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Data and code from: Temperature predicts relative mass but not size in passerines

Probst, Charlotte1; Berv, Jacob1; Zhou, Zhizuo2; Zhang, Mingyu1; Dias, Tiffany1; Alofs, Karen1; Fouhey, David3; Weeks, Brian1

Published Jun 25, 2026 on Dryad. https://doi.org/10.5061/dryad.gf1vhhn3b

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Jun 25, 2026 version files 1.44 MB

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Abstract

Aim: 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 structural size. We test for global gradients in mass relative to structural size (a proxy for insulation) and body size.

Location: Global.

Time period: Current.

Major taxa studied: Aves: Passeriformes.

Results: 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 body size, with species in colder climates having more mass than expected given their structural 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.

Main conclusions: 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 structural size across species. Our results suggest: (1) species in colder climates tend to have greater insulation (as indicated by their higher relative mass); (2) skeletal size in passerines does not strongly adhere to Bergmann’s Rule, suggesting spatial temperature-size relationships may be unlikely to predict temporal responses of size to climate change in this group; and (3) declines in relative mass may be a widespread cryptic response to global change.