Search

The effect of removal of the parietal eye (parietalectomy) was studied in free- living lava lizards at the Charles Darwin Research Station in the Galapagos Islands, and the results were compared with those obtained with parietalectomized temperate zone iguanids. Behavior, activity, body temperature, and Iodine-131 uptake by the thyroid gland were studied from Jan. 23 to Feb. 27, 1964. Parietalectomy had no detectable effect on any of the parame- ters investigated. Home ranges of adult male and female lizards were measured, and defensive and antagonistic behavior was noted. The home ranges of males often encompassed those of several females. Most adults arose and retired with the sun, and sharply reduced exposure to sunlight at midday. At night, lizards buried themselves in soil and leaf-litter, and the same bed sites were often regularly used. Individuals injected with Iodine-131 were located at night with a scintillation counter. Active lizards had body temperatures averaging 34 to 350C (range 22.6 to 39.00C), which showed a bi-modal distribution. Cloud cover was a significant factor in body temperature fluctuations. Sexual dimorphism, both in color and in size of adults, was marked. Females had red "cheek" patches and were smaller than males. Most males had sperm in their sperm ducts and some females contained large-shelled oviducal eggs. Mating males seized females with their jaws, and sometimes carried them some distance before attempting copulation. Posturing and perhaps the odor of refractory females discouraged males. The lizards were found to be insectivorous, but some plant material was also eaten. One small gecko (Phyllodactylus galapagoensis) was recovered from stomach contents.

As the earth warms, populations will be faced with novel environments to which they may not be adapted. In the short term, populations can be buffered against the negative effects, or maximize the beneficial effects, of such environmental change via phenotypic plasticity and, in the longer term, via adaptive evolution. However, the extent and direction of these population-level responses will be dependent on the degree to which responses vary among the individuals within them (i.e., within population variation in plasticity), which is, itself, likely to vary among populations. Despite this, we have estimates of among-individual variation in plastic responses across multiple populations for only a few systems. This lack of data limits our ability to predict the consequences of environmental change for population and species persistence accurately. Here, we utilized a 16-yr data set from climatically distinct populations of the viviparous skink Niveoscincus ocellatus tracking over 1,200 litters from more than 600 females from each population to examine inter- and intrapopulation variability in the response of parturition date to environmental temperature. We found that these populations share a common population-mean reaction norm but differ in the degree to which reaction norms vary among individuals. These results suggest that even where populations share a common mean-level response, we cannot assume that they will be affected similarly by altered environmental conditions. If we are to assess how changing climates will impact species and populations accurately, we require estimates of how plastic responses vary both among and within populations. publishedVersion

Although rapid evolution of body size on islands has long been known, the ecological mechanisms behind this island phenomenon remain poorly understood. Diet is an important selective pressure for morphological divergence. Here we investigate if selection for novel diets has contributed to the multiple independent cases of island gigantism in the Skyros wall lizard (Podarcis gaigeae) and if diet, predation, or both factors best explain island gigantism. We combined data on body size, shape, bite force, and realized and available diets to address this. Several lines of evidence suggest that diet has contributed to the island gigantism. The larger islet lizards have relatively wider heads and higher bite performance in relation to mainland lizards than would be expected from size differences alone. The proportions of consumed and available hard prey are higher on islets than mainland localities, and lizard body size is significantly correlated with the proportion of hard prey. Furthermore, the main axis of divergence in head shape is significantly correlated with dietary divergence. Finally, a model with only diet and one including diet and predation regime explain body size divergence equally well. Our results suggest that diet is an important ecological factor behind insular body size divergence, but could be consistent with an additional role for predation.

Changes in the time available for organisms to maintain physiologically preferred temperatures (thermal opportunity) is a primary mechanism by which climate change impacts the fitness and population dynamics of organisms. Yet, it is unclear whether losses or gains in thermal opportunity result in proportional changes in rates of energy procurement and use. We experimentally quantified lizard food consumption and energy assimilation at different durations of thermal opportunity. We incorporated these data in an individual-based model of foraging and digestion in lizards to explore the implications of nonlinear responses to shifts in thermal opportunity across a wide geographic range. Our model predicts that shifts in thermal opportunities resulting from climate change alter energy intake primarily through digestion rather than feeding, because simulated lizards were able to fill their gut faster than they can digest their food. Moreover, since rates of energy assimilation decelerate with increasing thermal opportunity, shifts in daily energetic assimilation would depend on the previous opportunity for thermoregulation. In particular, the same changes in thermal opportunity will have little impact on lizards from warm locations, while having a large impact on lizards from cold locations where thermoregulation is possible for only a few hours each day. Energy expenditure followed spatial patterns in thermal opportunity, with greater annual energy expenditure occurring at warmer locations. Our model predicts that lizards will spend more energy under climate change by maintaining higher body temperatures and remaining active longer. However, the predicted changes in energy assimilation following climate change greatly exceeded the predicted increases in energy expenditure. Simple models, which assume constant rates of energy gain during activity, will potentially mislead efforts to understand and predict the biological impacts of climate change.

