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Sexual size dimorphism (SSD), the difference in body size between males and females, is a widespread phenomenon and commonly attributed to variation in sex-specific patterns of selection. Using data from three populations of Polypedates megacephalus, we tested the hypotheses that sexual selection favoured large male body size and that fecundity selection favoured large female size. Females had a larger body size than males in all populations and thus exhibited a female-biased sexual size dimorphism. Standardized selection gradients showed that sexual selection for large male size was weak at best and hence unlikely to be an important driving force for SSD. As fecundity selection favouring large females was weak as well, it could not explain the female-biased SSD patterns found. However, a faster growth rate in females favoured a larger size, providing an explanation for female-biased SSD. Our findings suggest that female-biased SSD does not result from a phenotypic selection of female by size in P. megacephalus. [ABSTRACT FROM AUTHOR]

Rensch's rule describes a pattern of allometry whereby sexual size dimorphism (SSD) increases with body size when males are the larger sex, and whereby SSD decreases with body size when females are larger by intraspecific comparison. In groups of related species or sets of conspecific populations, Rensch's rule has so far largely been confirmed with females being the larger sex in small-sized populations and males the larger sex in large-sized populations. The toad-headed lizard Phrynocephalus vlangalii is a small viviparous agamid endemic to China. It exhibits either male- or female-biased SSD depending on the population, and is therefore a good model species to test Rensch's rule relative to SSD direction. Using morphological data from 38 populations across the species' range, we studied whether populations with male- and female-biased SSD both exhibit isometric relationships consistent with Rensch's rule. For populations with male-biased SSD, we reject the hypothesis that SSD consistent with Rensch's rule is driven by allometries in tail length (in this species the tail is used for visual signalling during territory defence). In populations with female-biased SSD we find significant evidence for fecundity selection favouring large females, but Rensch's rule is not supported in this species. Our findings suggest that the underlying evolutionary forces (sexual and fecundity selection) do not promote the direction of SSD consistent with Rensch's rule in both male- and female-biased SSD across populations within a species. [ABSTRACT FROM AUTHOR]