SEXUAL BIMATURATION AND SEXUAL SIZE DIMORPHISM IN ANIMALS WITH ASYMPTOTIC GROWTH AFTER MATURITY

Many animal taxa exhibit a positive correlation between sexual size di morphism and sex differences in age at maturity, such that members of the larger sex mature at older ages than members of the smaller sex. P revious workers have suggested that sexual bimaturation is a product o f sex differences in growth trajectories, but to date no one has teste d this hypothesis. The current study uses growth-based models to study relationships between sexual size dimorphism and sexual bimaturation in species with asymptotic growth after maturity. These models show th at sex differences in asymptotic size would produce sexual bimaturatio n even if both sexes approach their respective asymptotic sizes at the same age, mature at the same proportion of asymptotic size and have o therwise equivalent growth and maturation patterns. Furthermore, our a nalyses show that there are three ways to reduce sexual bimaturation i n sexually size-dimorphic species. (1) higher characteristic growth ra tes for members of the larger sex, (2) larger size at birth, hatching or metamorphosis for members of the larger sex or (3) smaller ratio of size at maturity to asymptotic size (relative size at maturity) for m embers of the larger sex. Of these three options, sex differences in r elative size at maturity are most common ill size-dimorphic species an d, in both male-larger and female-larger species, members of the large r sex frequently mature at a smaller proportion of their asymptotic si ze than do members of the smaller sex. Information about the growth an d maturation patterns of a taxon can be used to determine relationship s between sexual size dimorphism and sexual bimaturation for the membe rs of that taxon. This process is illustrated for Anolis Lizards, a ge nus in which both sexes exhibit the same strong correlation (r greater than or equal to 0.97) between size at maturity and asymptotic size, and in which the relative size at maturity is inversely related to asy mptotic size for both sexes. As a result, sexually size-dimorphic spec ies of anoles exhibit the expected pattern of a smaller relative size at maturity for members of the larger sex. However, for species in thi s genus, sex differences in the relative size at maturity are not stro ng enough to produce the same age at maturity for both sexes in sexual ly size-dimorphic species. Members of the larger sex (usually males) a re still expected to mature at older ages than members of the smaller sex in Anolis lizards.