Understanding processes that may stabilize ecological systems confronted with rapidly changing environmental conditions is a key issue in ecology. We studied a system of highly fluctuating populations, the moth Achyra rantalis feeding on the plant Sesuvium portulacastrum in a group of small subtropical islands of the Bahamas. The plant is a prostrate inhabitant of shorelines, and consequently moths are highly vulnerable to being consumed by the ground-foraging lizard Anolis sagrei. We measured the percent ground cover of Sesuvium and abundance of Achyra on 11 islands with lizards present and 21 islands without lizards annually for 10 consecutive years. Overall abundance of Achyra was 4.6 times higher on no-lizard islands than on lizard islands. The percent cover of Sesuvium exhibited lower temporal variability on lizard islands when the study site was undisturbed by hurricanes, and higher recovery rate on lizard islands following hurricanes. We suggest that both of these stabilizing phenomena are linked to a trophic cascade in which predatory lizards control herbivore populations, thereby suppressing outbreaks and enhancing plant recovery following physical disturbance.

The effect of predators on the abundance of prey species is a topic of ongoing debate in ecology; the effect of snake predators on their prey has been less debated, as there exists a general consensus that snakes do not negatively influence the abundance of their prey. However, this viewpoint has not been adequately tested. We quantified the effect of brown treesnake (Boiga irregularis) predation on the abundance and size of lizards on Guam by contrasting lizards in two 1-ha treatment plots of secondary forest from which snakes had been removed and excluded vs. two 1-ha control plots in which snakes were monitored but not removed or excluded. We removed resident snakes from the treatment plots with snake traps and hand capture, and snake immigration into these plots was precluded by electrified snake barriers. Lizards were sampled in all plots quarterly for a year following snake elimination in the treatment plots. Following the completion of this experiment, we used total removal sampling to census lizards on a 100-m2 subsample of each plot. Results of systematic lizard population monitoring before and after snake removal suggest that the abundance of the skink, Carlia ailanpalai, increased substantially and the abundance of two species of gekkonids, Lepidodactylus lugubris and Hemidactylus frenatus, also increased on snake-free plots. No treatment effect was observed for the skink Emoia caeruleocauda. Mean snout-vent length of all lizard species only increased following snake removal in the treatment plots. The general increase in prey density and mean size was unexpected in light of the literature consensus that snakes do not control the abundance of their prey species. Our findings show that, at least where alternate predators are lacking, snakes may indeed affect prey populations.

As a group, lizards occupy a vast array of habitats worldwide, yet there remain relatively few cases where habitat use (ecology), morphology, and thus, performance, are clearly related. The best known examples include: increased limb length in response to increased arboreal perch diameter in anoles and increased limb length in response to increased habitat openness for some skinks. Rocky habitats impose strong natural selection on specific morphological characteristics, which differs from that imposed on terrestrial species, because moving about on inclined substrates of irregular sizes and shapes constrains locomotor performance in predictable ways. We quantified habitat use, morphology, and performance of 19 species of lizards (family Scincidae, subfamily Lygosominae) from 23 populations in tropical Australia. These species use habitats with considerable variation in rock availability. Comparative phylogenetic analyses revealed that occupation of rock-dominated habitats correlated with the evolution of increased limb length, compared to species from forest habitats that predominantly occupied leaf litter. Moreover, increased limb length directly affected performance, with species from rocky habitats having greater sprinting, climbing, and clinging ability than their relatives from less rocky habitats. Thus, we found that the degree of rock use is correlated with both morphological and performance evolution in this group of tropical lizards.

