Avian thermoregulation in the heat: efficient evaporative cooling in two southern African nightjars.
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 (Ta) by measuring body temperature (Tb), resting metabolic rate (RMR) and total evaporative water loss (TEWL) at Taranging from 10 to 56 °C. Both species became hyperthermic at Ta> Tb. Lower critical limits of thermoneutrality occurred at Tabetween 35 and 37 °C, whereas we detected no clear upper critical limits of thermoneutrality. Between Ta≈ 37.0 and 39.9 °C, rates of TEWL increased rapidly with Ta. At Ta≥ 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 Ta≈ 56 °C in C. rufigena and 452% of MHP at Ta≈ 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 Taand 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.
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
RS
O'Connor
UNM Biology Department, University of New Mexico, MSC03-2020, Albuquerque, NM, 847131-0001, USA.
Blair O
BO
Wolf
Department of Biology, University of Regina, Regina, SK, S4S 0A2, Canada.
R Mark
RM
Brigham
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. aemckechnie@zoology.up.ac.za.
Andrew E
http://orcid.org/0000-0002-1524-1021
AE
McKechnie
D000818:Animals; D001481:Basal Metabolism; D001717:Birds / Q000502:physiology*; D001833:Body Temperature Regulation / Q000502:physiology*; D005260:Female; D006358:Hot Temperature; D008297:Male; D014870:Water Loss, Insensible / Q000502:physiology*
Caprimulgus rufigena; Caprimulgus tristigma; Evaporative water loss; Respirometry; Resting metabolic rate; Thermoregulation