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Reproduction

White Rhinoceros

White Rhino Reproductive Age

According to the North American Regional Studbook for white rhinos that includes data on 148 breeding females, 155 breeding males and their 555 calves (Capiro, 2024), the youngest white rhino to give birth was four years old which means she conceived at about two years eight months of age. By comparison, data from in situ populations indicate a somewhat later age of first reproduction, with average age of first reproduction occurring between 6.5 and 7.5 years of age in the wild (OwenSmith, 1973) and small private game reserves (Swaisgood et al., unpublished data). The youngest age of first birth reported in situ was 5.0 in a sample of 67 births (Swaisgood et al., unpublished data). Field studies also indicate little reproductive behavior before age five, with the mean age of first copulation at 6.1 years (Swaisgood et al., unpublished data). The oldest female white rhino to give birth was 41.5 years at the time of conception. The youngest and oldest male white rhinos to sire a calf were three years two months and 48 years eight months, respectively, at the time of conception. Therefore, it appears that white rhinos have the longest period of reproductive potential among rhino species starting at approximately three years of age and extending into their late 30’s and early 40’s. Although the oldest female at first reproduction was 30 years old (Capiro, 2024) at the time of the birth, only 3% of dams reproduced for the first time after the age of 20, indicating that first conception needs to occur before the females reach their late teens or fertility may be lost. To consider- the studbook is a database, but context such as what breeding situation the females were in is missing.

Seasonal Changes in Physiology Associated with Reproduction and Management Implications

The African white rhinoceros is not seasonally restricted in its breeding activity. Based on year-long fecal hormone metabolite analyses (Schwarzenberger et al., 1998; Patton et al., 1999; Brown et al., 2001), it appears that reproductively active female white rhinos are capable of exhibiting reproductive cycles and breeding throughout the year. Although there is variability in reproductive cycle lengths with some being ~35 days and others ~70 days, season does not appear to be associated with the expression of different cycle lengths. Additionally, many white rhinos are acyclic, but those that are do not cycle during any season of the year. Finally, North American studbook data confirm that white rhinos conceive and give birth every month of the year with only a slight skew towards successful matings in April to September (see figures below), which is almost certainly due to increased mating opportunities that result from the management strategies employed during the warmer weather.

Reproductive Cycle

Sound scientific data from both hormone metabolite monitoring and ultrasonography support the conclusion that white rhinoceroses can exhibit reproductive cycles of different lengths. Both 30-35 day (Hindle et al., 1992; Radcliffe et al., 1997; Patton et al., 1999; Brown et al., 2001) and 65-70 day (Schwarzenberger et al., 1998; Patton et al., 1999; Brown et al., 2001) cycles have been reported. The difference in the two cycle lengths cannot simply be attributed to individual animal differences because several females have been known to exhibit both cycle lengths over a period of months (Patton et al., 1999). The physiological mechanism differentiating these cycles is not yet understood, but based on incidence of pregnancy following matings, it is generally believed that the 30-35 day cycle can be fertile (Radcliffe et al., 1997; Schwarzenberger et al., 1998; Patton et al., 1999). Whereas, pregnancy has not yet been documented following a mating associated with a 65-70 day cycle (Schwarzenberger et al., 1998; Brown et al., 2001). In some cases, the longer cycles were associated with pregnancy and early embryonic death, indicating that elevated progesterone levels were the result of a lost pregnancy rather than an extended luteal phase (Radcliffe et al., 1997; Patton et al., 1999). An intensive field study documented 17 cycles circa 30 days in length but no longer cycle types, providing evidence supporting the shorter cycle as normal for the species (Swaisgood et al., unpublished data). The longer cycle length in white rhinoceroses is more prevalent in older females and may be a characteristic of aging animals that have not been reproducing consistently throughout their lives (Schwarzenberger et al., 1998; Patton et al., 1999; Brown et al., 2001). Another pattern that is cause for concern due to its prevalence within the captive white rhinoceros population is the acyclic or “flat liner” pattern, wherein progesterone remains at baseline. In studies conducted on relatively large sample sizes, close to 50% of the females exhibited this inactive pattern (Schwarzenberger et al., 1998; Patton et al., 1999; Brown et al., 2001). One hypothesis for cessation of reproductive activity is premature senescence resulting from oocyte depletion in animals that cycle continuously for years without becoming pregnant (Hermes et al., 2004). However, not all acyclic animals are aged, and some encouraging, anecdotal information suggests that cyclicity may resume in some acyclic females if they are introduced to a new male (Patton et al., 1999). The white rhinoceros is a spontaneous ovulator with a follicular phase that lasts about nine days. The preovulatory follicle reaches a size of ~30 mm in diameter and changes from spherical to pear-shaped about 48 hours before ovulation (Radcliffe et al., 1997). Ovulation has been confirmed 24 hours after breeding but cases of anovulation, hemorrhagic follicle formation and early embryo loss have also been reported in this rhino species (Radcliffe et al., 1997). Fecal progesterone metabolite concentrations increase six to nine days after mating and ovulation (Patton et al., 1999; Radcliffe et al., 1997).