Major abiotic disturbance can be an important factor influencing food-web dynamics, particularly in areas impacted by the recent increase in hurricane activity. We present a unique set of data on key food-web processes occurring on 10 small islands for three relatively calm years and then four subsequent years during which two hurricanes passed directly over the study site. Herbivory, as measured by leaf damage, was 3.2 times higher in the year after the first hurricane (2000) than in the previous year and was 1.7 times higher in the year after the second hurricane (2002) than in 2001. The effect of a top predator (the lizard, Anolis sagrei) on herbivory strengthened continuously after the first hurricane and overall was 2.4 times stronger during the disturbance period than before. Overall abundance of lizards was 30% lower during the disturbance period than before, and abundances of web spiders and hymenopteran parasitoids were 66% and 59% lower, respectively. We suggest that increased herbivory observed on all islands was caused, at least in part, by the overall reduction in predation by both lizards and arthropods, whereas magnification of the lizard effect on herbivory was caused by reduced compensatory predation by arthropods.

Physiological processes are optimized within a narrow range of body temperatures. Reptiles engage in behavioral thermoregulation to achieve the optimal body temperature range. Interestingly, however, thermoregulatory effort varies over time and across species. The cost–benefit model of thermoregulation of R. B. Huey and M. Slatkin is the only conceptual framework that attempts to explain the observed variation. The model postulates that reptiles should engage in thermoregulation when the costs incurred do not outweigh the benefits. The main cost is the thermal quality of the environment: thermoregulation is more costly in habitats of low thermal quality. We used thermal quality of the habitat, measured by the deviations of operative temperatures from the preferred body temperature range, as an indicator of costs and two quantitative indices of thermoregulation as indices of thermoregulatory investment. Regressions of the indices of thermoregulation on thermal quality of the habitat produced no pattern or ...

The relatively shorter growing seasons, lower temperatures, and severe winter weather associated with higher latitudes have long been assumed to limit the size of populations, and therefore to constrain the geographical ranges of species. In particular, conspicuous instances of catastrophic mortality during periods of severe winter weather suggest that losses to “winterkill” may often determine latitudinal range limits. However, actual comparisons of overwinter mortality among latitudinally separated populations have been exceedingly rare, and no such study has focused on temperate-zone lizards, one of the groups thought to experience greater overwinter mortality at higher latitudes. Here we examine latitudinal variation in overwinter mortality among seven populations of the side-blotched lizard (Uta stansburiana) along a transect spanning nearly 15° of latitude in the western United States. We used standard mark–recapture techniques to estimate daily mortality rates for the fall through spring (overwinte...

Previous laboratory studies have frequently determined the maximal running speeds of lizards. However, neither the extent to which animals use maximal speeds in nature nor the effects of arboreal habitat structure on undisturbed speeds of locomotion have been well documented. Furthermore, quantitative data describing the physical structure of arboreal habitats are lacking. Thus, we quantified available habitat, perch use, and the undisturbed locomotor behavior of four syntopic ecomorphs of Anolis lizards (A. sagrei, A. distichus, A. carolinensis, and A. angusticeps) in the field. Fifty percent of the cumulative perch length available in the habitat had values of diameter, length, and incline

The area traversed by an animal during food gathering, mating, and other routine activities is termed the home range (HR), and its size is believed to reflect behavioral and physiological requirements (e.g., access to mates and food). We evaluated both bio- logical and methodological factors that may affect estimated HR sizes of lizards. We com- piled a database of nearly 500 published sex 3 population data sets. These yielded usable data for 108 populations from 60 species, all with both sexes represented. Initial analyses indicated pervasive sex differences, so sexes were analyzed separately with both conven- tional and phylogenetically based statistical methods. First, we conducted conventional analyses of covariance, which treat each data point as statistically independent. Second, we repeated these analyses while comparing members of two major clades represented in our database, Iguania and Autarchoglossa. Third, we repeated the analyses after designating smaller clades, nested within major clades. Fourth, we performed a fully phylogenetic analysis via independent contrasts and a composite phylogeny assembled from published studies. All analyses showed highly significant effects of both biological (body size, diet, habitat type) and most methodological (calculation method and minimum number of sight- ings per individual, but not study duration) factors and covariates tested. The conventional analysis comparing Iguania and Autarchoglossa en toto indicated a significant difference, as has been suggested in the literature. The analysis with nested minor clades, however, revealed differences among these but not between the two major clades. The analysis with phylogenetically independent contrasts indicated no statistically significant differences ei- ther among minor clades or between major clades. We also used our database to compare the scaling of HR area to body mass, using recently published allometric equations for field metabolic rates of lizards. The similarity of allometric slopes suggests that, interspecifically, HR size scales directly with energetic requirements. Intraspecifically, however, we found that males in both Autarchoglossa and Iguania consistently had larger HRs than did females. This difference, presumably related to the need to maximize access to females, may impose a ''cost of reproduction'' on males.

