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Reproductive biology
An opportunistic breeder
Arabian oryx are opportunistic breeders in response to unpredictable rainfall. It seems that variations in temperature, rainfall and consequently food availability determine onset of breeding [Ismail, pers. com.]. Although calves are produced throughout the year a calving peak is recorded during winter [Strauss, 2002].
While many desert antelopes have adopted a strong seasonal reproduction pattern (births peak during the coolest months, allowing females to invest most of the available water resources in lactation and less in thermoregulation), the non-seasonal reproduction pattern observed in the Arabian oryx in captivity and in the reintroduced population of Mahazat as-Sayd remains a puzzle for biologists. When, in 1995, we released oryx in ‘Uruq Bani Ma’arid, a non-fenced protected area at the western edge of the Rub al-Khali, one of the driest area in the world, we did expect harsh resource conditions prevailing in ‘Uruq Bani Ma’arid to reveal the still poorly known reproduction strategy adopted by this species in the wild.
Fortunately for oryx but unfortunately for researchers trying to elucidate the determinant factors of reproduction, resource conditions between 1995 and 1998 were unexpectedly good due to above average rainfalls. The high productivity of the oryx herd observed by Maartin Strauss (2002), field biologist in the protected area at that time could unfortunately not bring any new lights on the reproduction issue, confirming however that when resource conditions are favorable, oryx can breed throughout the year.
Then in 1998-2000 drought hit the reserve and oryx productivity crashed. Resource quality and accessibility seemed to have had a direct impact on fecundity and temporal pattern of reproduction, although the mechanisms that contribute to reproduction control remain unresolved.
We hypothesised that body condition and associated physiological capacities are determinant factors for pregnancy. Good resource conditions following localized but significant rainfalls in April 2002 allowed us to test this hypothesis. As female body condition significantly improved by May 2002. We predicted according to the resource quality hypothesis that a calving peak should occur 8.5 month later (pregnancy duration) in February-March 2003. By the 15th February 2003 70% of the observed reintroduced females had calved and by the time we advertise this report nearly three-quarter of the reintroduced females observed had calved. These first results seem to confirm that body condition plays an important role in female fecundity and capacity to complete a pregnancy.
However more data are still needed to understand why oryx have maintained such “opportunistic” reproduction strategy while another ungulate species occurring in the area and relying on similar resource conditions have adopted a seasonal strategy, Arabian sand gazelles Gazella subgutturosa marica breeding during the cool months.
Female sexual activity
In captivity (NWRC), females reach sexual maturity between 13 and 18 month-old, depending on the presence of a male that stimulates puberty [Blanvillain et al., 1997]. The mean length of oestrus cycle is about 24 days [Sempéré et al., 1996], and female give birth to a single calf after a mean gestation length of about 260 days [Vié, 1996]. Interestingly, the mean calving interval is 295 days [Vié, 1996], which means that the majority of conceptions occur during the post-partum oestrus cycle. Similar results have been recorded in the wild [Strauss, 2002].
Male sexual activity
Males reach sexual maturity between 7 and 12 month-old [Ancrenaz et al., 1998]. They display a testosterone cyclicity, which in the wild may coincide with the oestrus cycle of the females. This trait may be an adaptation to reduce energy expenditure when females are not fertile.
Physiology: energy and water needs
The Arabian oryx can be considered as the quintessential desert ungulate, owing to its capacity to conserve water and reduce energy expenditures [Williams et al., 2001; Ostrowski et al., 2002].
Water flux
Among ungulates living in desert environments, the Arabian oryx has the lowest mass specific water turn-over rate, about four times lower than the legendary camel [Ostrowski et al., 2002]. The Arabian oryx may survive indefinitely in desert habitat without access to drinking water, covering its daily water expenses with the pre-formed water of the ingested forage (86%) and the metabolic water produced (14%) [Ostrowski et al., 2002].
Energy expenditures
Moreover, Arabian oryx have the lowest mass adjusted energy expenditure reported for eutherian mammals. They markedly reduce their energy expenditures during summer with respective value for spring and summer of about 22,000 kJ/day and 11,000 kJ/day [Williams et al., 2001]. This largest disparity recorded in an eutherian mammal is probably obtained both thanks to physiological and behavioural adjustments.
Recent studies
The National Wildlife Research Centre has recently entered into a collaborative project with the University of the Witwatersrand, Johannesburg, South Africa, to investigate the thermal biology of Arabian oryx (Oryx leucoryx) and Arabian sand gazelle (Gazella subgutturosa marica). The project forms part of the doctoral studies for Ms Robyn Hetem, a PhD candidate within the University’s Brain Function Research Group (BFRG).
The aim of the project was to extend the research on the thermoregulatory mechanisms employed by the Arabian oryx (Oryx leucoryx) to cope with the Arabian desert environment. Adaptive heterothermy is widely viewed as a key adaptation of ungulates to arid-zone life. It is defined as the storage of body heat during the day to reduce the thermal gradient with the environment, thus reducing evaporative heat loss and conserving body water. This stored heat can then be dissipated non-evaporatively during the colder night. An additional behavioural strategy hypothesised to be advantageous amid varying ecological conditions is cathemerality, defined as the seasonal flexibility in activity patterns. For a one year period (April 2006 – 2007) we measured body temperature and activity patterns, using implanted data loggers, in free-ranging Arabian oryx inhabiting the Mahazat As-Sayd Protected Area. We found that the oryx increased their amplitude of body temperature to nearly 6°C and shifted to a nocturnal activity pattern during hot dry periods. Arabian oryx therefore employ both adaptive heterothermy and cathemerality to survive in their extremely hot and dry environments. Interestingly, not only the environmental heat load, but also the availability of water appears to play an important role in the implementation of heterothermy.