Peak Breeding Season for White Rhinos in North America

Based on the North American studbook, an adult under managed care will breed throughout the year, and calves have been produced in every month of the year with no particular bias for a specific season. In situ populations also give birth throughout the year but have higher birth rates in the wet season when there are more resources available to support lactation (Owen-Smith, 1973; White et al., 2007). The distribution of births by month for southern white rhinos is shown in Figure 2.1 for the 555 white rhino births reported in the 2024 North American Regional Studbook (Capiro, 2024). Figure 2.2 shows the months of conception for white rhinos based on an approximate gestation of 16 months. These data demonstrate that white rhinos are fertile and conceive throughout the year. Although a slight seasonal effect on male rhino testosterone and mating activity has been reported for wild white rhinos (Kretzschmar et al., 2004), the trend towards more fertile matings from April to September in ex situ white rhinos could be management related since temperate zoos are more likely to pair their rhinos for mating during months with good weather.

Reproductive Monitoring

White rhinos breed best when maintained in larger social groups, comprised of at least one male and greater than two females, in large enclosures (Swaisgood et al., 2006; Metrione, 2010). Most males are fairly compatible with females, so reproductive monitoring is not typically necessary for timing introductions. However, hormone monitoring can be a valuable tool for determining if a rhino has truly stopped cycling when overt signs of estrus are not observed. Monitoring is also useful for assessing reproductive cycle length since white rhinos with the longer (~70 day) cycles are less likely to produce calves. Hormonal analyses are most commonly employed to diagnose and monitor pregnancy. Several hormones and their metabolites have been employed for monitoring reproductive activity in the African white rhinoceros (Table 2.7), with fecal progesterone metabolites most commonly chosen.

Introductions

Changing social groupings of rhinos through the introduction of additional individuals to an established individual, pair or group is a process requiring care and planning. Rhino species vary widely in social structure, and rhinos periodically vary their grouping patterns in the wild according to factors such as reproduction and the rearing of young. Social groupings in captivity, therefore, should also vary according to species, as well as to the circumstances within each institution. Rhinos may be very protective of their individual boundaries, but proper introduction procedures can minimize injury from conflict and aggression. The following section outlines general considerations for any rhino introduction and provides systematic descriptions of aggression, procedural recommendations, and descriptions of potential species-specific introduction types. Factors that must be considered in any introduction include individual animal personalities, staff experience and confidence level, and enclosure type (i.e., indoor/outdoor, public/off-exhibit, relatively small/large). Barrier types and temperature should also be considered. Introductions often result in aggression, and it should be noted that rhinos of both sexes have been the aggressors. Territorial defense is often limited to ritualized confrontations, in which two rhinos advance toward each other but stop nose-to-nose and engage in a staring contest to gauge each other’s size and strength. Also as part of this ritual, the two individuals may touch horns, back apart and wipe their horns on the ground (Nowak, 1991). More intensive conflicts involve head-on charges and the infliction of injuries by horning or ramming. In general, behaviors that have been noted during rhino introductions are listed in Table 2.3. It is important to note that what is often perceived as serious or dangerous aggression between rhinos is, in fact, normal behavior requiring no intervention of any kind. Along with increased size and thick skin comes decreased vulnerability compared with many other animals. Table 2.4 lists a descriptive hierarchy of aggression levels in rhinos.