Resource subsidies from external habitats can enhance the performance or population density of local consumers, altering their effects on in situ prey. Indirect effects of subsidies may be either positive or negative depending on the behavior of the shared consumer. Here we document strong links between riverine insects, riparian lizards (Sce- loporus occidentalis), and terrestrial invertebrates. We hypothesized that aquatic insects subsidize riparian lizard populations leading to higher growth rates of these lizards in near- river habitats, and that subsidies exert short-term positive effects on terrestrial resources as a result of diet shifts by lizards to aquatic insects. To test these hypotheses, we used 2 m high fences, or ''subsidy shields,'' to experimentally reduce aquatic insect flux to large (91 m 2 ) enclosures of lizards. Subsidy shields reduced aquatic insect flux by 55-65%. Growth rates of lizards were 73 higher in subsidized (no-shield) enclosures during the early summer but were not significantly different later in the summer, when ambient fluxes of aquatic insects dropped to 20% of their early season levels. Within the watershed, lizard growth rates (in mass) were positively correlated with the numerical abundance of aquatic insects. Thus, lizard growth rates tracked both seasonal and spatial availability of riverine insect subsidies during our experiment. Subsidies also had indirect effects on the ground- dwelling, terrestrial prey of lizards. Declines of diurnal terrestrial invertebrates were sig- nificantly higher in shield than no-shield enclosures, and the most common ground spider ( Arctosa sp. (Lycosidae)) disappeared completely from shield enclosures by the end of the experiment. Declines in terrestrial invertebrate abundance did not differ between no-shield enclosures and lizard exclosures. These data suggest that riverine insects subsidize riparian Sceloporus and, in the short term, reduce their predation on terrestrial arthropods.

Sexual differences in adult body size (sexual size dimorphism, or SSD) ultimately can be favored by selection because larger males are more likely to be successful competitors for females, because larger females bear larger clutches, or because intersexual size differences reduce resource competition. Natural selection during juvenile development can influence sexual dimorphism of adults, and selection on adults and juveniles may differ. Studies that address the relative contributions of adult body shape dimorphism and sexually dimorphic patterns of growth and maturity are particularly useful in understanding the evolution of size dimorphism, yet they are rare. We investigated three sympatric, congeneric lizard species with different degrees and directions of adult sexual dimorphism and compared their growth patterns, survival probabilities, and intersexual trophic niche differences. Different mechanisms, even within these closely related, sympatric species, acted on juvenile lizards to produce species differences in adult SSD. Both degree and direction of dimorphism resulted from differences between the sexes in either the duration of growth or the rate of growth, but not from differences in rates of survival or selection on juvenile growth rate. Species- and sex-specific trade-offs in the allocation of energy to growth and reproduction, as well as differential timing of maturation, thus caused the growth patterns of the sexes to diverge, producing SSD. The differences that we observed in the direction of SSD among these species is consistent with their different social systems, suggesting that differential selection on adult body size has been responsible for the observed species-specific differences in juvenile growth rates and maturational timing.

I examined the long-term consequences of a trade-off between predation risk and resource acquisition for the garden skink (Lampropholis guichenoti) by rearing hatchlings to maturity in outdoor enclosures covered with snake predator scent (mimicking high predator densities) or control scent (mimicking low predator densities). Open areas provided optimal foraging and basking sites but were covered with scent, whereas retreat sites provided suboptimal foraging and basking opportunities and were not scented. During the initial six months of the experiment, lizards reared in enclosures covered with scent from a natural predator became active later in the day, showed reduced mobility, and selected “safer” substrate microhabitats than did lizards raised in enclosures covered with control scent. These behavioral shifts reduced opportunities to forage and bask for lizards in the predator-scented enclosures. During the study, however, lizards from predator-scented enclosures became gradually less responsive to snak...

