Are animals shrinking due to climate change? Temperature-mediated selection on body mass in mountain wagtails
Climate change appears to affect body size of animals whose optimal size in part depends on temperature. However, attribution of observed body size changes to climate change requires an understanding of the selective pressures acting on body size under different temperatures. We examined the link between temperature and body mass in a population of mountain wagtails (Motacilla clara) in KwaZulu-Natal, South Africa, between 1976 and 1999, where temperature increased by 0.18 \(^\circ \)C. The wagtails became lighter by 0.035 g per year. Partitioning this trend, we found that only a small part of the effect (0.009 g/year) was due to individuals losing weight and a large part (0.027 g/year) was due to lighter individuals replacing heavier ones. Only the latter component was statistically significant. Apparently, the wagtails were reacting to selection for reduced weight. Examining survival, we found that selection was temperature-mediated, i.e., lighter individuals survived better under high temperatures, whereas heavier individuals survived better under low temperatures. Our results thus support the hypothesis that temperature drove the decline in body mass in this wagtail population and provides one of the first demonstrations of the selective forces underlying such trends.
KeywordsBody mass Climate change Survival Motacilla clara Bergmann’s rule
This paper is dedicated to the memory of Steven E. Piper who initiated this project and collected the data. This paper is largely based on a presentation Steven gave at the Pan-African Ornithological Conference in 2008. We thank the National Research Foundation of South Africa (Grants 85802 and 114696) and the Alliance for Collaboration on Climate and Earth System Sciences for funding. The NRF accepts no liability for opinions, findings, and conclusions or recommendations expressed in this publication.
Author contribution statement
ZB, HB, and RA conceptualized this study based on ideas of the late Steven Piper. ZB and HB collected data. JP, RA, and BE analysed the data. JP and RA wrote the manuscript. All authors contributed to revisions.
Compliance with ethical standards
All applicable institutional and national guidelines for the care and use of animals were followed.
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