Abstract
Sexual size dimorphism (SSD) is widespread within the animal kingdom. Rensch’s rule describes a relationship between SSD and body size: SSD increases with body size when males are the larger sex, and decreases with body size when females are the larger sex. Rensch’s rule is well supported for taxa that exhibit male-biased SSD but patterns of allometry among taxa with female-biased size dimorphism are mixed, there is evidence both for and against the rule. Furthermore, most studies have investigated Rensch’s rule across a variety of taxa; but among-population studies supporting Rensch’s rule are lacking, especially in taxa that display only slight SSD. Here, we tested whether patterns of intraspecific variation in SSD in greater horseshoe bats conform to Rensch’s rule, and evaluated the contribution of latitude to Rensch’s rule. Our results showed SSD was consistently female-biased in greater horseshoe bats, although female body size was only slightly larger than male body size. The slope of major axis regression of log10 (male) on log10 (female) was significantly different from 1. Forearm length for both sexes of greater horseshoe bats was significantly negatively correlated with latitude, and males displayed a slightly but nonsignificant steeper latitudinal cline in body size than females. We suggest that variation in patterns of SSD among greater horseshoe bat populations is consistent with Rensch’s rule indicating that males were the more variable sex. Males did not have a steeper body size–latitude relationship than females suggesting that sex-specific latitudinal variation in body size may not be an important contributing factor to Rensch’s rule. Future research on greater horseshoe bats might best focus on more comprehensive mechanisms driving the pattern of female-biased SSD variation.
Highlights
Sexual size dimorphism (SSD) is commonplace within the animal kingdom [1]
One primary objective is to test whether the pattern of SSD variation conforms to Rensch’s rule which predicts that when males are larger than females, SSD increases with increasing body size, but when females are larger than males, SSD decreases in larger species
The magnitude of SSD that we found is modest, the variation pattern in SSD among different greater horseshoe bat populations was consistent with Rensch’s rule, with male size being more variable than female size
Summary
Sexual size dimorphism (SSD) is commonplace within the animal kingdom [1]. Rensch’s rule predicts that male size varies more than female size, and male is purportedly the driver of size divergence whereas female size co-varies passively with that of males, thereby generating a pattern of allometry in SSD [5,6]. Rensch’s rule does not apply universally; while it is well supported for taxa that exhibit male-biased SSD or mixed SSD ([2,3,8,10], for the exception see [13]), patterns of allometry among taxa with female-biased size dimorphism are less clear and there is evidence both for [11,14,15] and against [4,16] the rule
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