food source and rabbits were even released on remote islands. Unfortunately the absence of predators allowed rabbit populations to rapidly reach pest numbers on some islands like New Zealand (O’Malley 2005).

Male rabbits

The head and body of the intact, adult, male rabbit is generally more thickset than the doe (Richardson 2000). The penile sheath is cylindrical and the penis can be easily extruded in rabbits over 2 months of age. The scrotal sacs sit craniolateral to the penis (Fig 2). The scrotum is oblong and partially hairless, and the testicles are relatively large with prominent epididymal fat pads (O’Malley 2005).

Figure 2. The oblong, partially hairless scrotal sacs sit craniolateral to the cylindrical penis. Photo credit: Dr. Matt Rosenbaum. Click image to enlarge.

The testicles descend sometime between 10 to 14 weeks of age. The first appearance can vary with the individual, the breed, and environmental temperature. The adult rabbit can retract the testes back up into the abdomen through the open inguinal ring when stressed or to regulate testicular temperature. Testes descend further on hot days and are brought closer to the body on cool days. This phenomenon can be distinguished from a true cryptorchid male by the absence of scrotal sac(s) (O’Malley 2005, Richardson 2000).

Prior to castration of the male rabbit, elevation of the hindquarters or gentle pressure on the caudal abdomen will cause testicles to fall back into the scrotum. A closed castration technique is preferred to minimize the risk of post-operative inguinal hernia. If an open surgical technique is performed, the large, superficial inguinal ring should be closed.

Female rabbits

Adult females, particularly medium and large breed does, often develop a dewlap or fold of skin beneath the chin. Older does tend to be larger than bucks of the same breed (Richardson 2000).

The ovaries are elongated, and are located relatively caudal in the rabbit. The oviducts are very long and coiled. The duplex uterus consists of two separate uterine horns separated along its length. There is no uterine body. The mesometrium is a site of significant fat storage in the rabbit, proportionally much greater than in other companion animals. The two uterine horns communicate with two cervices, which join to form a common vagina (O’Malley 2005). The rabbit vagina is relatively long and saccular (Vella 2012). The urethra enters the dorsal wall of the vagina; the clitoris sits on the ventral surface. The vulva appears triangular with a slit-like opening, and the appearance of this slit is used to distinguish juvenile females from juvenile male rabbits.

If the urinary bladder is expressed while the rabbit is in dorsal recumbency, the relatively flaccid vagina can potentially fill with urine. To minimize the risk of contamination during ovariohysterectomy, the bladder is expressed after the patient is anesthetized but before the animal is placed on its back (Jenkins 2012). The suspensory ligaments are relatively long, making exteriorization of the uterus relatively easy, however the large amount of fat in the broad ligament still makes rabbit spays relatively challenging procedures, even in young does. The double cervices are not routinely removed during ovariohysterectomy, however removal is indicated for patients with cervicitis, neoplasia, or endometriosis (Jenkins 2012, O’Malley 2005).

The female rabbit is an induced ovulator. There is no regular estrous cycle, instead ovulation occurs after mating. If coitus does not occur, the doe will vary in receptivity as ovarian follicles regress and new follicles mature. Periods of receptivity last anywhere from 5 to 14 days and are followed by one to two days in which the doe will refuse to mate. This cycle repeats until conception occurs, although ovarian activity decreases as photoperiod decreases during the late summer to winter months (Vella 2012, Klaphake 2012, O’Malley 2005).

Vaginal smear cytology is not useful for identification of doe receptivity (O’Malley 2005), however the appearance of the vulva can provide a helpful clue. When the doe is receptive, the vulva is more swollen and is often a pink-purple or reddish-purple color (Klaphake 2012, O’Malley 2005). During anestrus the vulva appears narrow and pale (O’Malley 2005).

Breeding rabbits

Both male and female rabbits can be quite territorial, and mating is best accomplished if the doe is placed in the buck’s enclosure or if the pair are introduced in neutral territory (Bays 2006, Richardson 2000).

Upon introduction, the buck follows the doe around, softly humming while sniffing and licking her for approximately 30 seconds (Vella 2012, Bays 2006). He may also spray the female with urine (Bays 2006). The receptive female will hop around in circles or flatten to the floor (Bays 2006). Lordosis is observed when pressure is applied to her back (Bays 2006), while the non-receptive doe will run away from the buck, and if cornered she may vocalize or even bite. Active mating begins when the buck grasps the female by the nape with his teeth. He then mounts the female rabbit, thrusting vigorously until ejaculation occurs relatively quickly. Afterwards the male emits a sharp cry or squeak, before falling onto his back or side while the doe either runs away or begins to bite and kick the male (Video 1)(Bays 2006, Richardson 2000).

Video 1. Rabbit breeding is a brief affair

Ovulation occurs 10 to 13 hours after mating (Vella 2012, O’ Malley 2005, Richardson 2000). Although a single mating is often sufficient stimulus to stimulate ovulation, breeders often allow mating to occur several times over a 30-minute period before returning the doe to her enclosure. Despite her fecundity, the doe should have no more than three litters in one year (Richardson 2000). Reproductive life varies with the breed, however bucks are typically bred for 5 to 6 years and does for approximately 3 years (Vella 3012).

