Little evidence for Bergmann's rule body size clines in passerines along tropical elevational gradients

AimTo test whether intra- and interspecific patterns in body mass along elevational gradients follow Bergmann's rule for a subset of tropical montane passerines. LocationTropical elevational gradients in New Guinea, Borneo, Peru and Costa Rica. MethodsI used linear regressions to assess intraspecific patterns in body mass along elevational gradients in common New Guinean passerines (2697 mist-netted individuals of 21 species). I then evaluated interspecific patterns using two data sets. First, I investigated differences in body mass in species pairs of elevational replacements, closely related species with minimal overlap along elevational gradients (species pairs; New Guinea: n=45, Borneo: n=22, Peru: n=58 and Costa Rica: n=30). Second, I used a comparative phylogenetic approach to test whether species' mid-point elevations predicted their masses within entire passerine avifaunas found along single elevational gradients (species; New Guinea: n=184, Peru: n=529 and Costa Rica: n=220). ResultsNew Guinean passerines exhibited minimal intraspecific variation in mass along elevational gradients. In two species, lower elevation individuals had significantly larger masses than upper elevation conspecifics. In species pairs of elevational replacements, there was no trend for upper elevation species to have larger masses than lower elevation species. Overall, species pairs tended to have positive mass disparities (mass of upper elevation species/mass of lower elevation species). However, contrary to predictions of Bergmann's rule, mass disparity was unrelated to elevational overlap. When considering entire passerine avifaunas along single elevational gradients, species' masses were uncorrelated with their mid-point elevational distributions. Main conclusionsI found little evidence that tropical montane passerines have larger body masses at higher elevations where temperatures are colder. This lack of pattern was consistent across evolutionarily independent avifaunas of different biogeographical regions. These results suggest mean temperature is not a generally important driver of body size evolution in tropical montane passerines.