Date Published: August 30, 2017
Publisher: Public Library of Science
Author(s): S. Streicher, H. Lutermann, N. C. Bennett, M. F. Bertelsen, O. B. Mohammed, P. R. Manger, M. Scantlebury, K. Ismael, A. N. Alagaili, Ulrich Joger.
Heterothermy, the ability to allow body temperature (Tb) to fluctuate, has been proposed as an adaptive mechanism that enables large ungulates to cope with the high environmental temperatures and lack of free water experienced in arid environments. By storing heat during the daytime and dissipating it during the night, arid-adapted ungulates may reduce evaporative water loss and conserve water. Adaptive heterothermy in large ungulates should be particularly pronounced in hot environments with severely limited access to free water. In the current study we investigated the effects of environmental temperature (ambient, Ta and soil, Ts) and water stress on the Tb of wild, free-ranging Arabian oryx (Oryx leucoryx) in two different sites in Saudi Arabia, Mahazat as-Sayd (MS) and Uruq Bani Ma’arid (UBM). Using implanted data loggers wet took continuous Tb readings every 10 minutes for an entire calendar year and determined the Tb amplitude as well as the heterothermy index (HI). Both differed significantly between sites but contrary to our expectations they were greater in MS despite its lower environmental temperatures and higher rainfall. This may be partially attributable to a higher activity in an unfamiliar environment for translocated animals in UBM. As expected Tb amplitude and HI were greatest during summer. Only minor sex differences were apparent that may be attributable to sex-specific investment into reproduction (e.g. male-male competition) during rut. Our results suggest that the degree of heterothermy is not only driven by extrinsic factors (e.g. environmental temperatures and water availability), but may also be affected by intrinsic factors (e.g. sex and/or behaviour).
Deserts sustain life, despite being hot and dry with low and unpredictable precipitation, intense solar radiation, low primary production and soil fertility, extreme span of ambient temperatures as well as an overall lack of free standing water [1–3]. Water availability is likely to be the most important factor affecting live in deserts and primary productivity often closely parallels rainfall in desert ecosystems . Similarly, since free standing water is usually scant in deserts, secondary productivity is usually indirectly linked to water availability as the rate of food consumption by desert herbivores tends to be linked to the water content of the plants consumed [5–8]. As a result animals living in desert environments make use of behavioural and physiological adaptations to reduce water loss and conserve energy [9,10]. Among mammals behavioural responses in the form of seasonal migrations, such as the large scale migrations of wildebeest (Connochaetus taurinus) in East Africa and springbok (Antidorcas marsupialis) in southern Africa but also the more localized movements of gemsbok (Oryx gazelle) in the Namib, are well documented . Migration is, however, usually restricted to lager mammals in which locomotion is not to costly and the use of retreats can provide short-term relief . Consequently, many desert-dwelling mammals exhibit nocturnal activity and use burrows during the day [9,11]. Their body size prevents large mammals from the use of the latter strategy. Instead shade seeking behaviour is commonly observed while in the absence of shade ungulates have also been observed to orientate themselves in a way that the least body surface is exposed to the sun [9,12,13]. Although their larger body size is associated with a reduction in evaporative water loss (EWL), the more limited benefits of the behavioural strategies employed, probably means that they have a to rely on physiological mechanisms to a greater extent for water and energy conservation than small mammals .
Our study provides the first continuous body temperature measurements for an entire calendar of free-living Arabian oryx in UBM within the ‘Empty Quarter’ in Saudi Arabia. Our data show that, as has previously been reported for MS , oryx in UBM display the characteristics typically associated with adaptive heterothermy—an increase in the daily amplitude of Tb rhythm where maximum Tb increased and minimum Tb decreased, particularly during the dry and hot months (i.e. summer). As predicted the two methods we used to quantify body temperature fluctuations, Tb amplitude and HI, both confirmed that oryx in UBM employ adaptive heterothermy. The use of HI, suggested to provide and objective reference for heterothermy measurements, has been criticized because of its reliance on the upper mode of the bimodal temperature distribution typical for small mammals that may not necessarily reflect the optimal temperature and does not account for variation in MR [18,47,51,52]. However, although the inclusion of simultaneous MR and Tb measurements has been suggested to provide a more accurate indication of the use of heterothermy this is logistically challenging in studies like ours . The similar results obtained for Tb amplitude and HI furthermore suggest that the disadvantages of the latter are negliable in our study.