Extreme water efficiency of Cape gannet Morus capensis chicks as an adaptation to water scarcity and heat stress in the breeding colony

Abstract

Cape gannet Morus capensis chicks depend entirely on fish prey and metabolic water for water requirements during development. Water loss through evaporative cooling due to heat stress is substantial. We measured water flux and field metabolic rates (FMR) of Cape gannet chicks and adults to determine if gannets developed water saving strategies. The water economy index (WEI, g kJ⁻¹) decreased with chick age according to the model WEI = 0.676 – 0.272 × log₁₀(t), indicating that water efficiency increased with age. At fledging, the WEI of chicks was at the level expected of adult desert birds. Desert birds maintain a low WEI by also having a low FMR, whereas Cape gannet chicks have FMR comparable to other seabird species’ nestling requirements. We propose that maintaining low WEI is adaptive for Cape gannets because (1) chicks need to balance water loss through evaporative cooling, (2) fledglings need to overcome a period of up to a week when they cannot ingest any water and (3) adults spend extended periods in the breeding colony during which water can become a limiting factor. Understanding the physiological mechanism of maintaining low WEI will become increasingly important with future rising temperatures.

Keywords:

• Field metabolic rate
• physiological adaptations
• seabirds
• water economy index (WEI)
• water flux

Acknowledgements

We are grateful to the following individuals and institutions: Dr. Rob Crawford and his team at Oceans and Coasts, Department of Environmental Affairs, for logistical support during the field work at Malgas Island;Pierre Nel and team at West Coast National Park for providing logistical support; Mrs Berthe Verstappen-Dumoulin at Centre for Isotope Research (CIO), University of Groningen, for doing the isotope analyses. Valuable contributions to this manuscript were made by Dr. Lorien Pichegru, Prof. John Arnould and four anonymous reviewers, who enhanced the quality of this paper considerably. All methods used for studying and handling gannets during this study were approved by the Animal Use and Care Committee of South African National Parks, who also granted the research permit to conduct this study at Malgas Island, West Coast National Park.