Gestation

When compared to hares with their 40 to 50 day pregnancy, rabbits have a relatively short gestation period averaging 31 days. Gestation can range from 28 to 35 days (Vella 2012, Bays 2006, O’Malley 2005), however the risk of stillbirth increases by Day 32 (O’Malley 2005). Litter size ranges from four to 12 kits (Vella 2012, Bays 2006). Small breed rabbits tend to produce smaller litters that are born after a relatively long gestation period (Vella 2012). Larger litters are generally born after a shorter gestation period (O’Malley 2005). The fetus is palpable by Day 12 to 14 (Richardson 2000).

Despite their high fertility rates, rabbits tend to suffer from a high incidence of embryonic mortality. There are many potential reasons for this problem including infection, heredity, trauma, drug use, poor nutrition, as well as social or environmental stress. Fetal death and resorption is most likely to occur in subordinate does (Vella 2012, Klaphake 2012). The fetus is also at increased risk on Day 13, when placentation changes from yolk sac to hemochorial, and on Day 21 when there is a temporary reduction in blood flow as the fetus changes in shape and size (Klaphake 2012, O’Malley 2005).

Parturition

The doe begins to nest several days to a few hours before parturition. Hair epilates more easily as estrogen levels rise and progesterone levels fall, and the doe plucks hair from her abdomen, sides, and dewlap. She then uses the fur to line her nest of hay and straw (Fig 3) (Vella 2012, O’Malley 2005).

Figure 3. The doe interweaves plucked hairs with hay and straw to create her nest. Photo credit: Andre Mouraux via Flickr Creative Commons. Click image to enlarge.

Kindling usually occurs during the early morning hours and normally takes about 30 minutes (Vella 2012). The doe begins a fertile postpartum estrus within 24 hours of kindling, however her receptiveness decreases once lactation begins and this lack of interest in breeding continues until after weaning (O’Malley 2005).

Newborn rabbit

Unlike the precocial young of hares, rabbits deliver altricial young that normally weigh 40 to 50 grams at birth (O’Malley 2005, Harkness 1995). The kit is born hairless, with sealed eyelids and ear canals. Anogenital stimulation is required for elimination of feces and urine (Table 1) (Bays 2006).

Despite their relatively helpless state, the young are not brooded by the mother (Bautista 2013, Bays 2006). Brown fat levels are highest during the first 2 weeks of life. Brown fat produces heat or “non-shivering thermogenesis” because of its extensive capillary network and its reserves are unaffected by the animal’s nutritional state (O’Malley 2005).

Until fur begins to grow in at Day 10, kits also depend heavily on the warmth and insulation provided by littermates (Bautista 2013, Bays 2006, Hull 1982). Central litter positions are associated with higher body temperature, higher milk intake, heavier body weights, and faster growth rates (Bautista 2013), however behavioral differences have also been identified based on litter position during the first week of life. Adult animals that occupied the periphery showed an “enhanced survival instinct” being “more proactive” than “intermediate” or “central” littermates (Reyes-Meza 2011).

Kits rely heavily on their sense of smell at birth. The mother is recognized by the smell of her feces, and suckling is stimulated by a pheromone secreted by a gland near the nipple (Vella 2012, O’Malley 2005). Does also scent mark their kits, and females will pursue and even kill kits from other colonies. Successful cross-fostering of neonatal domestic rabbits requires camouflaging the new kit’s scent by rubbing the newcomer in nest bedding and/or placing the kit on the bottom of the litter pile (Bays 2006).

Rabbit milk

Rabbits usually possess eight mammary glands that extend over their thoracic and inguinal region. Only the doe has nipples. She may possess as many as 10 nipples, and the presence of accessory nipples is favored by rabbit breeders (Vella 2012, O’Malley 2005).

Rabbit milk is very rich, so rich in fact that the doe needs only nurse her young once or twice a day (O’Malley 2005, Cheeke 1987). Rabbit milk is very high in fat (9%) with unusually low lactose levels (1%) and very high protein (13%) (Table 2) (O’Malley 2005, Cheeke 1987, FAO). Physiologically, lactation is a very demanding time for the doe. Water consumption increases ten-fold during lactation as does cecotroph consumption (O’Malley 2005).

The doe spends approximately 3 to 5 minutes at a time nursing her young (O’Malley 2005, Cheeke 1987). The free-ranging doe then carefully covers the nest burrow with soil each time she leaves (Bays 2006). This “stop” disguises the nest from predators (Thompson 1994).

