Clinal variation in body size and sexual dimorphism in an Indian fruit bat, Cynopterus sphinx (Chiroptera : Pteropodidae)

Geographic variation in body size and sexual dimorphism of the short-nosed fruit bat (Cynopterus sphinx) was investigated in peninsular India. Bats we re sampled at 12 localities along a 1200 km latitudinal transect that paral leled the eastern flanks of the Western Ghats. The geographic pattern of va riation in external morphology of C. sphinx conforms to the predictions of Bergmann's Rule, as indicated by a steep, monotonic dine of increasing body size from south to north. This study represents one of the first conclusiv ely documented examples of Bergmann's Rule in a tropical mammal and confirm s that latitudinal dines in body size are not exclusively restricted to tem perate zone homeotherms. Body size was indexed by a multivariate axis deriv ed from principal components analysis of linear measurements that summarize body and wing dimensions. Additionally, length of forearm was used as a un ivariate index of structural size to examine geographic variation in a more inclusive sample of bats across the latitudinal transect. Multivariate and univariate size metrics were strongly and positively correlated with body mass, and exhibited highly concordant patterns of clinal variation. Stepwis e multiple regression on climatological variables revealed that increasing size of male and female C. sphinx was associated with decreasing minimum te mperature, increasing relative humidity, and increasing seasonality. Althou gh patterns of geographic size variation were highly concordant between the sexes, C. sphinx also exhibited a latitudinal dine in the magnitude and di rection of sexual size dimorphism. The size differential reversed direction across the latitudinal gradient, as males averaged larger in the north, an d females averaged larger in the south. The degree of female-biased size di morphism across the transect was negatively correlated with body size of bo th sexes. Canonical discriminant analysis revealed that male- and female-bi ased size dimorphism were based on contrasting sets of external characters. Available data on geographic variation in the degree of polygyny in C. sph inx suggests that sexual selection on male size may play a role in determin ing the geographic pattern of sexual size dimorphism. (C) 2001 The Linnean Society of London.