Data from: Thermoregulation in free-ranging ground woodpeckers Geocolaptes olivaceus: no evidence of torpor

Heterothermic responses characterised by pronounced hypometabolism and reductions in body temperature (Tb) 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 Tb patterns in free-ranging individuals and measured Tb 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 Tb characteristic of classic endothemic homeothermy, with a mean rest-phase Tb of 37.9 ± 0.2 °C and no data below 37.0 °C. The mean circadian amplitude of Tb was 4.2 °C, equivalent to approximately twice the expected value. There was some evidence of seasonal acclimatisation in Tb, with a small decrease in rest-phase Tb with the onset of the austral winter. Captive birds showed patterns of resting metabolic rate and Tb 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.以显著代谢减退与体温（Tb）降低为特征的异温反应（heterothermic responses），是小型内温动物抵消维持内温性恒温的能量成本的最有效途径之一。因此，为何日常蛰伏（daily torpor）与冬眠（hibernation）仅局限于少数鸟类支系，目前仍未明晰。为进一步阐明鸟类蛰伏的系统发育分布规律，我们对南非地啄木（Geocolaptes olivaceus）的冬季体温调节开展了研究。我们认为该物种是探究异温现象的理想候选类群，因其常年栖息于冬季寒冷的山地环境，且以小型外温性猎物为食。我们记录了野外自由活动个体的体温模式，并对圈养个体的体温与代谢率进行了测定。无论是野生个体还是圈养个体，均未表现出任何日常蛰伏甚至浅静息期低体温的迹象。所有个体均呈现典型内温性恒温动物的双峰分布体温模式，静息期平均体温为37.9 ± 0.2 ℃，未出现低于37.0 ℃的体温数据。体温的平均昼夜节律振幅为4.2 ℃，约为预期值的两倍。有证据表明该物种存在体温的季节性驯化现象，随着南半球冬季的来临，其静息期体温会出现小幅下降。圈养个体的静息代谢率与体温模式均符合经典内温性恒温模型。南非地啄木（Geocolaptes olivaceus）看似缺乏蛰伏能力，这支持了如下观点：与哺乳动物不同，许多本可从深度异温中获益的鸟类类群，实际上并不会使用该策略。