Sexual dimorphism and natural history of the Western Mexico Whiptail, Aspidoscelis costata (Squamata: Teiidae), from Isla Isabel, Nayarit, Mexico

Lizard populations found in insular environments may show ecological and morphological characteristics that differ from those living in continents, as a result of different ecological and evolutionary processes. In this study, we analyzed sexual dimorphism, reproduction, and diet in a population of the whiptail lizard, Aspidoscelis costata, from Isla Isabel, Nayarit, Mexico, sampled in 1977 and 1981. Males and females from Isla Isabel showed no sexual dimorphism in many morphological structures, such as snout-vent length (SVL), but they did in femur length (FL) and tibia length (TL). Both sexes displayed synchronous reproductive cycles. However, males had smaller SVL (47.0 mm) in average than females (59.0 mm) at sexual maturity. The average clutch size was 2.4 eggs, but there was no correlation between egg number and female's SVL. The diet of A. costata consisted of insects, arachnids, crustaceans, and plant material. Insects were the most consumed prey item, being mostly represented by orthopterans, coleopterans, and hymenopterans. Diet breadth of males was higher (Levin's formula, B = 0.596) than in females (B = 0.358), and diet overlap between sexes was relatively high (Pianka's index, O_jk = 0.822), with an overlap ranging from 66.2 to 100%. The population of A. costata from Isla Isabel shows different morphological and ecological characteristics from their continental counterparts, but future studies on whiptail species with island and mainland populations are required to determine the ecological and evolutionary patterns.