We studied the effects of lizard size, and availability of C3 and C4 plants, on stable carbon-isotope (δ13C) ratios of lizards in an Amazonian savanna. Availability of C3 and C4 plants was estimated from 500-point quadrats in each of 38 plots of 3.75 ha. Ratios of δ13C in muscle tissue were used to estimate the contributions of C3- and C4-based food chains to the diets of four species of lizards: Cnemidophorus lemniscatus, Kentropyx striata, Ameiva ameiva, and Anolis auratus. The relative contributions of C3 and C4 plants to the diets of the lizards were affected by the extent of shrub cover in C. lemniscatus, A. auratus, and possibly in K. striata. However, there was a strong effect of lizard size among and within lizard species. These results are at variance with a previous evaluation of diet overlap among size classes of these species that was based on overlap in prey taxonomic categories. This indicates that species and/or size classes of predators on arthropods may obtain their energy from food chain...

The primary objective of this study was to test the hypothesis that lizards intensify interactions among spider species. The most abundant lizard and web-spider spe- cies (Anolis sagrei and Metepeira datona, respectively), were manipulated within field enclosures over a 30-mo period. The design was a 2 X 2 factorial: A. sagrei present and removed, crossed with M. datona present and removed. A. sagrei reduced abundance of Eustala cazieri, the second most abundant web spider, and tended to reduce abundance of all rare spider species combined. M. datona had no significant effect on abundance of E. cazieri or of all rare species combined. In each statistical analysis of abundance, the A. sagrei X M. datona interaction term was not significant, indicating that lizards did not intensify (or reduce) interactions among spider species. Furthermore, data on spider food supply, body length, and web height showed no evidence for two proposed mechanisms in which interactions among spider species are stronger with lizards than without. Thus, this experiment supports the hypothesis that direct predation is the major mechanism by which lizards reduce the abundance and species richness of spiders.

We examined how autotomy–induced shifts in behavior of a diurnal lizard (Lampropholis guichenoti) and two natural snake predators influence the outcome of predatory interactions. During staged encounters in seminatural enclosures, we estimated the consumption order of lizards with recently autotomised tails (“tailless”) and intact tails (“tailed”). Tail loss increased a lizard's chances of being consumed first by a diurnal snake (Demansia psammophis), but not by a nocturnal species (Rhinoplocephalus nigrescens). We presented snakes with an inert tailed lizard vs. tailless lizard, scoring which was consumed first. Neither predator actively selected tailless prey. We quantified variation in the behavior of tailed and tailless lizards that determines the outcome of predator–prey interactions. Tailless lizards were not easier to detect visually or via chemoreception than were tailed lizards. Instead, the higher consumption rate of tailless lizards by the diurnal snake probably reflects reduced locomotor perfo...

Current paradigms relating to reptilian temperature regulation are derived almost entirely from data on diurnally active species, although many reptile species are nocturnal. We investigated the extent and behavioral mechanisms of temperature regulation in the nocturnal, rock-dwelling marbled gecko Christinus marmoratus during spring and summer. During the daytime, we recorded body temperatures of lizards sheltering beneath rocks, as well as operative temperatures (the equilibrium body temperatures that animals would attain in given microclimates) beneath randomly chosen rocks available to the lizards. These measurements were compared with reference to the lizard's set-point (or target) temperature range (Tset), which we determined in a laboratory thermal gradient. In both seasons, lizard body temperatures were closer to set than were randomly sampled operative temperatures, despite mean operative temperatures being 7?C below and 6?C above Tset in spring and summer, respectively. The mean body temperature of geckos was 7?C higher, and the accuracy and effectiveness of temperature regulation were also much higher, during summer than during spring. The more severe physiological consequences of thermocon- formity during summer, as well as a more constraining thermal environment during spring, are advanced as reasons for this seasonal pattern. At least two behavioral mechanisms allow C. marmoratus to achieve a regulated body temperature. First, operative temperatures within retreat sites selected by geckos were closer to Tset than were operative temperatures beneath available rocks, suggesting retreat-site selection as a thermoregulatory mechanism. Second, lizard body temperatures were closer to Tset than were operative temperatures at random positions within selected retreat sites. This observation, together with correlative evidence relating lizard body temperature to the maximum and minimum operative temperatures beneath selected retreat sites, indicates that positional and/or postural adjustments were also employed as thermoregulatory mecha- nisms. This study suggests that nocturnal lizards are capable of regulating body temperature to an extent that is comparable to that of diurnal lizards. Our findings have implications for understanding the thermal physiology and ecology of nocturnal ectotherms.

