Reciprocal sexual size dimorphism and Rensch’s rule in Toad-headed lizards (Phrynocephalus vlangalii)

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 in interspecific comparisons. In groups of related species or sets of conspecific populations, Rensch’s rule has so far largely been confirmed when females will be the larger in small-sized populations and males would be 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 under study, and is therefore a good model species to examine Rensch’s rule depending on SSD direction. Using morphological data from 38 populations across the species’ ranges, we studied whether populations with both male- and female-biased SSD 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 defense). In populations with female-biased SSD we find a significant evidence for fecundity selection on large females, but Rensch’s rule is not supported in this species. Our findings suggest that the underlying evolutionary forces (sexual selection and fecundity selection) cannot promote the direction of SSD consistent with Rensch’s rule in both male-biased and female-biased SSD across populations within a species