Temperature Predicts Relative Mass but Not Size in Passerines

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Abstract: Predictable relationships between temperature and animal size and shape are the basis for classic rules in macroecology. Body size is thought to covary negatively with temperature (Bergmann’s Rule), broadly attributed to thermoregulatory demand for larger size in colder climates. However, ecophysiological principles suggest that changes in insulative materials may be a more effective thermoregulatory adaptation than changes in overall skeletal size. We test for global gradients in mass relative to skeletal size (a proxy for insulation) and body size. Combining a dataset of mass and skeletal traits for 1958 passerine species, we use Bayesian phylogenetic comparative analyses to test the extent to which temperature predicts skeletal size and relative mass. Across species, we find no evidence that temperature predicts skeletal size; rather, the effects associated with phylogeny account for nearly all of the variation in size. In contrast, we find a global gradient in mass relative to skeletal size, with species in colder climates having more mass than expected given their skeletal size. For 24 species, we assess these relationships within species and find that relative mass has declined through time but is not predicted by temperature. Mass relative to skeletal size is expected to exert a significant effect on insulation and energy stores, and we find it is more labile than skeletal size across species. Our results suggest that species in colder climates tend to have greater insulation, skeletal size in passerines does not strongly adhere to Bergmann’s Rule, and declines in relative mass may be a widespread cryptic response to global change.

Recommended citation: C. M. Probst, J. S. Berv, Z. Zhou, M. Zhang, T. Dias, K. Alofs, D. F. Fouhey, B. C. Weeks. 2026. Global Ecology and Biogeography 35(6):e70272. https://doi.org/10.1111/geb.70272