Geographic Variation in the Life History of the Lizard Anolis carolinensis and Support for the Pelvic Constraint Model

Abstract

The green anole (Anolis carolinensis) occurs throughout the southeastern United States and its distribution defines the northern limit of the range of this genus of neotropical lizards. The variation in female, egg, and hatchling sizes is quantified for eight populations of the green anole that range from south Florida to the mountains of east Tennessee. Six populations are composed of the typical red-throated (dewlap) morph of this species, one of the gray-throated morph, and one population is representative of a variant of the red-throated morph. Among populations of the red-throated morph, female, egg, and hatchling sizes all increased significantly with increasing latitude. Additionally, females in the north had disproportionately larger eggs and offspring than did females to the south. We propose that increased selection pressure for large hatchling body size associated with decreasing winter temperatures (i.e., in- creasing latitude) results in the observed latitudinal diversification in the sizes of green anole eggs, juveniles, and adults. Whereas egg size was largely independent of female body size in southern popu- lations, there was a significant positive relationship between egg and female sizes in northern populations. The relationships between egg width and pelvic aperture width, relative to female body size, were examined for lizards from three populations that varied in the degree to which egg size was related to female body size. The size of the pelvic girdle aperture appeared to constrain the egg size of northern females to a greater extent than it did in southern females. Taken together, these data suggest that egg size may be optimized by natural selection in southern populations, but not in northern populations. Optimization of egg size may not be possible for lizards in northern populations due to this apparent morphological constraint on egg size, which is consistent with predictions of the pelvic constraint model. The environ- mental, physiological, and morphological factors that may combine to limit the continued northward range expansion of this successful colonizer are discussed. The genus Anolis is an ecologically and mor- phologically diverse group of approximately 300 species and subspecies of neotropical lizards in the family Iguanidae. Most species of Anolis are arboreal, but several are either saxicolous, ter- restrial, or semi-aquatic. Maximum adult female body size may be as little as 2 g in some species to over 50 g in one Puerto Rican anole (Andrews and Rand, 1974). Whereas species of Anolis have diverged in body size and habitat preference,