Proximate mechanisms that contribute to female-biased sexual size dimorphism in an anguid lizard

Abstract

Various proximate mechanisms have been proposed to explain sexual size dimorphism (SSD) in vertebrates. Identifying the proximate causation of SSD allows insight into the ultimate reasons why SSD exists. I explored whether differential growth rates and (or) mortality explain SSD in the lizard Elgaria coerulea (Baird and Girard, 1852). I estimated growth parameters for males and females using the logistic-by-weight growth curves and determined survivorship using two complimentary methods: standard life-table calculations and capture–recapture methods. The former calculated age-specific survivorship, whereas the latter tested for differences in survivorship between males and females while considering differences in their recapture rates. I considered age-specific SSD as further evidence of SSD independent of differential mortality. Differences in growth asymptote, not intrinsic growth rate, contribute to SSD in this population. SSD is not due to differential mortality, as there is no difference in survivorship of males and females over 3 years of age. In addition, there is age-specific SSD with females larger than males for individuals 4 years of age and greater. The female-biased SSD may be a result of selection for large body size, although further studies are necessary to identify the ultimate cause of SSD in this species.