A Comparative Study of Population Density and Sexual Size Dimorphism in Lizards

Two hypotheses predict a positive relationship between population density and sexual size dimorphism (SSD) among species of Anolis lizards; these hypotheses focus on food competition and male-male competition for breeding territories, respectively. We first tested an underlying assumption of the food competition hypothesis, using data on the prey consumed by Anolis aeneus. This assumption is that SSD reduces intersexual food competition relative to the amount of competition expected if males and females are the same size. Contrary to this assumption, estimates of food competition were lower if males and females were the same size than if males were larger than females or vice versa Next, we tested the prediction from both hypotheses that SSD should be positively related to female density, using data from 25 taxa (24 species) of anoles. Statistically significant relationships between these two variables were obtained in the vast majority of potentially correct phylogenies for the species in this data set, based on either a gradual or a speciational model of evolution. In addition to documenting a relationship between SSD and density, this study shows how comparative questions can be pursued in taxa that currently lack a definitive phylogeny.