Thermoregulation in free‐ranging ground woodpeckers Geocolaptes olivaceus: no evidence of torpor

Heterothermic responses characterised by pronounced hypometabolism and reductions in body temperature (T_b) are one of the most effective ways in which small endotherms can offset the energetic cost of endothermic homeothermy. It remains unclear, therefore, why daily torpor and hibernation are restricted to only a subset of avian lineages. To further our understanding of the phylogenetic distribution of avian torpor, we investigated winter thermoregulation in the southern African ground woodpecker Geocolaptes olivaceus. We considered this species a good candidate for heterothermy, because it is resident year‐round in mountainous regions with cold winters and reliant on small ectothermic prey. We recorded T_b patterns in free‐ranging individuals and measured T_b and metabolic rates in captive individuals. Neither free‐ranging nor captive woodpeckers showed any indication of daily torpor or even shallow rest‐phase hypothermia. All birds maintained bimodally distributed T_b characteristic of classic endothemic homeothermy, with a mean rest‐phase T_b of 37.9 ± 0.2°C and no data below 37.0°C. The mean circadian amplitude of T_b was 4.2°C, equivalent to approximately twice the expected value. There was some evidence of seasonal acclimatisation in T_b, with a small decrease in rest‐phase T_b with the onset of the austral winter. Captive birds showed patterns of resting metabolic rate and T_b consistent with the classic model of endothermic homeothermy. The apparent absence of torpor in G. olivaceus supports the notion that, unlike the case in mammals, many avian taxa that may a priori be expected to benefit from deep heterothermy do not use it.