Results of this study showed that both males and females of A. nebulosus from the island population (SPI) were larger in SVL and in other morphometric characteristics than those from the mainland population (BFSCH). A similar pattern was found in males of A. lineattissima, but not in females, which exhibited similar morphological characteristics at both populations. The latter pattern in female A. lineattissima can be explained in that the female growth rate of A. lineattissima is similar at both CI and Xametla, and females reach sexual maturity at a similar body size (Table 2); a similar pattern has been observed in other lizard species with a broad distribution (Losos et al. ). Another explanation could be that the mortality rate in both populations of A. lineattissima is higher in females than in males and that females are responding to this pressure by growing quickly and reaching sexual maturity at a smaller SVL, a strategy observed in other vertebrate groups (Stearns and Koella ).
On the other hand, larger body sizes in both males and females of A. nebulosus and in males of A. lineattissima from island populations as compared to mainland populations could be explained in that the insular populations have evolved to attain a large size as a function of the absence of mainland predators, an hypothesis that has been tested in other lizards from island and mainland habitats (Andrews ; Lister and García ). The absence from SPI of golden-orb weaver spiders (Nephila), known predators of A. nebulosus, as well as various avian and snake predators of A. lineattissima from CI (Ramírez-Bautista and Vitt ; Ramírez-Bautista et al. ) could be a factor influencing the evolution of larger body sizes. By reducing the number of predators, lizards can eliminate time and energy costs associated with escape behavior and therefore direct a greater amount of energy toward growth (Tinkle and Ballinger ; Sinervo et al. ). Another explanation could be that lizards with a large body size have a higher survival rate during periodic food shortages because they are able to store more energy as fat bodies (Michaud and Echternacht ). This idea is also related to the hypothesis that lizards of a large body size (by energy storage) can better withstand catastrophic weather events such as hurricanes and storms over longer periods, events that are generally more intense on islands than on the mainland (Whittaker and Fernández-Palacios ).
Allometric analysis of males and females of both populations of A. nebulosus and A. lineattissima revealed a positive or negative trend in growth of the morphometric characters with respect to body size. In lizards, both types of allometry are considered to be adaptive responses to the selective pressures of the environment, such as predation, diet, and microhabitat (Vitt and Congdon ; Meiri ; Feldman and Meiri ).
In both populations of A. nebulosus, the allometric relationships of HL and HW, along with anatomical structures that provide agility, such as FL and TL, were negative, which could indicate that in both populations, individuals exhibit limited movement, primarily reduced to foraging, or escape from predators. This suggests that A. nebulosus is highly sedentary, a strategy that favors optimal energy expenditure, unlike in other groups such as whiptail lizards, which have large claws, sturdy limbs, and a long tail to escape in an agile way from their predators (Aguilar-Moreno et al. ). Studies of species in the genus Anolis have documented that microhabitat use and/or perch types (e.g., shrubs, brush, trunk-ground, trunk-rock, branches, and large rocks) are correlated with different ectomorphs that vary in body size, length of fingers, claws, and number of digital lamellae (Butler and Losos ). It has also been proposed that selection of microhabitats by lizards is associated with different types of allometric growth of the locomotory morphometric structures; which could explain differences in A. nebulosus populations from the island and mainland (Vitt and Cooper ).
On the other hand, in females of some species of Anolis, the relationships between SVL and PGL may have a stable (stable = positive) allometric pattern (Michaud and Echternacht ; Butler and Losos ); for example, in A. nebulosus, this morphometric characteristic (PGL) revealed a high allometric growth rate in relation to body size. In some species of Anolis, the pelvic girdle is positively correlated with egg size or volume and with the quantity and quality of food resources in the environment (Michaud and Echternacht ). For example, in A. carolinensis, there is a positive allometric relationship between SVL and aperture of the pelvic girdle; in other words, at greater SVL, there is a greater amplitude of the pelvic girdle. In the northernmost populations, the environment is subject to relatively low temperatures for extended periods of time; such populations have relatively large SVL, perhaps to allow for more storage of energy as fat bodies to help survive harsh winters (see Michaud and Echternacht ).
In A. lineattissima, most coefficients of allometry were positive for HW, FOL, AL, and TL, which could indicate that the anterior and posterior limbs are physiologically and/or ecologically more functional than those of A. nebulosus, enabling greater speed to escape successfully from predators and also to better explore the litter layer and ground during foraging, a behavior that has been observed in most whiptail lizards (Aguilar-Moreno et al. ; Mata-Silva et al. ). On the other hand, a positive allometric relationship in HW with respect to SVL in males from both populations could be related to sexual dimorphism (Aguilar-Moreno et al. ). Males exhibited larger heads and jaws than females, which could be explained by male-male combat and/or divergence in prey selection (e.g., males and females may specialize on prey of different sizes).
According to Andrews (), sexual dimorphism in lizard species inhabiting islands is more common and conspicuous than in lizard species from mainland habitats. This asymmetry has been called ‘ecological relaxation’ and has been associated with the decreased interspecific competition and low intensity of predation characteristic of islands (Stone et al. ). Ecological relaxation has been observed in some lizard species of the genus Anolis and Microlophus inhabiting in some islands of the Greater Antilles and the Galapagos; in these species, males are larger than females (Butler and Losos ; Stone et al. ). In this study, we found that males of A. nebulosus and A. lineattissima, from SPI and CI, respectively, were larger than conspecific females in all morphometric variables measured (Table 5). In contrast, males of A. lineattissima from the mainland were larger than females only in HL, HW, FOL, and TL. We observed sexual dimorphism just in HL for A. nebulosus from the mainland.
Sexual dimorphism of species from the island and mainland could be explained under the assumption that if resources (e.g., space and food) are limited and male populations are locally dense, then there may be intrasexual competition; lizards of large body sizes are better competitors for these resources. The hypothesis of niche divergence (Schoener ; Hierlihy et al. ) suggests that a degree of overlap in diet between males and females results in a strong competition for food; thus, a divergence in eating habits may be indicative of sexual dimorphism (Ramírez-Bautista and Pavón ; Aguilar-Moreno et al. ; Hierlihy et al. ). In addition, if the sex ratio is skewed toward males, then agonistic interactions between males for access to females during reproductive season should be intense; thus, sexual dimorphism would be explained by sexual selection (Hierlihy et al. ). Consequently, a large body size and head are morphometric attributes that could generate higher fitness in males, a pattern observed in gonochoristic Aspidoscelis species and in most species of Anolis from island and mainland populations (Anderson and Vitt ; Losos et al. ; Aguilar-Moreno et al. ). Finally, if body size at sexual maturity is larger in males than in females, then males should have a higher growth rate and are faster to defend territory and reproduce successfully, which would explain why males of A. nebulosus and A. lineattissima from islands are larger and have higher growth rates than males and females from mainland populations (Andrews ).
The absence of sexual dimorphism in A. nebulosus from mainland populations could be due to food resources are sufficiently abundant in the environment that competition is limited. This pattern has been observed in other species of Anolis from the Greater Antilles (Losos et al. ) and in species of the genus Sceloporus (from Central Mexico; Ramírez-Bautista and Pavón ; Ramírez-Bautista et al. ).
In general, lizards from insular environments are larger than lizards from conspecific mainland populations. However, we still need to evaluate the role of other factors in generating differences between island and mainland populations, such as distance between islands and mainland populations, variation in reproductive characteristics, and differences in feeding habits among populations.