Abstract
Two patterns commonly emerge when animal body size is analyzed as a function of latitudinal distribution. First, body size increases with latitude, a temperature effect known as Bergmann's rule, and second, the converse to Bergmann's rule, a pattern in which body size decreases with latitude. However, other geographic patterns can emerge when the mechanisms that generate Bergmann's and the converse to Bergmann's clines operate together. Here, we use phylogenetic comparative analysis in order to control for phylogenetic inertia, and we show that bumblebees exhibit the converse to Bergmann's rule. Bumblebee taxa are distributed worldwide in temperate and tropical regions. The largest species are found in places with high water availability during the driest time of the year. Nonetheless, large body size is constrained by extreme temperatures. Bumblebees’ body size could be related to a higher extent to the size of food rewards to be harvested than to the energetic advantages of thermoregulation. Moreover, we found that the body size of eusocial and cuckoo species responded in the same way to environmental variables, suggesting that they have not diverged due to different selective pressures.
Highlights
Body size has profound consequences on fitness, female fecundity, and male mating success, which usually increase with body size (Alcock and Thornhill 1983; Wiklund and Karlsson 1988; Honek 1993; Andersson 1994)
Some insect species can attain a large adult body size when they grow at high temperatures (Atkinson 1994; Mousseau 1997; Horne et al 2015)
Despite the fact that bumblebees from cold climates are larger than bumblebees from temperate ones, the geographic variation in Bombus’ body size indicated that this group follows the converse to Bergmann’s rule: a decrease in body size with latitude
Summary
Body size has profound consequences on fitness, female fecundity, and male mating success, which usually increase with body size (Alcock and Thornhill 1983; Wiklund and Karlsson 1988; Honek 1993; Andersson 1994). Body size depends on the regulation of growth rates and development times (Blanckenhorn et al 2007), and it can be influenced by environmental temperature, season length, productivity, and mortality (Blanckenhorn and Demont 2004; Chown and Gaston 2010). Ectotherms (Ray 1960; Stevenson 1985; Barlow 1994; Hawkins and Lawton 1995) and endothermic animals (Cushman et al 1993; Blackburn and Gaston 1994a,b) tend to attain larger adult body sizes. Growth rates increase, development times are shorter, and adults reach a smaller body size
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