A COMPARATIVE-STUDY OF POPULATION-DENSITY AND SEXUAL SIZE DIMORPHISM IN LIZARDS

Two hypotheses predict a positive relationship between population dens ity and sexual size dimorphism (SSD) among species of Anolis lizards; these hypotheses focus on food competition and male-male competition f or breeding territories, respectively. We first tested an underlying a ssumption of the food competition hypothesis, using data on the prey c onsumed by Anolis aeneus. This assumption is that SSD reduces intersex ual food competition relative to the amount of competition expected if males and females are the same size. Contrary to this assumption, est imates of food competition were lower if males and females were the sa me size than if males were larger than females or vice versa. Next, we tested the prediction from both hypotheses that SSD should be positiv ely related to female density, using data from 25 taxa (24 species) of anoles. Statistically significant relationships between these two var iables were obtained in the vast majority of potentially correct phylo genies for the species in this data set, based on either a gradual or a speciational model of evolution. In addition to documenting a relati onship between SSD and density, this study shows how comparative quest ions can be pursued in taxa that currently lack a definitive phylogeny .