The maximal running speeds of lizards have commonly been measured using level racetracks in a laboratory, but the normal speeds of locomotion of lizards and most animals in their natural habitats are not well documented. Thus we quantified the locomotor activity of the lizard Uma scoparia in its natural sand-dune habitat to determine how commonly animals run near their maximal speed and how inclines in the environment might affect habitat use and the speed of locomotion. We used leaf blowers to erase footprints in the sand in three 40 m x 100 m plots differing in surface topography and vegetation and then measured attributes of the paths and tracks left by U. scoparia that were not disturbed (by observers) during their period of peak activity. Laboratory observations allowed us to estimate speed for each field measurement of stride length (n = 5993). The frequency distribution of stride speed had two distinct modes (mean values 0.25 and 2.00 m/s), for which only the lower one is likely to be aerobically sustainable. Bipedal locomotion was extremely rare (

In many ectotherms, incubation temperature has profound effects on the timing of hatching and size of hatchlings, but the mechanisms underlying these effects are poorly understood. We studied the energetics of embryonic development and growth in the lizard Sceloporus undulatus. Eggs were incubated at six constant temperatures, ranging from 288 to 388C, and embryonic metabolism, incubation period, and body size at hatching were determined. The duration of embryonic development decreased significantly from 55 da t 28 8 to 40 da t 32 8C but did not differ significantly between 328 and 348C. Embryos incubated at temperatures above 348C did not survive to hatching. Metabolic rate at specific stages of development (percentage of total incubation period) did not differ among embryos incubated at 288 ,3 0 8, and 348C. As a result, the total amount of energy expended during the incubation period at 288C (2.0 kJ) was greater than that at 308-348C (1.7-1.8 kJ). However, the difference in energy expenditure did not affect body size at hatching; neither snout-vent length nor body mass varied significantly with incubation temperature, and both were the same as those of hatchlings collected in the field. Thus, there was no apparent trade-off between hatching date and body size of lizards at hatching. In a natural population in New Jersey, USA, we quantified soil temperatures at potential nesting sites and studied the thermoregulatory behavior of gravid females to examine the possible consequences of female behavior for hatchling phenotypes. In females and at potential nest sites, embryos would experience temperatures that resulted in high mortality in the laboratory experiment ( .328C). Gravid females had a field body temperature of 33.98C (95% CI 5 0.88C) and selected a body temperature of 33.38C (95% CI 5 1.08C) when placed in thermal gradients in the laboratory. Soil temperatures rose above 328C for several hours each day. Embryos must be able to survive intermittent exposure to tem- peratures that were lethal under conditions of chronic exposure in the laboratory. Selection of relatively high body temperatures by gravid females, coupled with tolerance of acute exposure to relatively high soil temperatures, would reduce the incubation period without a concomitant reduction in body size at hatching.

Mothers may be able to manipulate the phenotypes of their progeny not only via direct pathways (e.g., allocation of nutrients and energy), but also indirectly, by inducing particular developmental pathways through selection of nest sites and thus, incubation conditions. In the field, female skinks (Bassiana duperreyi) in montane southeastern Aus- tralia select nest sites with specific thermal characteristics-especially, a high mean tem- perature and a high diel range in temperature. Monitoring of 14 natural nests throughout most of the incubation period revealed considerable differences among nests in both means and variances of thermal regimes. Laboratory experiments on this species show that both the mean and the variance of incubation temperatures profoundly influence developmental rates (and hence, incubation periods) as well as the body shape, activity levels, thermo- regulatory behavior, and running speeds of the hatchlings. Hence, a female's selection of a particular nest site can substantially modify both the time of emergence of the hatchlings, and the morphology and behavior of her offspring. Even within a single nest, minor dif- ferences among eggs in their depth below the ground surface will directly affect thermal variance, and so may strongly influence rates of embryogenesis and the phenotype of the hatchling. The effects of thermal variance (independent of the mean) on embryonic de- velopment rates and hatchling phenotypes suggest that data from constant-temperature incubation in the laboratory should be interpreted with caution.

Among iguanian lizards, males are often larger than females. As a result, much attention has been directed toward understanding the evolution of sexual size dimorphism (SSD) in this group. SSD can result from a number of developmental, physiological, and demographic processes, which may cause sex differences in growth trajectories, timing of maturity, and adult age distributions. Knowledge of these proximate mechanisms can improve our understanding of the evolution of SSD. Only recently, however, have efforts been directed toward understanding proximate mechanisms. I examined the proximate causes of SSD in a population of a South American iguanian lizard, Microlophus occipitalis. The majority of the SSD in this population resulted from continued postmaturity growth in males and reduced postmaturity growth in females. Additionally, adult males tended to live longer than females, which affected adult age and size distributions. The proportion of young individuals in samples had strong effects on temporal fluctuations in SSD in the population. I discuss the importance of studying proximate mechanisms for evolutionary analyses of SSD in organisms that continue to grow after reaching maturity.

