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Data: Generalized evidence for Bergmann’s rule body size variation in a cosmopolitan owl genus


Romano, Andrea; Séchaud, Robin; Roulin, Alexandre (2020), Data: Generalized evidence for Bergmann’s rule body size variation in a cosmopolitan owl genus, Dryad, Dataset,


Aim: The eco-geographic Bergmann’s rule predicts that animals have smaller body size in warmer regions than in cold environments because of thermoregulatory reasons. Although this rule has been widely investigated, intraspecific analyses on cosmopolitan taxa are rare. We examined whether geographic variation in wing length, a proxy of body size, shows a Bergmannian pattern and can be explained by three mechanisms known to affect animal body size (heat conservation, resource availability and starvation resistance) in seven species of nocturnal raptors of the genus Tyto.

Location: World.

Taxon: Genus Tyto.

Methods: We measured wing length of 9033 museum specimens covering the entire distributional range of each species and linked it with geographic (absolute latitude, elevation) and climatic predictors associated with heat conservation, resource availability and starvation resistance hypotheses of spatial variation in body size.

Results: All the species show a trend of increasing wing length with increasing latitude and/or elevation, and in five of them either or both geographic predictors are statistically significant. In all the species showing a Bergmannian pattern, wing length significantly decreases with temperature, thus supporting the heat conservation hypothesis. Conversely, we found less generalized support for the other hypotheses, although in some species significant trends between wing length and proxies of climatic seasonality and/or primary productivity emerged.

Main conclusions: Consistent clines in body shrinking in warm environments are observed in species living in different continents at different latitudinal and temperature ranges, as well as exploiting different habitats. These findings thus support the hypothesis that body size is, at least partly, selected for heat maintenance depending on the thermal environment, even in nocturnal species which are not directly exposed to solar radiation. However, different selective pressures may also have concomitantly acted to promote body size evolution in this bird group.