Journal
AMERICAN NATURALIST
Volume 178, Issue 2, Pages 202-213Publisher
UNIV CHICAGO PRESS
DOI: 10.1086/660834
Keywords
animal model; climate change; heritability; Parus major; microevolution; selection
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Funding
- University of Edinburgh
- Wolfgang Pauli Institute, University of Vienna
- European Commission
- Wolfgang Pauli Institute
- University of Natural Resources and Applied Sciences in Vienna
- NWO (Netherlands Organization for Scientific Research)
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Bergmann's rule predicts a decrease in body size with increasing temperature and has much empirical support. Surprisingly, we know very little about whether Bergmann size clines are due to a genetic response or are a consequence of phenotypic plasticity. Here, we use data on body size (mass and tarsus length) from three long-term (1979-2008) study populations of great tits (Parus major) that experienced a temperature increase to examine mechanisms behind Bergmann's rule. We show that adult body mass decreased over the study period in all populations and that tarsus length increased in one population. Both body mass and tarsus length were heritable and under weak positive directional selection, predicting an increase, rather than a decrease, in body mass. There was no support for microevolutionary change, and thus the observed declines in body mass were likely a result of phenotypic plasticity. Interestingly, this plasticity was not in direct response to temperature changes but seemed to be due to changes in prey dynamics. Our results caution against interpreting recent phenotypic body size declines as adaptive evolutionary responses to temperature changes and highlight the importance of considering alternative environmental factors when testing size clines.
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