Table 2.4. Levels of aggression in rhinos (Fouraker and Wagener, 1996).

Level of Aggression Definition

1 Rhinos are charging each other but do not make physical contact.

2 Rhinos are charging each other with physical contact resulting in some cuts and scrapes to the facial area.

3 Rhinos are charging each other with physical contact resulting in cuts and scrapes to the facial area and body.

4 Charging and/or pursuit proceeds to the point that one or both rhinos are knocked down at least once. Scrapes and cuts are deeper and more numerous.

5 Aggression and pursuit proceed to the point that one or both rhinos have subcutaneous wounds or arterial blood flow.

a It should be noted that one animal might break away from the confrontation and attempt to escape. The aggressor often will pursue and begin horn-prodding the underbelly of the escapee as the two run around the enclosure. Often a rear leg is hooked and held aloft while pursuit continues. If the escapee does not stop and resume a defensive posture, the animals might continue until heat or exhaustion becomes a critical factor. Aggression at this point is more serious.

In some cases, aggression may proceed to a point at which management should intervene to prevent serious injury. Rhino managers should allow some aggression during an introduction but be prepared to intervene in the event that aggression threatens the lives of one or more animals. Protocols for intervening may vary across institutions, but in general, careful consideration should be given to intervening in an introduction before aggression reaches Level 5 (Table 2.4). Stopping an introduction at a level prior to this will not lessen aggression during a subsequent introduction attempt. Animals that are allowed to “settle their differences” will establish some territorial boundaries and will usually not engage in serious aggression again, with the exception of a male attempting to approach an estrous female. In sum, moderate aggression is commonplace in any rhino introduction; sparring and fighting will occur and result in minor injuries (cutaneous wounds). However, in most cases, aggression levels prior to Level 5 may be allowed to continue using the discretion of management.

Table 2.3. Behaviors noted during rhino introductions (Fouraker and Wagener, 1996).

Non-aggressive Behaviors Ritualized Confrontations Potential Stress-Related Aggressive Behaviors

Follow Head sweep Pacing Charge/chase

Touch/rub/lick Face-to-face stare Running (excessive) Open-mouth threat

Anal or genital Space-maintenance and Sparring

investigation threat vocalizations Goring

(excessive)

Diarrhea

Head Bob

The introduction process requires much planning and cooperation among managers. Table 2.5 outlines recommended steps for rhino introductions. Familiarization through visual, olfactory and tactile contact should be permitted if at all possible prior to a full-scale introduction. If the facility permits, this may be accomplished by first placing individuals in the same barn or in nearby outdoor lots, or by providing olfactory cues such as urine, feces, or skin rubbings from the animal being introduced. As the animals acclimate, managers may move them to adjacent barred stalls or fenced outdoor yards. These barriers prevent confrontations leading to serious injury but allow acclimation and familiarization prior to introduction. The actual introduction should be attempted in the largest available enclosure. Enclosures should be large enough to allow ample space for shading, mock-fighting, aggression and defense. The enclosure should contain visual barriers such as brush or earth piles or boulders (“run-arounds”), which give rhinos places to hide without becoming cornered or trapped. These features may lessen overt aggression if a rhino is able to escape the sight-line of another. An enclosure should not contain dead ends in which an individual may become trapped by an aggressor. The enclosure should allow for the use of high-pressure fire hoses, CO2 fire extinguishers and/or vehicles to aid in separating individuals.

