The female oryx has no particular breeding season, but the frequency of same age calves observed in a group suggest that the female come into estrus around the same time (Estes 1991). The sexual maturity of a female usually begins around age 2, and gestation period lasts around 9 months with the new born calve weighting anywhere between 9 and 15 kg (Fahey1999). A female can be receptive again within a few weeks of giving birth (Anonymous 1999, Estes 1991, and Kingdon 1982).
Oryx sexual behavior begins with the courtship turn called the “mating whirl around”. The female at this point can refuse copulation by clashing frontally with the male. Eventually, the female submits and will try to move behind the bull as a sign of appeasement. The bull then attempts to sniff the back of the cow and assume an erect position of dominance. The female responses with a head low posture and urinate for the male. Upon determining that the female is in estrus through the urine test, the male will proceed to mount and inseminate the female.
Near the time of calving, the female will isolate itself from the rest of the group like the wildebeest. Staying in a herd would in this case make the pregnant female more vulnerable to her predators, thus offers no advantage. After the birth of her offspring, she will remain near the vicinity but never too close to draw predator attention to the immobile calf. Estes (1991) has observed that even a subject comes within two meters of the calf, it will not move or stir. In another case, a female oryx who just gave birth moved away from the calf quickly upon sensing the approach of a predator. It has been argued that this immobility in the calf serve as the best defense mechanism against predators by minimizing their detection. The calf joins the maternal herd within six weeks of birth and weaned after 3.5 months (Estes 1991 and Fahey 1999). Because of this isolation from the herd, it is not necessary for the male oryx to use and defend its territory as a nesting territory. The calf presumably has the intrinsic predation protection mechanism that does not require the physical defense of the territorial male.
A young oryx
Photo courtesy of Chris Jameson
As mentioned already, the gemsboks live in mixed, nursery, female, and bachelor herds or as solitary bulls. A pronounced hierarchy exists within these groups especially the mixed and bachelor herds. Use of aggression and ritualized displays enforce this social structure. Studies observing mating behaviors reported that only the dominant male of a mixed herd breed with the female (Anonymous 1999, Walther 1978, and Walther 1977-78). With respect to solitary male gemsboks, a single bull will defend a small territory. He will herd a mixed, female, or nursery herd into his territory and only he will mate with the receptive cows (Fahey 1999, Anonymous 1999). In order for this strategy to work, the territory bull must be able to suppress the dominant bull of the mixed herd. In one case of the gemsboks of the Serengeti National Park, the solitary territorial bull and the dominant bull were about the same size, and after a severely long fight, the herd moved away from the solitary territory bull. The females did not mate with the solitary male (Walther 1978).
Photo courtesy of Steve Barrett http://www.liv.ac.uk/~sdb/Safari-2001/Images/427%20Oryx.jpg
The difference in mammalian sexual morphology forces animals to choose different types of mating strategies to ensure reproductive success and propagation of their genes. Male favors copulating as often as possible, while the female prefer to mate with the best candidate to ensure the survival of her offspring. The mating system each species of animal chooses ultimately depends on the size of the animal and how that character determines their food dispersion and anti-predator behaviors.
As mentioned in the introduction, the oryx gemsboks are non discriminating grazers and feeds on a variety of grass species, roots, and tubers. Kingdon (1982) noted that the Tsavo oryx prefer high protein grasses such as Brachiaria and shrubs Grewia. Some studies have also noted the flexibility in gemsboks choice of food (Hamilton 1977 and Taylor 1969). Under normal conditions, with adequate vegetation, the gemsbok will graze on plants with high water content. During the dry season in which the temperature may peak above 40°C, the gemsbok will graze on the dry shrubs of genus Disperma. Taylor (1969) found that these plants contain around 1% of water content during the day. However, the gemsbok usually feed on them during the night, at which the plant will absorbs up to 30% of water when the desert temperature drops. According to Jarman (1974), the body size of an ungulate negatively correlates with the level of selectivity in their food choice (Jarman 1974). Classified as Class E antelopes, oryx are relatively unselective in their diet. Because of its size, the oryx lacks the dexterity that is necessary to obtain the more nutritious parts of a plant, thus they must graze less selectively on grass and lower quality vegetation (Jarman 1974 and Brashares 2000) The versatility in their choice of food makes it unnecessary for a female to depend on the male for resources, which rules out monogamy and resource defense polygyny as the gemsboks’ mating strategy.
The abundance and dispersion of the food resource also play a role in determining specie’s mating system. Like the wildebeest, the oryx has been documented as nomadic antelopes following seasonal rainfalls into areas of new grass (Hamilton 1977). During the rainy season, the abundance of grass makes the food indefensible, thus further argues against the gemsbok adopting a monogamous or resource defense polygynous mating system. As drier seasons approach, this relatively abundant source of food becomes less available and more unpredictably distributed throughout their habitat. The same argument for a mating only territory can be applied here. Once the gemsbok finishes a patch of grass, he cannot wait for the patch of land to regenerate food, thus he must find another feeding ground. The inability to maintain enough resource in his territory for himself and his mates forces the male to defend a herd of females instead. Emlen and Oring (1977) defined this type of mating system as a female defense polygyny, where the male directly defend a group of female and keeps them in his territory for the purpose of mating (Berger 1996).
