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

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.