Animal personality and disposition should always be considered in introductions. A subordinate animal should be introduced to a more dominant animal in an enclosure familiar to the subordinate. In the case of multiple-animal introductions (such as introduction of a new female to an established male-female group, discussed below), the most subordinate animal should be introduced to the next most subordinate, and so on up the dominance hierarchy. Greater aggression may be noted in some individuals in the presence of an estrous female; therefore, any introduction attempt at this time should be especially well-monitored or possibly avoided if the attempt involves a male

Table 2.5. Steps in the introduction process

Step Description

Animals in the same barn or multiple outdoor lots should have olfactory and auditory exposure to each other. If the animals are not housed near each other (i.e., enclosures on opposite sides of the zoo, etc.), they should be moved to the same exhibit area.
Animals should be given visual contact with each other in addition to the above sensory modalities. Animals may be shifted within a barn or in adjacent outdoor lots. If at any point during this process the animals display symptoms associated with stress (e.g., pacing, diarrhea, excessive vocalizations) for more than 2-3 hours, the introduction should return to the previous step.
If animals are not already positioned adjacent to each other, they should be moved closer together (e.g., to adjacent stalls or adjacent outdoor enclosures).
The actual introduction (full tactile exposure) should take place in the largest enclosure available and follow guidelines stated in this chapter. Preferably, the enclosure should be familiar to the least dominant animal and include ample “run-arounds”.
Within institutions in which rhinos can be left together 24 hours per day, they should be separated during the first several nights or until they show only minor aggression.

Appropriate personnel for first-time introductions include the primary animal manager, a vet with immobilization equipment, and the curator and keepers most familiar with rhinos. Other staff also may be needed at critical points around the enclosure’s perimeter so that the animals may be observed at all times in case separation becomes necessary. It should be noted that if a barn is opened and used to separate individuals, only one individual should be allowed inside the barn, and it must not be trapped inside by an aggressor. White rhinos are preferably maintained in herd-like (male and multiple female) groups in captivity; therefore, many types of introductions may need to be attempted. Following are recommended protocols for potential white rhino introduction types.

Introduction of a New Male to a Female (or Vice Versa) to Form a Pair

The introduction should occur in the largest lot available, following the general introduction protocols above. If a single large lot is not available, adjoining lots should be opened to form a large area for the introduction. If the latter strategy is used, care should be taken to modify any resulting dead ends in the exhibit where a rhino may become trapped during an aggressive interaction.

Introduction of a New Female to an Established Male-Female Group

If given the opportunity, female white rhinos will establish social bonds with one another. New females should be introduced to a group one female at a time. As each subgroup of females becomes stable, additional females may be introduced one at a time. Finally, the stable female group (including the new female) should be introduced to the male. The time required for stable integration ranges from one to ten weeks.

Introduction of a New Male to an Established Female Group

A group of females to which a male is to be introduced should be compatible prior to the introduction of the male. Unlike the introduction of a female to a same-sex group, the male should be introduced to the group as a whole rather than to one individual at a time. The reported time required for stable integration has been estimated at five weeks. If the females are too aggressive to the new male, it may be necessary to reduce the numbers of females at introduction. If splitting the females into subgroups, it is useful to keep together those females that seem to be behaviorally bonded, particularly if there are four or more females. If only three females, that group should not be broken up to introduce the male.

Reintroduction of a Female with New Calf to a Male-Female Group

Following parturition, the reintroduction of a female and her new calf to a group should be treated as a first-time introduction of a female to an established group. The two should be allowed to acclimate to one female at a time, successively forming larger and larger female-female/calf subgroups. The final step is introducing the entire female group (including the new female and her calf) to the male. Some institutions have placed infants and their mothers back with the herd or single male as early as two weeks after birth

Pregnancy and Parturition

Table 2.6 lists behaviors associated with pregnancy and impending birth. In general, pregnant white rhinos will cease behaviors associated with estrus and exhibit a lack of breeding behavior. In all species, there may be a mucus discharge, noticeable weight gain or increase in girth size, as well as increased frequencies of defecation and urination throughout gestation. Pregnant white rhino females have been observed isolating themselves from other individuals. If pregnancy is confirmed (pregnancy fecal hormone tests are available, or through ultrasound), and/or the breeding date is known, the physical separation of the pregnant female from the bull/herd should take place as early as 30 days and as late as 24 hours prior to birth. Institutions with very large enclosures have had successful births in the yard with the male present; however, the female(s) and any males must be watched very closely. The onset of labor often takes place at night or in the early morning and may last one to three hours. Parturition usually lasts ten to 12 hours from breaking of the water, though first-time mothers may take longer to calve. The presentation of a calf is generally head-first, although rearfeet presentations do occur and may take longer than head-first births, but usually deliver without assistance. Capabilities for monitoring births remotely through closed-circuit television or other means are advisable.

