Brain Temperature in Heat-Stressed, Water-Deprived Desert Phasianids: Sand Partridge (Ammoperdix heyi) and Chukar (Alectoris chukar sinaica)

Although sand partridges (Ammoperdix heyi) and chukars (Alectoris chukar sinaica) of the Negev desert are sympatric in some areas, sand partridges are endemic to arid regions, whereas chukars are primarily mesophilous. We reasoned that one thermoregulatory adaptation that might favor sand partridges over chukars in arid environments would be a better ability to regulate brain (hypothalamic) temperature, especially during heat stress and water deprivation. We therefore exposed both species to ambient temperatures ($T_{a}$) from 28.5 to 44.5 C before and after water deprivation. We simultaneously measured colonic and hypothalamic temperatures ($T_{c}$, and $T_{h}$ respectively) and the frequencies (f) of breathing, panting, and gular flutter. The $T_{c}$, increased directly with $T_{a}$ to the same extent in both watered and water-deprived groups of both species. However, sand partridges could thermoregulate and maintain $T_{h}$ lower than $T_{c}$, even when $T_{a}$ exceeded $T_{c}$, whereas chukars could not. Both species responded to water deprivation by reducing f increasing $T_{c}$ and $T_{h}$ thresholds for panting, and changing the patterns and/or frequencies of panting and gular flutter. As a result, $T_{c} - T_{h} (i.e., \DeltaT)$ in waterdeprived birds decreased with increasing $T_{c}$ (i.e., ΔT) in water-deprived birds decreased with increasing $T_{c}$ until panting began. At that point ΔT began to increase with $T_{c}$. In chukars, ΔT was influenced by $T_{a}$, by water deprivation, and by the interaction between them, whereas ΔT in sand partridges was influenced only by the interaction of water deprivation with $T_{a}$. Our findings suggest that sand partridges are better able to regulate body and brain temperatures during heat stress and water deprivation than are chukars and that this may contribute to their ability to live in very arid areas from which chukars are absent.