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

Abstract

AimThe eco‐geographical 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 geographical 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.LocationWorld.TaxonGenus Tyto.MethodsWe measured wing length of 9,033 museum specimens covering the entire distributional range of each species and linked it with geographical (absolute latitude, elevation) and climatic predictors associated with heat conservation, resource availability and starvation resistance hypotheses of spatial variation in body size.ResultsAll the species show a trend of increasing wing length with increasing latitude and/or elevation, and in five of them either or both geographical 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 conclusionsConsistent 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.

Geographic variation in body size and plumage colour according to diet composition in a nocturnal raptor

“…It adds to the limited known cases of adaptations of predator body size according to the size distribution of prey, which was previously observed in other vertebrate taxa, such as mammals (Erlinge 1987) and snakes (Schwaner 1985, Forsman 1991. This result also helps to understand and interpret better our recent finding about the lack of any clear variation in wing length along geographic and climatic gradients in T. alba, a contrasting result when compared to other species of the same genus (Romano et al 2021a). Wing length seems therefore more strongly affected by diet composition, and thus on food availability, than climatic factors, at least in this taxon.…”

“…In this taxon, body size (i.e. wing length) does not vary along geographic or climatic gradients according to Bergmann's rule (Romano et al 2021a), thus suggesting that other selective pressures may drive its spatial variation. Conversely, bill length has been shown to vary consistently with Allen's rule, with populations living at higher latitudes and altitudes (and smaller temperatures) showing a smaller beak than those inhabiting closer to the equator (Romano et al 2020a).…”

Evolution of wing length and melanin‐based coloration in insular populations of a cosmopolitan raptor

“…Wing length is considered a good proxy for body size, as the vast majority of the studies investigating body size variation in birds used this trait as a surrogate for size (e.g. a meta‐analysis by Ashton, 2002; see also Romano et al, 2020). Importantly, even Bergmann himself used mainly wingspan as a predictor of size in the original formulation of the homonym biogeographic rule (Bergmann, 1847, see also Salewski & Watt, 2017).…”

“…Indeed, it is impossible to realize a correct phylogeny among individuals for this taxon because individuals formally belonging to distinct and geographically isolated subspecies are usually more genetically related to each other than individuals sampled in the same population (Uva et al., 2018). For this reason, and following previous studies (Romano et al., 2019, 2020, 2020a; 2020b), analyses were performed separately for T. alba , T. furcata and T. javanica because they are not only geographically isolated but also monophyletic evolutionary lineages (Uva et al., 2018). From an evolutionary perspective, these lineages can be therefore considered as adaptive radiations occurring simultaneously in geographically separated regions, and producing a wide range of local adaptations, including insular phenotypes (Uva et al., 2018).…”

Resource and seasonality drive interspecific variability in a Dynamic Energy Budget model

“…In general, empirical evidence in endotherms supports both patterns (e.g., for Bergman's rule see Ashton et al 2000, Cardillo 2002, Ashton 2002, Freckleton et al 2003, Blackburn and Hawkins 2004, Rodríguez et al 2006, Olalla-Tárraga and Rodríguez 2007, Ramirez et al 2008, Olson et al 2009, Morales-Castilla et al 2012a, b, Clauss et al 2013, Torres-Romero et al 2016, Romano et al 2020 and reviews in Meiri and Dayan 2003, Watt et al 2010, Huston and Wolverton 2011. For Lack's rule see Moreau 1944, Lack 1947, Ashmole 1963, Cody 1966, Ricklefs 1980, Kulesza 1990, Iverson et al 1993, Griebeler and Böhning-Gaese 2004, Evans et al 2005, Jetz et al 2008, Mesquita et al 2016, Meiri et al 2020and review in Boyer et al 2010.…”