Table 2.6. Physiological and behavioral indicators of impending parturition.

30 Days Prior to Birth 2 Weeks Prior to Birth 24 to 48 Hours Prior to Birth

Increase in teat size Nipples enlarge Udders increase dramatically in size

Beginnings of milk production Nipples develop wax Inappetance

Milk may be expended with pressure on the teats Vulva swelling occurs Becomes irritable and aggressive to stimuli, including staff

Female may prolapse Mucus plug forms vaginally when defecating Increased vulva dilation

Increased restlessness, lies down often

Calf Development

A single calf is generally the rule. Few data are available on birth weights, but in general, calves weigh 54 to 70 kg (range=120-155 lb; n=5). Immediately following birth, the newborn calf is usually cleaned by its mother and stands for the first time within 30 minutes to five hours of birth. A newborn calf may require a substrate that allows traction to help steady it. Suitable materials may include sand, gravel, straw, hay or rubber matting. In all cases, both the dam and calf should be monitored closely to prevent ingestion of the substrate. A calf should begin nursing within one to two hours of standing (though in a single case, a calf removed from its dam for medical intervention nursed 16 hr post-birth). The dam will nurse her calf while standing or lying on her side. Infants less than two months old may nurse hourly, while older calves nurse at intervals of about 2.5 hours. It has been reported that calves may gain up to 4.54 kg (10 lb) per day for the first ten days. The first defecation has been reported at two to ten days of age (n=2). Calves may nurse for up to two years, although they have been observed first sampling solid food at less than one week to one month of age. Calves may be offered supplemental feedings of milk if the dam is believed to be a poor milk producer or the calf is not gaining weight (see Nutrition chapter).

Infant rhinos have been successfully pulled from their mothers because of rejection, medical issues related to the mother or infant, or from a failure to nurse. Otherwise, it should be noted that weaning for management purposes can be accomplished if necessary at six months, but one year is preferable. One attempt to use a surrogate mother was unsuccessful; however, hand-reared infants have been assimilated into existing groups and have shown reproductive success. Keeping the calf with the dam or the entire herd even longer helps to facilitate social learning. Male calves are usually weaned by the dam at an earlier age than female calves, but even male calves could stay with the white rhino herd until two to three years of age without problems. Depending on the facility, a white rhino dam and her newborn calf may be reintroduced to the male/herd after two weeks. White rhino calves chase and mock-fight with any male, female, calf or juvenile in the herd. Non-aggressive sexual behaviors may be exhibited at as early as 18 months of age in males. In general, the long-term social effects of removing rhino calves from dams should be investigated. For all species, weaning or permanent separation of the calf except for medical reasons should not occur before one year of age. A calf can, however, be temporarily separated from its mother at as early as one month of age for short periods of time (e.g., re-breeding of dam).

Generally, the procedure is to separate the calf for short periods of time (e.g., 15-20 min during cleaning) and gradually increase the separation time. If a dam is not going to be re-bred, her calf may remain with her until it reaches sexual maturity (at approximately 4.5-5 yr of age). It should be noted that available data indicate that nursing does not inhibit conception. In a herd-like situation, a female white rhino may temporarily abandon her calf during estrus and rejoin it immediately after breeding until the birth of her next calf. The first calf may be forced away before parturition of the second calf as the dam seeks to isolate herself. Following the birth of the second calf, the first calf, then a sub-adult, may rejoin its mother and her new calf in a social group for up to four years or until it reaches sexual maturity.

