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

Abstract

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.

Variation in reproductive investment increases body temperature amplitude in a temperate passerine

“…Free-living white-browed sparrow-weavers ( Plocepasser mahali ) in the Kalahari Desert, South Africa, had a body temperature amplitude of 4.5 °C (Smit et al 2013 ). Studies on non-passerines in sub-tropical environments also report body temperature amplitudes of between 4.2 and 5.5 °C (McKechnie and Smit 2010 ; Kemp et al 2017 ). Thus, high body temperature amplitudes have mostly been described for birds living in sub-tropical and/or in arid environments.…”

Further miniaturisation of the Thermochron iButton to create a thermal bio-logger weighing 0.3 g

“…These commercially available devices weighing ∼2.9 g, either implanted internally or attached to the skin, are capable of recording temperatures at a resolution of 0.0625°C in real time, at user-defined intervals. Thermochron iButtons have been used to record body temperatures in varied taxa, including marine and freshwater fish (Nowell et al, 2015;Peat et al, 2015), avian and non-avian reptiles (Kemp et al, 2017;Harlow et al, 2010), and mammals ranging in size from elephants (Loxodonta africana; Kinahan et al, 2007) to elephant shrews (Elephantulus myurus; Mzilikazi et al, 2002;Mzilikazi and Lovegrove, 2004). However, as far as we are aware, studies that have used iButtons to measure body temperature have been confined to vertebrates, as most invertebrates of interest are presumably too small to carry a device of this size.…”