Bergmann's Body Size Rule Operates in Facultatively Endothermic Insects: Evidence from a Complex of Cryptic Bumblebee Species

• Published in: PloS one Vol. 11; no. 10; p. e0163307
• Main Authors: Scriven, Jessica J, Whitehorn, Penelope R, Goulson, Dave, Tinsley, Matthew C
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 14.10.2016; Public Library of Science (PLoS)
• Subjects: Animal behavior; Animals; Bees; Bees - physiology; Biological evolution; Biology and Life Sciences; Body size; Body Size - physiology; Bombus; Bombus lucorum; Bombus terrestris; Bumblebees; Case studies; Climate; Earth Sciences; Ecosystem; Endothermic reactions; Environmental science; Female; Food availability; Heat; Insects; Interspecific; Invertebrates; Male; Males; Medicine and Health Sciences; Models, Biological; Morphology; Queens; Reptiles & amphibians; Seasons; Spatial distribution; Species; Starvation; Temperature; Thermoregulation; Trends; Workers (insect caste); Austria; United Kingdom > UK; North America; Ireland; Europe
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0163307

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.