Photo courtesy of Bruce J. Hayward (photographer)
c/o University of Alaska Museum
The predators of the Oryx gazella are lions, cheetahs, spotted hyenas, and the wild dogs. The gemsbok can use their powerful horns to defend themselves and lions have been found to die of oryx wounds (Kingdon 1982). However, fighting is costly for the gemsbok and happens only under extreme circumstances. As one of the larger groups of the antelopes of Africa, oryx chooses group living as a means to avoid predators. The likelihood of a fellow group mate falling prey to a predator is greater in a group of twenty compared to a group of two. (Berger 1996 and Jarman 1974). Unlike smaller ungulates such as the dik dik, oryx will always be conspicuous to predators regardless of where they hide because of their size. Their defense against predators are fleeing, fighting, both of which are probably more prominent in solitary male bulls, and staying in large groups. Why not form larger groups such as those of the wildebeest then? Predator avoidance may induce the need to remain in a herd for the oryx, but resources, especially food and water, limit the size of their herds (Jarman 1974). Although they may have abundant vegetation during the rainy season, the extreme conditions of the dry season limits the availability of food and water, thus formation of large herds comparable to those of the wildebeest would not be feasible. Like the African buffalo, large bovids such as the oryx live in large migratory herds, find safety in number, and have a polygynous mating system based on male dominance hierarchies (Berger 1996). However, beta males of the African buffalo do mate if more female are in estrus at the same time, while such observations has not been made with the oryx (Berger 1996). Predation pressure leading to the aggregation of the oryx in mixed herd most likely caused the development of a polygynous mating system. Furthermore, the indefensible resources ultimately caused the oryx to adopt the female defense polygyny strategy.
Although both the resource dispersion and predator avoidance strategy suggest female defense polygyny for the oryx, some discrepancies remain when compared to other typical polygynous species. The sexual selection theory explains why most polygynous mammailian species, such as the African buffalo, have sexual dimorphism (Loison 1999). In a polygynous society, the individual males who are capable of defending either a resource or a group of female gain access to mate, thus natural selection favors those male who are bigger and stronger. Such selection does not exist in the female species of these groups of animals. Overtime, physical differences between the two sexes increases, thus making most polygynous species exhibit sexual dimorphism. However, the oryx has minimal sexual dimorphism and yet seem to adopt the female defense polygyny mating strategy. No clear sexual dimorphism exist in the oryx imply the equal ability for both male and female to defend against predators. The male would then be protecting the female against other male oryx. Natural selection would seem to favor those individual male who can best defend his harem, most likely through the use of aggression. Increase aggression usually correlates positively with body size, yet the male and female oryx do not differ significantly in size from each other. How is this minimal level of dimorphism maintained if a selection pressure seems to be in place for the two sexes to diverge?
One possible explanation could be that evolution has not worked quickly enough for us to see the effects of sexual selection. Perhaps in the future, oryx sex dimorphism will become more apparent. The benefits from maintaining the same body size among the sexes, such as group synchrony suggested by the activity budget hypothesis, may work against the forces of sexually selection (Ruckstuhl 2000). The extreme conditions of these animals’ habitat may have contributed to this tug of war between dimorphic and non-dimorphic species. Kingdon (1982) suggests that the female hierarchy contributed to the maintenance of the lack of sexual dimorphism within this specie of antelope. Current researches seem to lack sufficient information and evidence to explain this phenomenon.
Instead of sexual dimorphism in the body size, the difference of horn between the male and female gemsboks may be associated with the polygynous mating system. Male gemsboks have horns that are more developed and thicker at the base compared to those of the female (Anonymous 1999, Moller 1995). Parker (1983) argues that the use of horns differ between female and male species thus they evolved into different size and/or length. The male gemsboks use their horns the majority of the time to defend their territory against other male, while the female mainly use horns to defend her and the offspring from predators. Straighter and thinner horns of female oryx suggest their exclusive use for predator defense (Parker 1983). Gemsboks with more symmetric horns dominate a herd more often than those with asymmetric horns (Moller 1995). If symmetric horns influence dominance, and dominance positively correlates to mating success, then symmetric horns would imply mating success in the gemsbok. It may be possible that sexual selection favors those with more symmetric horns instead of bigger body size, which then explains the lack of sexual dimorphism in this female defense polygynous specie.
Other mating systems
Because of such pronounced hierarchy within the gemsboks, hierarchical promiscuity could be another possible mating system they adopt. Although the groups formed seem to remain the same throughout lifetime, no long term relationship between the mating male and the female forms after copulation (Berger 1996). The male oryx will also mate with any female within his territory that comes into estrus and move away from the female shortly after copulation (Berger 1996 and Estes 1991). Characteristics such as the exclusive mating rights of the alpha bull both overlap the definitions of a promiscuous and polygynous mating system. However, more support seems to lean towards female defense polygyny with observations made of “harem” groups in the oryx populations of the Tarangire National Park and the Serengeti (Walther 1978 and Walther 1977-78).
Photo courtesy of South Africa Online Travel Guide