Abstract
According to Bergmann’s rule we expect species with larger body size to inhabit locations with a cooler climate, where they may be well adapted to conserve heat and resist starvation. This rule is generally applied to endotherms. In contrast, body size in ectothermic invertebrates has been suggested to follow the reverse ecogeographic trend: these converse Bergmann’s patterns may be driven by the ecological constraints of shorter season length and lower food availability in cooler high latitude locations. Such patterns are particularly common in large insects due to their longer development times. As large and facultatively endothermic insects, bumblebees could thus be expected to follow either trend. In this investigation, we studied body size of three bumblebee species over a large spatial area and investigated whether interspecific trends in body size correspond to differences in their distribution consistent with either Bergmann’s or a converse Bergmann’s rule. We examined the body size of queens, males and workers of the Bombus lucorum complex of cryptic bumblebee species from across the whole of Great Britain. We found interspecific differences in body size corresponding to Bergmann’s rule: queens and males of the more northerly distributed, cool-adapted, species were largest. In contrast, the mean body size of the worker caste did not vary between the three species. These differences in body size may have evolved under selection pressures for thermoregulation or starvation resistance. We suggest that this case study in facultatively endothermic insects may help clarify the selection pressures governing Bergmann rule trends more generally.
Highlights
The study of large-scale spatial variation in organismal traits has long been of interest to biologists, especially those studying ecology and evolution
Thorax width was recorded for 1,095 bees belonging to the lucorum complex species, which comprised 575 B. lucorum, 330 B. cryptarum and 190 B. magnus individuals (S1 Table)
Posthoc tests revealed that both B. cryptarum and B. magnus queens were significantly larger than B. lucorum queens (Fig 2a): the mean thorax breadths were 3.3% larger in B. cryptarum (t = 3.2, df = 80.8, P = 0.005) and 9.6% larger in B. magnus (t = 3.2, df = 21.4, P = 0.01)
Summary
The study of large-scale spatial variation in organismal traits has long been of interest to biologists, especially those studying ecology and evolution. Perhaps the best known is Bergmann’s rule, which predicts that endothermic vertebrate species inhabiting cooler climates will be larger than related species from warmer climates [3]. Bergmann originally used this rule to describe interspecific trends but it was later redefined to explain intraspecific variation [2,3,4,5]. This rule is generally applied to endothermic organisms and has been shown to hold for many species of bird and mammal [6,7,8]. The mechanism first proposed to explain these patterns was that in endotherms heat generation capacity increases with body volume, whereas heat loss increases with surface area; larger organisms, with relatively lower surface area, are favoured in cooler environments [3,4]
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