Abstract:
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).
