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Anolis cooki and Anolis cristatellus are sympatric lizards engaged in intense interspecific competition. We have identified and analyzed a critical area of competitive interference by measuring various niche dimensions and behavioral interactions under natural conditions. Four study areas of similar habitat composition were established within 5 km of each other: one allopatric for A. cooki, one allopatric for A. cristatellus and two sympatric areas. The variables of cloacal temperatures, perch height, perch diameter and habitat type were compared between lizards in and among the study areas. No significant differences in the variables were found between the sexes of a species or between species on the allopatric areas, nor were there differences between sympatric and allopatric A. cristatellus. Sympatric A. cooki males and females, however, exhibited a significant divergence from their allopatric counterparts by utilizing standing dead vegetation and small bushes rather than larger and more complex microhabitat. This shift was most likely in response to competitive interference from A. cristatellus. Except for sympatric A. cooki, adult males in both the allopatric and sympatric conditions showed a significant and positive correlation between snout-vent lengths and the relative complexity and height of their immediate microhabitat. This suggests that there is intraspecific competition among males, with the larger conspecific males controlling the taller and more complex habitat. Thus when the sympatric male A. cooki occupy small types of microhabitat, they are only likely to encounter the smaller size classes of A. cristatellus males. Intruder-release experiments showed that encounters between male A. cooki and A. cristatellus were very aggressive, similar in intensity to conspecific interactions. Less than one out of two times were resident A. cooki able to evict A. cristatellus intruders of similar size. Thus by occupying smaller habitat types, sympatric A. cooki appears to decrease the likelihood of interacting with large A. cristatellus, against whom they would have little chance of holding a disputed territory. From current evidence, A. cooki is under competitive pressure from the more ubiquitous A. cristatellus and has no apparent refugium.

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