The neonatal rabbit stomach has a pH of approximately 5.0 to 6.5. A stomach at this pH, full of milk curd, would normally make an ideal substrate for bacterial overgrowth. Fortunately the kit’s stomach contains a protective antimicrobial factor called “milk oil” during the first 3 weeks of life. Milk oil is a mixture of octanoic and decanoic fatty acids and is produced by an enzymatic reaction that occurs when does milk comes into contact with enzymes in the kit’s digestive tract. Hand-raised rabbits lack this protective antimicrobial factor making them susceptible to infection (O’Malley 2005, Harkness 1995).

Weaning

Kits begin to leave the nest and eat solid food at approximately Day 18-21. Weaning is generally achieved by Day 42. The doe is usually removed during the weaning process so the young can remain in a familiar enclosure.

Weaning is a critical time in the rabbit’s life when the young are vulnerable to illness. Kits begin to eat cecotrophs passed by the doe at approximately 2 weeks of age. By the time the protective effect of milk oil ends at 4 to 6 weeks, the gut has not been completely colonized by healthy microbes and stomach pH has not reached a mature adult level of 1 to 2. “Bad bacteria”, like coliforms and Clostridia spp., can proliferate causing rapid enterotoxemia, particularly when the rabbit is fed a low fiber, high carbohydrate diet (O’Malley 2005, Cheeke 1987).

Puberty

Until sexually mature, wild rabbits live furtively and often alone. By 3 to 4 months of age the youngsters may form pair bonds and take their place in a colony’s social hierarchy. Since males can outnumber does, male rabbits sometimes live solitary lives as “satellites” to the colony (Thompson 1994). Mortality rates can be as high as 90% during the first year of life (Harriman 1995).

Body weight is more important than age in determining sexual maturity. The juvenile rabbit reaches puberty just after it undergoes a maximal rate of growth; therefore the age of onset for sexual maturity varies with the rabbit breed. Small breed rabbits typically develop faster and become sexually mature at an earlier age (Table 3). Does generally reach puberty before bucks (Vella 2012).

Optimal sperm production occurs 40 to 70 days after puberty is reached (Vella 2012). Male rabbits should be housed separated from females by at least 16 weeks of age to avoid unplanned pregnancies.

Sexual behavior

Negative sexual behaviors, such as territoriality and aggression, are most intense at the height of adolescence. Observed behaviors can include chinning, circling, honking or oinking, and mounting and humping. During puberty, both males and females that were previously litter trained may also urinate and defecate outside of the box to mark their territory (Bays 2012). Nesting behavior, such as frantic digging and chewing, is also commonly observed in does (Bays 2012, Bays 2006). Visit Behavior Basics: The European Rabbit for additional details on sexual behavior in the pubescent rabbit.

Fortunately neutering improves the pet quality of house rabbits, and responsible rabbit owners elect to have their pets spayed or neutered. Viable sperm can persist post-castration, so it is prudent to keep the buck separate from the doe for 4 to 6 weeks to be safe.

Pathology

Uterine adenocarcinoma is the most common neoplasia of the domestic rabbit (Varga 2013, Klaphake 2012, Vinci 2010, Walter 2010). The incidence of uterine tumors is independent of the doe’s breeding history (Klaphake 2012). Age is considered the most important risk factor for this slowly developing tumor, and the incidence of disease is highest in middle aged to older rabbits (Klaphake 2012, Saito 2002). Uterine adenocarcinoma is present in approximately 60% of females after 4 years of age (Varga 2013); the mean age of affected rabbits at presentation is 6.1 years (Walter 2010).

Early clinical signs of uterine adenocarcinoma can include subtle, non-specific signs of illness such as anorexia, and loss of body condition. Infertility, vaginal discharge, and hematuria can also be observed (Fig 4). Pale mucous membranes will develop with persistent or heavy hemorrhage (Varga 2013, Walter 2010). As disease advances, additional findings can include gastrointestinal stasis, swollen, painful mammary glands, and abdominal swelling. Multiple masses may be palpable cranial to the urinary bladder on physical exam. Does are sometimes presented for dyspnea secondary to pulmonary metastases or excessive uterine enlargement (Varga 2013).

Figure 4. Hemometra due to adenocarcinoma in a doe. Note the duplex uterus. Photo credit: Uwe Gille via Wikimedia Commons. Click image to enlarge.

Uterine neoplasia is best managed through prevention. Does not intended for breeding should be spayed early. The owners of intact females should be educated on the early signs of disease, and instructed to present the doe regularly for physical examination (Klaphake 2012).

Other pathologic conditions commonly reported in the doe include endometrial hyperplasia, pyometra, endometritis, and pseudopregnancy (Varga 2013, Klaphake 2012, Walter 2010, O’Malley 2005). Pseudopregnancy, also known as pseudocyesis or “false pregnancy”, can occur even in does housed alone although the most common causes include an infertile mating or the presence of a nearby male rabbit (Klaphake 2012, O’Malley 2005). As the mature corpus luteum (CL) secretes progesterone, enlargement of the uterus and mammary glands is most pronounced during the first 10 days. By Day 16, organs begin to involute. The CL begins to degenerate after Day 18. As progesterone levels fall, the doe will pluck fur to make a nest after 18 to 22 days (O’Malley 2005).

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References