Abstract
Hot, dry summer conditions impose physiological stress on endotherms, yet we have a poor understanding of how endotherms seasonally adjust their costs of thermoregulation under hot conditions. We determined whether seasonal phenotypic plasticity in evaporative cooling capacity at high temperatures explained how the range-restricted Cape Rockjumper (Chaetops frenatus; hereafter ‘Rockjumper’), copes with hot and dry summer temperatures of the temperate mountain peaks of southwest South Africa. We measured evaporative water loss (EWL), resting metabolic rate (RMR), and body temperature at high air temperatures (30–42 °C) of individuals from a wild population of Rockjumpers during winter and summer (n = 11 winter, 4 females, 7 males; n = 10 summer, 6 females, 4 males). We found Rockjumper evaporative cooling in summer imposes higher EWL (i.e. greater water costs) compared to winter, although an accompanying lack of change in RMR resulted in increased summer cooling efficiency. These patterns are similar to those observed in species that inhabit regions where summer temperatures are routinely high but the species are not water stressed. Our findings indicate that avian seasonal physiological adjustments to heat can be diverse. Further seasonal studies on thermoregulation in response to heat will greatly improve our knowledge of the functional value of traits such as evaporative cooling efficiency and heat tolerance and how they contribute to the physiological stress organisms experience in heterogenous environments.
Zusammenfassung
Saisonale physiologische Reaktionen auf Hitze bei einer alpinen Vogelart mit begrenztem Verbreitungsgebiet: der Kapfelsenspringer ( Chaetops frenatus )
Heiße und trockene Bedingungen im Sommer bewirken physiologischen Stress bei warmblütigen Tieren. Dennoch haben wir ein begrenztes Verständnis, wie Endotherme die Kosten für die Thermoregulation unter Hitzebedingungen saisonal anpassen. Wir bestimmten, inwiefern die saisonale phänotypische Plastizität in der Fähigkeit zur Verdunstungskühlung bei hohen Temperaturen erklärt, wie die in ihrem Verbreitungsgebiet begrenzten Kapfelsenspringer (Chaetops frenatus) mit den heißen und trockenen Sommertemperaturen der gemäßigten Berggipfel im südwestlichen Südafrika zurechtkommen. Gemessen wurden der Wasserverlust durch Verdunsten, der Ruhestoffwechsel sowie die Körpertemperatur bei hohen Lufttemperaturen (30-42 °C) von Individuen einer Population von Kapfelsenspringern im Winter und im Sommer (n = 11 Winter, 4 Weibchen, 7 Männchen; n = 10 Sommer, 6 Weibchen, 4 Männchen). Wir stellten fest, dass die Verdunstungskühlung im Sommer einen höheren Wasserverlust (und damit höheren Wasserbedarf) im Vergleich zum Winter bedingt. Wenngleich eine damit einhergehende fehlende Veränderung im Ruhestoffwechsel in einer gesteigerten Kühleffektivität im Sommer resultiert. Diese Muster sind vergleichbar mit denen von Arten, die Regionen mit üblicherweise hohen Sommertemperaturen, aber ohne Wasserstress besiedeln. Unsere Ergebnisse zeigen, dass saisonale physiologische Anpassungsstrategien an Hitze bei Vögeln sehr divers sein können. Weitere saisonale Studien zu Thermoregulation bei Hitze würden die Kenntnisse über die funktionalen Eigenschaften wie zum Beispiel die Effizienz der Verdunstungskühlung und Hitzetoleranz deutlich erweitern und Informationen darüber liefern, wie diese Eigenschaften zum physiologischen Stress von Organismen in einer heterogenen Umwelt betragen.
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Acknowledgements
We firstly thank the Lee family for allowing us to conduct research on their property. We would also like to thank A. E. McKechnie for lending us the RH-300 water vapour analyser for the duration of the study. We are indebted to the many volunteers who spent hours and days helping us to catch Rockjumpers: Audrey Miller, Jenny Tartini, Alacia Welch, Gavin Emmons, Cristina Ebneter, Nicolas Pattinson, Cuen Muller, and Maxine Smit. Special thanks to Mark Brigham and two anonymous reviewers for commenting on drafts of this manuscript. This study was funded by a National Research Foundation (S. A.)-Thuthuka Grant (B. S.) and a Nelson Mandela Metropolitan University Research Themes Grant (B. S.). All experimental procedures were approved by the Research Ethics Committee: Animal (A15-SCI-ZOO-007) at Nelson Mandela Metropolitan University with a bird capture permit issued by Cape Nature, Western Cape, South Africa (0037-AAA041-00060).
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Communicated by L. Fusani.
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Oswald, K.N., Lee, A.T.K. & Smit, B. Seasonal physiological responses to heat in an alpine range-restricted bird: the Cape Rockjumper (Chaetops frenatus). J Ornithol 159, 1063–1072 (2018). https://doi.org/10.1007/s10336-018-1582-8
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DOI: https://doi.org/10.1007/s10336-018-1582-8