The distribution of births by month for African white rhinos is shown in Figure 2.1 for the 845 white rhino births reported in the 2011 North American Regional Studbook (Christman, 2011). Figure 2.2 shows the months of conception for white rhinos based on an approximate gestation of 16 months. These data demonstrate that white rhinos are fertile and conceive throughout the year. Although a slight seasonal effect on male rhino testosterone and mating activity has been reported for wild white rhinos (Kretzschmar et al., 2004), the trend towards more fertile matings from April to September in ex situ white rhinos could be management related since temperate zoos are more likely to pair their rhinos for mating during months with good weather.

Reproductive Monitoring

White rhinos breed best when maintained in larger social groups, comprised of at least one male and greater than two females, in large enclosures (Swaisgood et al., 2006; Metrione, 2010). Most males are fairly compatible with females, so reproductive monitoring is not typically necessary for timing introductions. However, hormone monitoring can be a valuable tool for determining if a rhino has truly stopped cycling when overt signs of estrus are not observed. Hormonal analyses are most commonly employed to diagnose and monitor pregnancy. Several hormones and their metabolites have been employed for monitoring reproductive activity in the African white rhinoceros (Table 2.7), with fecal progesterone metabolites most commonly chosen.

Table 2.7 Reproductive Hormones monitored in the African white rhino

Sample Estrogen Progesterone Testosterone

Urine f3 f3

Feces f1,4,6,7,8, m1,5

Serum m2,5

Saliva

* Table adapted from Roth, 2006.

1 Berkeley et al., 1997; 2Brett et al., 1989; 3Brown et al., 2001 ; 4Christensen et al., 2009; 5Czekala and Callison, 1996;

6 Garnier et al., 2002; 7Hindle et al., 1992; 8Hodges and Green, 1989; 9Radcliffe et al., 2001; 10Ramsay et al., 1987;

11Schwarzenberger et al., 1993; 12Schwarzenberger et al., 1996

Pregnancy Detection and Loss

In the white rhino, pregnancy can be detected by rectal ultrasonography as early as 15 days after ovulation (Radcliffe et al., 1997) if the female is conditioned to stand for the procedure. However, many white rhinos are not conditioned for ultrasound exams. Therefore, fecal or serum progesterone concentrations often are evaluated to diagnose and monitor pregnancy in white rhinos. Following mating, sustained elevated progesterone is a good indicator of pregnancy, but pregnancy can only be confirmed approximately three months into gestation when progesterone concentrations increase above post-ovulatory luteal concentrations (Patton et al., 1999; Brown et al., 2001). Early pregnancy loss, abortions and stillbirths have all been reported in the white rhino (Radcliffe et al., 1997; Patton et al., 1999; Hildebrandt et al., 2007) and the incidence of early pregnancy loss could be quite high. It has been suggested that many of the longer (~70 day) reproductive cycles could be the result of early pregnancy followed by embryonic loss, which extends the luteal phase of the cycle. However, until more white rhinos are monitored regularly by ultrasound to confirm the presence of an embryo followed by its loss, we can only speculate about the prevalence of early embryonic death in white rhinos.

Reproductive Technologies (Semen Collection, Artificial Insemination, etc.)

Viable white rhino semen samples can be collected by electroejaculation (Hermes et al., 2005; Roth et al., 2005) or harvested from the epididymis post-mortem (Roth, unpublished). White rhino sperm obtained by either method can survive processing and cryopreservation, but post-thaw motility is significantly reduced (Hermes et al., 2005; 2009b), indicating more research is needed to improve the cryopreservation protocols employed. Regardless, the successful production of a white rhino calf following AI with frozen-thawed semen proves that some white rhino sperm are fully functional post-thaw (Hermes et al., 2009b). Artificial insemination (AI) techniques have been developed for the white rhino. To date, five calves have been produced (Hildebrandt et al., 2007; Hermes et al., 2009b), and four survived. Three of the five pregnancies were in the same female white rhino maintained at the Budapest Zoo. Numerous attempts to produce pregnancies in many other white rhinos at zoos across Europe, North America and Australia have failed to yield additional offspring. Therefore, although tremendous progress has been made in establishing that AI can be successful, more research is needed to improve the efficiency of AI for facilitating the management of this rhino species. There are no reports of in vitro fertilization attempts in the white rhino. Sperm sorting to segregate X- versus Y-bearing sperm has been conducted and appears to be possible in white rhinos (Behr et al., 2009; O’Brien et al., 2011). However, maintaining adequate sperm quality throughout the collection, transport, staining, sorting and cryopreservation process has proven challenging, and the sorting rate can be slow. Therefore, additional research and refinement of techniques will be necessary before samples of sufficient concentration and quality for AI can be produced.

