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Endotherms allocate large amounts of energy and water to the regulation of a precise body temperature (Tb), but can potentially reduce thermoregulatory costs by allowing Tb to deviate from normothermic levels. Many data on heterothermy at low air temperatures (Ta) exist for caprimulgids, whereas data on thermoregulation at high Ta are largely absent, despite members of this taxon frequently roosting and nesting in sites exposed to high operative temperatures. We investigated thermoregulation in free-ranging rufous-cheeked nightjars Caprimulgus rufigena and freckled nightjars Caprimulgus tristigma in the southern African arid zone. Individuals of both species showed labile Tb fluctuating around a single modal Tb (Tb-mod). Average Tb-mod was 39.7°C for rufous-cheeked nightjars and 39.0°C for freckled nightjars. In both species, diurnal Tb increased with increasing Ta. At Ta ≥ 38°C, rufous-cheeked nightjar mean Tb increased to 42°C, equivalent to 2.3°C above Tb-mod. Under similar conditions, freckled nightjar Tb was on average only 1.1°C above Tb-mod, with a mean Tb of 40.0°C. Freckled nightjars are one of the most heterothermic caprimulgids investigated to date, but our data suggest that during hot conditions this species maintains Tb within a narrow range above Tb-mod, possibly reflecting an evolutionary tradeoff between decreased thermal sensitivity to lower Tb but increased sensitivity to high Tb. These findings reveal how general thermoregulatory patterns at similar Ta can vary even among closely related species. [ABSTRACT FROM AUTHOR]

Many studies have shown that endotherms become more heterothermic when the costs of thermoregulation are high and/or when limited energy availability constrains thermoregulatory capacity. However, the roles of many ecological variables, including constraints on foraging opportunities and/or success, remain largely unknown. To test the prediction that thermoregulatory patterns should be related to foraging opportunities in a heterothermic endotherm, we examined the relationship between the lunar cycle and heterothermy in Freckled Nightjars (Caprimulgus tristigma), which are visually orienting, nocturnal insectivores that are dependent on ambient light to forage. This model system provides an opportunity to assess whether variation in foraging opportunities influences the expression of heterothermy. The nightjars were active and foraged for insects when moonlight was available but became inactive and heterothermic in the absence of moonlight. Lunar illumination was a much stronger predictor of the magnitude of heterothermic responses than was air temperature (Ta). Our data suggest that heterothermy was strongly related to variation in foraging opportunities associated with the lunar cycle, even though food abundance appeared to remain relatively high throughout the study period. Patterns of thermoregulation in this population of Freckled Nightjars provide novel insights into the environmental and ecological determinants of heterothermy, with the lunar cycle, and not Ta, being the strongest predictor of torpor use. [ABSTRACT FROM PUBLISHER]