Does size matter? Comparison of body temperature and activity of free-living Arabian oryx (Oryx leucoryx) and the smaller Arabian sand gazelle (Gazella subgutturosa marica) in the Saudi desert

Abstract

Heterothermy, a variability in body temperature beyond the normal limits of homeothermy, is widely viewed as a key adaptation of arid-adapted ungulates. However, desert ungulates with a small body mass, i.e. a relatively large surface area-to-volume ratio and a small thermal inertia, are theoretically less likely to employ adaptive heterothermy than are larger ungulates. We measured body temperature and activity patterns, using implanted data loggers, in free-ranging Arabian oryx (Oryx leucoryx, ±70 kg) and the smaller Arabian sand gazelle (Gazella subgutturosa marica, ±15 kg) inhabiting the same Arabian desert environment, at the same time. Compared to oryx, sand gazelle had higher mean daily body temperatures (F(1,6) = 47.3, P = 0.0005), higher minimum daily body temperatures (F(1,6) = 42.6, P = 0.0006) and higher maximum daily body temperatures (F(1,6) = 11.0, P = 0.02). Despite these differences, both species responded similarly to changes in environmental conditions. As predicted for adaptive heterothermy, maximum daily body temperature increased (F(1,6) = 84.0, P < 0.0001), minimum daily body temperature decreased (F(1,6) = 92.2, P < 0.0001), and daily body temperature amplitude increased (F(1,6) = 97.6, P < 0.0001) as conditions got progressively hotter and drier. There were no species differences in activity levels, however, both gazelle and oryx showed a biphasic or crepuscular rhythm during the warm wet season but shifted to a more nocturnal rhythm during the hot dry season. Activity was attenuated during the heat of the day at times when both species selected cool microclimates. These two species of Arabian ungulates employ heterothermy, cathemerality and shade seeking very similarly to survive the extreme, arid conditions of Arabian deserts, despite their size difference.

Towards a mechanistic understanding of the responses of large terrestrial mammals to heat and aridity associated with climate change

“…Surprisingly, patterns of activity across 24 h and across seasons were almost identical in the large, grazing Arabian oryx and the smaller mixed-feeding Arabian sand gazelle. These data therefore do not support widely-held views that the feeding time of herbivores, reflected by time active, is tightly associated with body mass or feeding style [96].…”

“…A similar pattern is evident in free-living large mammals experiencing water shortage. Free-living Arabian oryx and Arabian sand gazelles without access to water exhibited larger daily changes in body core temperature, allowing 24 h maximum body temperature to increase above that recorded when the animals were hydrated [96,120]. As shown in Fig.…”

“…In contrast, in a period of extreme heat and aridity, Arabian oryx (Fig. 3) and Arabian sand gazelles shifted from a biphasic or crepuscular pattern of activity to a nocturnal pattern, without reducing total 24 h activity [91,96]. Surprisingly, patterns of activity across 24 h and across seasons were almost identical in the large, grazing Arabian oryx and the smaller mixed-feeding Arabian sand gazelle.…”

“…A reduction in body core temperature also was evident in Arabian oryx (Fig. 3, [120]) and Arabian sand gazelles in summer [96,143], when environmental heat load was high but food was scarce. Similarly, free-living western grey kangaroos had lower 24 h minimum body core temperatures in summer, a dry season in their Australian habitat with reduced plant growth, than in winter [144].…”

“…Kudu antelope (Tragelaphus strepsiceros), despite being much smaller than an elephant, sought shade at a similar globe temperature when exposed to summer heat, with access to drinking water [95]. Similarly, despite being about four-fold smaller in size than Arabian oryx, Arabian sand gazelles (Gazella subgutturosa marica) in the same desert environment as the oryx sought shade at the same globe temperature (28°C) as did the oryx [96]. Red and western grey (Macropus fuliginosus) kangaroos of similar body mass retreated to shade at different radiant heat loads, with red kangaroos apparently more tolerant of radiant heat [97].…”