Thermal Biology and Temperature Selection in Juvenile Lizards of Co-occurring Native and Introduced Anolis Species

—Microhabitat selection is critical to thermoregulation in ectotherms, particularly in small-bodied organisms for which low thermal inertia can impose rapid acquisition of thermal equilibrium with the environment. Although thermal microhabitat selection is often included in studies of niche partitioning among lizard species these investigations typically address only adult thermoregulation and, therefore, potentially neglect age classes in which thermal microhabitat selection could be especially consequential. We measured thermal time constants for juveniles of the lizard Anolis carolinensis and compared thermal microhabitat selection of this species with that of an invasive co-occurring congener, Anolis sagrei. In keeping with the exceptionally low mass of these lizards, heating and cooling rates were found to be extremely high and more similar to those of some insects than to other terrestrial vertebrates. On laboratory thermal gradients juvenile A. carolinensis tended to select warmer temperatures than A. sagrei, a pattern that is opposite to that observed for adults of these species. Because the rate of core temperature change was found to be an order of magnitude higher in juveniles than in adults and because the interspecific pattern of temperature selection is categorically different, these results suggest that investigation of juvenile anole thermoregulation could be of particular importance in developing an accurate characterization of resource partitioning among these model species. In ectotherms, temperature is a principal quantity linking whole animal biological functioning and the environment and, therefore, has been a preeminent concern in ecology (Angilletta, 2009). Heat exchange governs behavioral, physiological, and chemical processes connected to resource acquisition (Beaupre et al., 1993; Belliure et al.; 1996; Ayers and Shine, 1997), growth (Avery, 1984), development (Bull, 1980; Georges et al., 2005), performance (Bennett, 1990; Angilletta et al., 2002), and ultimately survival and reproduction (Dawson, 1975; Spotila and Standora, 1985). In some cases, environmental temperature appears to be a primary direct determinant of animal movement, habitat selection, and territory establishment (Kearney, 2002; Kearney et al., 2003; Downes and Bauwens, 2004). Additionally, via indirect effects arising through its influence on sympatric species, environmental temperature can mediate a large portion of an animal’s ecological interactions. Environmental temperature, therefore, serves as a major axis defining the fundamental multidimensional niche (Hutchinson, 1957; Magnuson et al., 1979). Caribbean lizards of the genus Anolis have been studied in regard to ecological differentiation and multidimensional niche partitioning (Roughgarden, 1995; Losos, 2009). This work has resulted in the classic conceptualization of Anolis species recurrently evolving toward occupancy of a distinct set of niches (Losos et al., 2003), each associated with a specific ecomorph, or suite of morphological and ecological characteristics (Williams, 1983). Sympatric Anolis appear to partition resources predominantly along only three axes: prey size, structural habitat, and thermal environment (Schoener, 1974). In general, optimal range of body temperatures and precision of thermoregulation varies according to season, species, sex, reproductive status, and age (Huey and Pianka, 1977; Patterson and Davies, 1978; Van Damme et al., 1986; Ming-Chung and Hutchison, 1994; Diaz et al., 2006). Most studies of niche partitioning among anoles, and particularly those regarding thermal habitat, however, have focused exclusively on adults (Hertz, 1992; Jenssen et al., 1996; Huey and Webster, 1976). Because body size is expected to have major effects on heat flux with the environment (Stevenson, 1985), thermoregulation in other age classes could differ substantially from that described in current characterizations of Anolis ecology and niche