Sublethal fitness costs of chronic exposure to hot weather vary between sexes in a threatened desert lark
Ryno Kemp, Marc T. Freeman, Barry van Jaarsveld, Zenon J. Czenze, Shannon R. Conradie, Andrew E. McKechnie
Abstract
Behavioural trade-offs between foraging and thermoregulation result in negative relationships between diurnal body mass (Mb) gain and daily maximum air temperature (Tmax) in arid-zone birds. However, it remains unclear how these trade-offs are affected by habitat type or sexually dimorphic behaviour. Using the South African arid-zone endemic Red Lark (Calendulauda burra) as a model taxon and focusing solely on non-breeding adults, we evaluated predictions that a) time-activity budgets are more strongly affected by air temperature (Ta) than is the case for species occurring in arid savanna habitats with greater access to shaded microsites, and b) the effects of Tmax on diurnal Mb gain differ between males and females. Behavioural observations revealed an effect of Ta on activity, and sex differences in the temperature-dependence of resting and wing-spreading. Moreover, Mb data obtained from habituated larks trained to perch on an electronic balance revealed that in males, but not females, diurnal Mb gain was negatively related to Tmax, with diurnal Mb gain equivalent to average overnight Mb loss when Tmax = 33.5°C and zero diurnal Mb gain (i.e. net 24-hr Mb loss of ~ 5 %) when Tmax = 40.0°C. Under recent climate conditions, male Red Larks rarely, if ever, experienced consecutive days with Tmax > 40.0°C but, under an unmitigated climate change scenario, will experience 10–15 consecutive days per summer by the end of the 21st century. Our findings suggest that this threatened lark species is unlikely to persist across much of its current range by 2100.