Fertility Assessment

Male fertility assessments should include: 1) an ultrasound examination of the testicles to evaluate tissue consistency and size, 2) electroejaculation to confirm sperm production, and 3) serum testosterone analysis. It is important to note that a positive result from electroejaculation is very meaningful, but a negative result should not be used alone to diagnose a male with infertility because electroejaculation is not always successful in producing good quality sperm samples, even when conducted on proven bulls. Female rhinos should be examined by rectal ultrasound to determine if any masses or cysts exist in the vagina or uterus. These types of pathologies appear to be very prevalent in older and even mid-age white rhinos (Hermes et al., 2006). Ultrasonography can also be used to determine if the ovaries are active, however, any fertility assessment should include some form of longitudinal endocrine analysis which is more informative for assessing ovarian activity than a single ultrasound exam of the ovaries. These assessments can be conducted using serum or fecal samples (Table 2.7). Given that some white rhinos exhibit reproductive cycles that last 70 to 75 days in length, samples should be collected three times a week for at least three months to evaluate ovarian activity.

Genome Resource Banking

Several white rhino sperm banks already exist at the Cincinnati Zoo’s Center for Conservation and Research of Endangered Wildlife, San Diego Zoo’s Institute for Conservation Research and at the SeaWorld/Busch Gardens Reproductive Department. The latter primarily contains samples that have been sorted for X- and Y-bearing sperm, whereas the former contain samples comprised of a natural mixture of X- and Y-bearing sperm collected either by electroejaculation or post-mortem from the epididymis. Fibroblast cell lines from many white rhinos are banked in San Diego Zoo’s Frozen Zoo and efforts are underway to develop white rhino stem cells. Oocyte and embryo cryopreservation techniques in the white rhino are still in the experimental stages.

Challenges

There are three significant challenges to white rhino reproduction in North American zoos: 1) a significant proportion of the females are acyclic; 2) the F1 generation of female rhinos appears to be reproductively active and mating but many are not producing calves; and 3) the prevalence of uterine pathology is quite high, even in females less than 20 years old. The cause(s) of acyclicity in female white rhinos remains unknown. There is speculation that it is the result of old age or premature reproductive aging, but to-date, there is no robust, scientific evidence to support this hypotheses. Reproductive suppression by dominant females is unlikely. The presence of other females appears to increase reproduction in both zoos (Swaisgood et al., 2006; Metrione 2010) and small game reserves (Swaisgood et al., unpublished data), and subordinate females were not more likely than dominant females to be nulliparous or acyclic (Metrione and Harder, 2011). Exogenous hormone treatments have been effective in stimulating ovarian activity in acyclic rhinos (Hildebrandt et al., 2007; Hermes et al., 2009a) but to date they have not been very effective in returning the female to a state of natural reproductive cyclicity. In contrast, the F1 female white rhinos appear to be exhibiting ovarian activity and behavioral estrus, are mating with the bulls, but are not producing calves (Swaisgood et al., 2006), suggesting that they are either failing to ovulate appropriately, failing to conceive or are conceiving and losing the pregnancies early in gestation. Reproductive tract pathologies have been reported in many species and it is pretty well accepted that the development of pathology can be avoided if the female is allowed to breed and does so successfully. However, some species are more prone to develop reproductive tract pathologies than others, and white rhinos appear to be more susceptible than black rhinos. How early in life and how often the females have to become pregnant to avoid developing reproductive tract cysts and masses are unanswered questions, but it is probably best to try and produce calves from females before they reach ten years of age.