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（pronounced hypometabolism）和体温（body temperature, Tb）降低为特征的异温反应（heterothermic responses），是小型内温动物（endotherms）弥补内温恒温性（endothermic homeothermy）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）仅局限于部分鸟类支系（avian lineages），目前仍未明确。为加深我们对鸟类蛰伏系统发育分布（phylogenetic distribution）的理解，我们针对南非地啄木（Geocolaptes olivaceus）的冬季体温调节展开了研究。我们将该物种视为异温性研究的优质候选对象，原因在于其全年栖息于冬季寒冷的山地生境，且依赖小型外温动物作为猎物。我们记录了野生自由活动个体的体温模式，并测定了圈养个体的体温与代谢率。无论是野生自由活动还是圈养的地啄木，均未表现出任何日常蛰伏甚至轻度休息期体温过低（rest-phase hypothermia）的迹象。所有个体均维持着经典内温恒温动物特有的双峰分布体温，休息期平均体温为37.9 ± 0.2 °C，未出现低于37.0 °C的体温数据。体温的平均昼夜节律振幅为4.2 °C，约为预期值的两倍。有证据表明该物种存在体温的季节驯化（seasonal acclimatisation）现象：随着南半球冬季（austral winter）来临，休息期体温会出现小幅下降。圈养个体的静息代谢率（resting metabolic rate）与体温模式均符合经典内温恒温模型。南非地啄木明显缺乏蛰伏能力，这支持了如下观点：与哺乳动物的情况不同，许多本可被先验地认为能从深度异温中获益的鸟类类群，实际上并未使用该策略。