Adaptive thermoregulation during summer in two populations of an arid‐zone passerine

Abstract

Abstract. Heterothermy plays an important role in lowering the costs of thermoregulation in endotherms by reducing water and energy requirements. We tested predictions that birds in arid habitats should express fine-scale variation in their thermoregulatory patterns as a function of prevailing climatic conditions. We assessed effects of air temperature (T air ) and water vapor pressure deficit (D) on body temperature (T b ) in free-living White-browed Sparrow-Weavers (Plocepasser mahali ) during summer in two arid habitats in the Kalahari Desert, South Africa, using data from a dry period at a hot, desert site (n ¼ 7 birds), and during a dry period (n ¼ 4 birds) and a wet period (n ¼ 5 birds) at a milder, semi-desert site. The desert birds maintained a significantly higher set-point T b (41.58 6 0.28C, mean 6 SD) than semi-desert birds (40.28 6 0.28C). During the warmest part of day (12:00-18:00 hours), T b increased significantly during periods of high T air and/or high humidity, and mean and maximum T b were up to 1.48 and 2.38C, respectively, above normal levels. However, as T air increased, birds at the desert site maintained T b at or below set-point levels for a greater proportion of the time than birds at the semi-desert site. Birds at the desert site also expressed a greater magnitude of daily heterothermy (heterothermy index, HI ¼ 2.48 6 0.38C, mean 6 SD) than birds at the semi-desert site: the latter population showed a greater magnitude of heterothermy during a dry period (HI ¼ 2.18 6 0.38C) than during a wet period (HI ¼ 1.68 6 0.28C). Birds continued foraging throughout the warmest part of the day, despite the fact that heat dissipation (percentage of time spent panting and wing-spreading) increased significantly with increasing T air . Our findings reveal that populations can vary in their thermoregulatory responses in both space and time and suggest that small changes in T air can have significant effects on thermoregulation in free-ranging desert birds, even when T air , T b . These data have important implications for assessing vulnerability of species to climate change, suggesting that sensitivity should be assessed at the population, rather than species, level.

Seasonal and geographical variation in heat tolerance and evaporative cooling capacity in a passerine bird

“…In contrast to these free-ranging populations, our laboratory data for P. mahali ( present study) and those of Trost (1972) on E. alpestris reveal lower T b in desert than in mesic populations at high T a . Moreover, free-ranging sparrow-weavers commenced panting at T a >28°C (Smit et al, 2013), whereas our limited behavioural observations indicated that birds in the laboratory started panting at a substantially higher T a across all sites (mean T a at onset of panting=36.8±2.7°C; lowest T a =32.0°C). These differences likely reflect the very low chamber humidities in the present study and the effects of solar radiation on the operative temperatures experienced by free-ranging sparrow-weavers.…”

“…It is also possible that maintaining a lower T b has an adaptive value by providing a greater capacity for heat storage before lethal T b limits are reached (McNab and Morrison, 1963;Tieleman et al, 2002a). Smit et al (2013) found that in two free-ranging populations of P. mahali in the Kalahari Desert, the T b set-point was significantly higher in a desert (41.5±0.2°C) versus semi-desert population (40.2±0.2°C), but that the desert population did not have a greater capacity for hyperthermia (i.e. T b >modal T b values).…”

“…These challenges are often manifested as consequential trade-offs affecting behaviour, body condition and reproductive decisions (e.g. du Plessis et al, 2012;Smit et al, 2013;Tieleman et al, 2008), which are likely to be strongly affected by the increases in maximum T a (T a,max ) and duration and frequency of extreme heat waves predicted to occur during the 21st century as a result of anthropogenic climate change (Intergovernmental Panel on Climate Change, 2012).…”

“…Glanville et al, 2012;Smit et al, 2013). Understanding intraspecific variation in heat tolerance and evaporative cooling capacity, and the relative contributions of local adaptation versus phenotypic plasticity to inter-population differences, is directly relevant to testing the assumption implicit in climate-envelope modelling studies that species cannot occupy habitats hotter than those within which they currently occur (Boyles et al, 2011;Pearson and Dawson, 2003).…”

Avian torpor or alternative thermoregulatory strategies for overwintering?

“…However, only a handful of studies have documented the nocturnal energetic strategy of free-ranging passerines, and even fewer have measured core T b . White-throated sparrows (Zonotrichia albicollis) have a rest-phase hypothermia, with a 3.4°C reduction in T skin between active and rest phase (Dolby et al, 2004) and the diel variation in T b of the arid-dwelling white-browed sparrow-weaver (Plocepasser mahali) is attributed to seasonal environmental conditions (Smit et al, 2013). The only comparable measurement of continuous, nocturnal T b of an Australian passerine in the cold is for captive-raised juvenile dusky woodswallows (Artamus cyaneurus; Maddocks and Geiser, 2007), measured in outdoor aviaries.…”