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Avian thermoregulation in the heat: efficient evaporative cooling in two southern African nightjars

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Abstract

Nightjars represent a model taxon for investigating physiological limits of heat tolerance because of their habit of roosting and nesting in sunlit sites during the heat of the day. We investigated the physiological responses of Rufous-cheeked nightjars (Caprimulgus rufigena) and Freckled nightjars (Caprimulgus tristigma) to high air temperatures (T a) by measuring body temperature (T b), resting metabolic rate (RMR) and total evaporative water loss (TEWL) at T a ranging from 10 to 56 °C. Both species became hyperthermic at T a > T b. Lower critical limits of thermoneutrality occurred at T a between 35 and 37 °C, whereas we detected no clear upper critical limits of thermoneutrality. Between T a ≈ 37.0 and 39.9 °C, rates of TEWL increased rapidly with T a. At T a ≥ 40 °C, fractional increases in mass-specific TEWL rates were 78–106% of allometric predictions. Increasing evaporative heat dissipation incurred only small metabolic costs, with the RMR of neither species ever increasing by more than 20% above thermoneutral values. Consequently, both species displayed extremely efficient evaporative cooling; maximum evaporative heat dissipation was equivalent to 515% of metabolic heat production (MHP) at T a ≈ 56 °C in C. rufigena and 452% of MHP at T a ≈ 52 °C in C. tristigma. Our data reiterate that caprimulgids have evolved an efficient mechanism of evaporative cooling via gular fluttering, which minimizes metabolic heat production at high T a and reduces total heat loads. This likely aids in reducing TEWL rates and helps nightjars cope with some of the most thermally challenging conditions experienced by any bird.

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Abbreviations

EHL:

Evaporative heat loss

M b :

Body mass

MHP:

Metabolic heat production

RMR:

Resting metabolic rate

T a :

Air temperature

T b :

Body temperature

T e :

Operative temperature

TEWL:

Total evaporative water loss

T lc :

Lower critical limit of thermoneutrality

T uc :

Upper critical limit of thermoneutrality

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Acknowledgements

We thank Duncan MacFadyen and E. Oppenheimer & Son for allowing us to conduct research on their properties, Pieter and Verencia Benade for their hospitality and access to their farm. Cathy Bester, Ryno Kemp and Pieter Erasmus provided valuable assistance in the field. Lastly, we thank Bruce Woodroffe and Awesome Tools (Cape Town, South Africa) for discounted lighting equipment. This paper was improved through the helpful comments of two anonymous reviewers. Funding was provided by the DST-NRF Center of Excellence at the Percy FitzPatrick Institute and University of Pretoria. This material is based on work supported by the National Science Foundation under IOS-1122228 to B.O.W. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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Authors and Affiliations

  1. DST-NRF Center of Excellence at the Percy FitzPatrick Institute, Department of Zoology and Entomology, University of Pretoria, Private Bag X20, Hatfield, 0028, South Africa

    Ryan S. O’Connor & Andrew E. McKechnie

  2. UNM Biology Department, University of New Mexico, MSC03-2020, Albuquerque, NM, 847131-0001, USA

    Blair O. Wolf

  3. Department of Biology, University of Regina, Regina, SK, S4S 0A2, Canada

    R. Mark Brigham

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  1. Ryan S. O’Connor
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  2. Blair O. Wolf
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  4. Andrew E. McKechnie
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Correspondence to Andrew E. McKechnie.

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All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. This study was approved by the University of Pretoria Animal Ethics Committee (Project EC068-13) and complies with current South African laws.

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Communicated by I. D. Hume.

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O’Connor, R.S., Wolf, B.O., Brigham, R.M. et al. Avian thermoregulation in the heat: efficient evaporative cooling in two southern African nightjars. J Comp Physiol B 187, 477–491 (2017). https://doi.org/10.1007/s00360-016-1047-4

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