Intraspecific and interspecific behavioral interactions in adult male Anolis sagrei and gray-throated Anolis carolinensis (Sauria : Iguanidae)

Members of the Anolis carolinensis (Sauria: Iguanidae) complex co-occur with A. sagrei on many Caribbean islands. Where the two species occur together, A. carolinensis occupies a trunk-crown niche, whereas A. sagrei is considered a trunk-ground species. Until the recent introduction and subsequent dispersal of A. sagrei in Florida, A. carolinensis was the only representative of the genus in North America. There it had exhibited ecological release, occupying a ground-trunk-crown niche. With the spread of A. sagrei, A. carolinensis has been excluded from habitats it once occupied in disturbed areas of south Florida or has exhibited dramatic population declines. However, in coastal areas of southwest Florida, the two species appear to coexist without dramatic reductions in the density of A. carolinensis. Here A. carolinensis males exhibit a gray throat or dewlap in contrast to the red dewlap found throughout other areas of Florida and the southeastern United States. These morphs may differ behaviorally as well as physically. The purpose of this study is to assess the role of aggressive behavioral interference in the suspected competition between sympatric populations of A. sagrei and the gray-throated morph of

A. carolinensis

Intra- and interspecific encounters were staged in the laboratory between the two species. Analyses of the presence, absence, frequency, and latency of behaviors normally associated with anoline aggression indicate that interspecific aggression does occur and at more intense levels than were previously reported in studies using the red-throated morph. However, the majority of interspecific encounters resulted in low levels of aggression and eventual coexistence. Neither species appeared dominant in these staged interspecific encounters; however, results indicate that A. carolinensis escalates to a higher level of aggression than A. A. sagrei.

Field studies conducted in southwestern Florida indicate that interspecific aggression also occurs under natural conditions, although the majority of interactions were nonaggressive and resulted in coexistence. When interspecific aggression occurred, A. carolinensis was the aggressor. Anolis carolinensis successfully defended its territory interspecifically with attack, chase, and supplantation behavior. Time since initial interspecific contact may provide a possible explanation for these present-day observations. Initially, gray-throated A. carolinensis may have recognized A. sagrei as an interspecific competitor. The recent introduction of another anoline species into an otherwise competitor-free environment may have prompted elevated levels of aggression in A. carolinensis. In contrast, A. sagrei is a recent arrival from Cuba where it coexists with A. carolinensis in a system of successful niche partitioning, and, therefore, exhibits little interspecific aggression.

Low levels of interspecific aggression may indicate that A. sagrei and gray-throated A. carolinensis are presently in a dynamic phase of ecological niche partitioning. Predominant coexistence suggests that niche divergence has occurred in certain geographic areas. A decrease in the density of A. carolinensis in southern Florida may more likely be precipitated by competitive resource exploitation.