Although abiotic and biotic factors can interact to shape the spatial niche of a species, studies that explore the interactive effects of both at a local scale are rare. We demonstrate that one of the main axes (perch height) characterizing the spatial niche of a common lizard, Anolis sagrei, varies according to the interactive effects of weather and the activity of a larger predatory lizard, Leiocephalus carinatus. Results were completely consistent: no matter how favorable the weather conditions for using the ground (mainly characterized by temperature, humidity, wind speed, rain), A. sagrei did not do so if the predator was present. Hence, great behavioral plasticity enabled A. sagrei to adjust its use of space very quickly. To the best of our knowledge, these results constitute the first field demonstration for anoles (and possibly for other animals as well) of how time-varying environmental conditions and predator presence interact to produce short-term changes in utilization along a major niche axis., This study was supported by the EU Seventh Framework Programme ([FP7/2007‐2013] under grant agreement no. 221638) in the form of a Marie Curie International Outgoing Fellowship to M. Lopez‐Darias and National Science Foundation grants to T. W. Schoener and J. B. Losos. We thank the Bahamas Ministry of Agriculture and the Bahamas Environment, Science and Technology (BEST) Commission of the Ministry of the Environment for permission to conduct this research.

This study considered the effects of insectivorous Anolis lizards on a large, complex food web of arthropods and associated herbivory in a tropical rain forest canopy. We excluded Anolis lizards for 6 mo from 20-30 m high tree crowns in Puerto Rican rain forest. Simultaneous with lizard exclusion, we sampled orb spiders, airborne arthropods, and leaf arthropods in lizard removal crowns and in controls. We also sampled herbivory at the end of the experiment. Lizard removal had strong, statistically significant, positive effects on arthropods >2 mm in length and weak negative effects on arthropods 2 mm, both predatory, i.e., orb spiders and parasitic Hymenoptera, and nonpredatory forms, i.e., Diptera, Coleoptera, Orthoptera, and Blattaria, showed strong significant and positive responses to lizard removal. Large Psocoptera, Homoptera, leaf spiders, and ants did not show significant overall responses to lizard removal. Frequency of herbivore damage on new leaves was positively correlated with abundance of Orthoptera and Blattaria. This damage was sig- nificantly greater in lizard removal crowns than in controls, indicating an indirect effect of anoles on plants. The indirect effect of lizards on small arthropods through the predatory arthropod pathway appeared weak. Results of lizard removal shown by this study corrob- orate other lizard removal studies from more xeric, ground-level habitats with simpler food webs in the West Indies, particularly with respect to orb spiders and herbivory. Taken together with the results of similar experiments performed in trophically less complex systems, this experiment suggests that food web size is less important than body size in determining interaction strength between community members.

The frillneck lizard Chlamydosaurus kingii is an arboreal lizard that is a conspicuous component of the reptile fauna of the wet-dry tropics of northern Australia during the wet season. During the dry season, however, they are secretive, and a previous study revealed that during this season they remain perched in trees and have field metabolic rates only 28% of the wet season levels. Body temperatures (Tb's) of the lizards were measured by radio telemetry throughout the day during the wet and dry seasons. The midday Tb's during the wet season were high (grand mean = 36.70C) and typical for heliothermic lizards, but the dry season midday Tb's were significantly lower (grand mean = 32.80C). Microclimatic data and physical characteristics of the lizards were used in a biophysical model to calculate the operative temperatures (Te) of lizards in the shade, in the sun on a horizontal plane, and normal to the sun at each hour of the day for the two seasons. The Te's revealed the physical possibility for the lizards to achieve much higher Tb's during the dry season than were measured. Thus, the lower Tb's in the dry season represent a shift in preference rather than an inability to attain a high Tb during the cooler dry season. Inspection of the Tb's and Te's revealed that although the lizards remained cooler in the dry season, they did not thermoregulate at the lowest possible Tb's. During both seasons the lizards basked in the sun early and late in the day, but during the dry season the lizards stopped intensive basking at a Tb -40C lower than in the wet season. An index of the extent to which the lizards exploit the available thermal environment indicates that they thermoregu- late carefully in both seasons. Tb's were also measured in a laboratory thermal gradient during both seasons, and the Tb's selected during the dry season were significantly lower than those selected in the wet season. This suggests that the seasonal shift in thermal preference is an acclimatization response or an endogenous seasonal cycle rather than a response to a simple thermal cue. The lower Tb's in the dry season result in a conservation of energy and water during a season when these resources are relatively scarce. However, the fact that the lizards do not thermoregulate at the lowest possible Tb's suggests that the dry season Tb's represent a compromise between conservation of resources and the ability to perform other functions such as escape predators and/or digest food.

