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.

以显著代谢减退和体温（body temperature，缩写Tb）降低为特征的异温反应，是小型内温动物补偿内温性恒温代谢成本的最有效方式之一。因此，为何日常蛰眠与冬眠仅局限于少数鸟类支系，这一问题至今仍未得到明确解答。为加深我们对鸟类蛰眠系统发育分布的认识，我们对南非地啄木（Geocolaptes olivaceus）的冬季体温调节展开了研究。我们认为该物种是开展异温性研究的理想候选类群：其一，它常年栖息于冬季寒冷的山地环境；其二，其依赖小型外温动物作为猎物。我们记录了野生个体的体温模式，并测量了圈养个体的体温与代谢率。无论是野生还是圈养的地啄木，均未表现出任何日常蛰眠迹象，甚至连轻度休息期低体温都未出现。所有个体均维持着典型内温性恒温动物的双峰分布体温模式，休息期平均体温为37.9±0.2℃，未出现低于37.0℃的体温记录。体温的日均昼夜波动幅度均值为4.2℃，约为预期值的两倍。存在一定证据表明该物种存在体温的季节驯化现象：随着南半球冬季的来临，其休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（G. olivaceus）明显不具备蛰眠能力，这一结果支持了如下观点：与哺乳动物不同，许多原本被认为可从深度异温中获益的鸟类类群实际上并未使用该策略。