McKechnie Andrew E, Gerson Alexander R, McWhorter Todd J, Smith Eric Krabbe, Talbot William A, Wolf Blair O
DST-NRF Centre of Excellence at the Percy FitzPatrick Institute, Department of Zoology and Entomology, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa
Department of Biology, University of Massachusetts, Amherst, MA 01003, USA.
J Exp Biol. 2017 Jul 1;220(Pt 13):2436-2444. doi: 10.1242/jeb.155507. Epub 2017 Apr 28.
Evaporative heat loss pathways vary among avian orders, but the extent to which evaporative cooling capacity and heat tolerance vary within orders remains unclear. We quantified the upper limits to thermoregulation under extremely hot conditions in five Australian passerines: yellow-plumed honeyeater (; ∼17 g), spiny-cheeked honeyeater (; ∼42 g), chestnut-crowned babbler (; ∼52 g), grey butcherbird (; ∼86 g) and apostlebird (; ∼118 g). At air temperatures () exceeding body temperature (), all five species showed increases in to maximum values around 44-45°C, accompanied by rapid increases in resting metabolic rate above clearly defined upper critical limits of thermoneutrality and increases in evaporative water loss (EWL) to levels equivalent to 670-860% of baseline rates at thermoneutral Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.20 to 2.17, consistent with the known range for passerines, and well below the corresponding ranges for columbids and caprimulgids. Heat tolerance limit (HTL, the maximum tolerated) scaled positively with body mass, varying from 46°C in yellow-plumed honeyeaters to 52°C in a single apostlebird, but was lower than that of three southern African ploceid passerines investigated previously. We argue this difference is functionally linked to a smaller scope for increases in EWL above baseline levels. Our data reiterate the reliance of passerines in general on respiratory evaporative heat loss via panting, but also reveal substantial within-order variation in heat tolerance and evaporative cooling capacity.
蒸发散热途径在鸟类目之间存在差异，但在目内蒸发冷却能力和耐热性的变化程度尚不清楚。我们量化了五种澳大利亚雀形目鸟类在极热条件下的体温调节上限：黄羽吸蜜鸟（约17克）、刺颊吸蜜鸟（约42克）、栗冠细尾鹩莺（约52克）、灰伯劳（约86克）和使徒鸟（约118克）。在气温超过体温时，所有五个物种的体温都升高到44 - 45°C左右的最大值，同时静息代谢率在明显高于热中性上限的临界值时迅速增加，蒸发失水（EWL）增加到相当于热中性时基线水平的670 - 860%。最大冷却能力，量化为通过蒸发消散的代谢产热的比例，范围在1.20至2.17之间，与雀形目已知范围一致，且远低于鸽形目和夜鹰目的相应范围。耐热极限（HTL，即耐受的最高体温）与体重呈正相关，从黄羽吸蜜鸟的46°C到一只使徒鸟的52°C不等，但低于先前研究的三种南部非洲织雀科雀形目鸟类。我们认为这种差异在功能上与高于基线水平的EWL增加范围较小有关。我们的数据重申了雀形目鸟类一般依赖通过喘气进行呼吸蒸发散热，但也揭示了目内热耐受性和蒸发冷却能力的显著差异。
