COS 32-2 - Cryptic evidence of Allen's Rule: heat dissipation demand links temperature and structure in bird wings

Background/Question/Methods

Classic ecogeographic rules describing general spatial relationships between temperature and morphology are hypothesized to predict widespread biotic responses to climate change. However, spatial correlations between temperature and key thermoregulatory aspects of morphology (e.g. size and appendage length) have been alternatively attributed to the demands of cold tolerance or heat dissipation, complicating the connection between spatial patterns and temporal responses to global warming. We take advantage of bird wings to test whether selection for greater heat dissipation capacity alone can drive variation in appendage length. In birds, wings are the primary sites of heat dissipation during flight, when the dominant thermoregulatory challenge is dissipating excess heat; conversely, heat loss from wings is negligible when birds are not active and heat retention is potentially important. We use computer vision to measure wing bones from museum skeletal specimens spanning ~33% of passerine birds. We then used phylogenetic mixed models to test whether the length of these bones, which are expected to be highly correlated with the heat dissipation capacity of the wings, are predicted by temperature after controlling for a range of life history variables and the aerodynamic properties of the wing that determine flight efficiency and maneuverability.

Results/Conclusions

We find that across 1,603 species of passerine birds, after controlling for the range of ecological, geographic and flight aerodynamic variables expected to influence wing morphology, warmer temperatures are associated with increases in the length of the thermodynamically active component of the wing (P < 0.05). This previously undescribed macroecological pattern demonstrates the power of thermoregulatory demands to shape even the most critical functional components of morphology, and is evidence that at a global scale, bird wings conform to Allen's Rule. Given that this component of the wing is likely only important to heat dissipation when excess heat needs to be released into the environment (i.e. they do not have a role in heat conservation), our results provide a clear link between widespread spatial correlations between temperature and appendage length and predicted temporal trends in morphology as the world warms.