We studied the relationship between daily active time and growth rate in hatchling sagebrush lizards (Sceloporus graciosus), a mainly montane species, and western fence lizards (Sceloporus occidentalis), which inhabit a broader range of thermal environments. The study involved two populations of each species along in altitudinal gradient in southern California and one population of S. occidentalis from Oregon. In the field daily activity time varied seasonally: activity periods were short during spring and fall and longer during the summer. Activity patterns also varied geographically: in montane and high latitude environments, hatchlings experienced reduced opportunity for growth both daily and seasonally, because fewer, shorter days are available before the end of the activity season. We conducted laboratory experiments to measure the effect of the thermal environment on hatching growth rates. Laboratory—incubated hatchlings had access to radiant energy for part of each day and were kept at 15°C (too lo...

To determine the effects of lizards and web spiders on species in lower trophic levels, we manipulated their abundances within large field enclosures on Staniel Cay, Bahamas, from May 1989 to March 1992. The experimental design (2 x 2 factorial) measured the separate effects of lizards and spiders and compensatory predation (lizard x spider interaction). In treatments where web spiders were unaltered, mean number of web spider individuals was 1.4 times higher in enclosures with lizards removed than in those with lizards present at natural densities. Total biomass of aerial arthropods caught in sticky traps was 1.4 times higher in treatments with web spiders removed than in treatments with web spiders unaltered. Lizards had no significant effect on aerial arthropods. Total amount of herbivore damage on sea grape leaves was 3.3 times higher in treatments with lizards removed than in treatments with lizards unaltered. Web spiders had no significant effect on leaf damage. The lizard x spider interaction was not significant in each analysis, in- dicating that compensatory predation was weak. Our results support a model in which the interaction between top predators (lizards) and herbivores is strong, whereas the interaction between intermediate predators (web spiders) and herbivores is weak. Consequently, the net effect of top predators on producers is positive.

Geographic variation in life history phenotypes between populations of a single species is often assumed to reflect genetic divergence caused by natural selection. The relative contribution of genetic and environmental sources of phenotypic variation is rarely determined, especially for vertebrates. However, distinguishing between phenotypic plasticity induced by proximate environmental variation and genetic divergence is fun- damental to understanding the ecological and evolutionary significance of geographic vari- ation. We used a reciprocal transplant experiment to uncover the relative importance of population-specific (genetic) and environmental sources of variation in individual growth rates between two populations of the fence lizard, Sceloporus undulates. Our study revealed a population x environment interaction, consistent with a genotype x environment in- teraction that would result if differences between populations were genetically based. The growth rates of Nebraska lizards, normally twice that of New Jersey lizards, were reduced by half when the animals were transplanted to New Jersey. However, New Jersey lizards showed no increase in growth rates when transplanted to Nebraska. Comparisons of relative food availability (arthropod abundance) indicated that more food was available in New Jersey during the experiment. Estimates of potential activity day length using hollow copper models were from 2 to 2.5 h longer in Nebraska. This result suggests that the thermal biophysical environment may have reduced growth rates of Nebraska lizards transplanted to New Jersey, perhaps by reducing foraging time or by limiting physiological processes supporting growth. Our results are consistent with previous studies of S. undulates that assumed genetic differences in life histories between populations and implicated the thermal biophysical environment as an ecological source of variation.

Density dependence plays an important role in the regulation of most pop- ulations. Descriptive studies provide only limited evidence, while density manipulations are thought to be a more powerful tool. Here, we describe such a manipulation. We experimentally analyzed demographic responses to changes in density in common lizard (Lacerta vivipara) populations. Two neighboring sites were studied from 1986 to 1988. At the end of the first year, density was artificially decreased at one site and increased at the other. The evolution of demographic parameters over time (before and after ma- nipulation) was compared between the two sites. We found that: (1) density in each age and sex class quickly converged to pre-experimental values at both sites; (2) survival rates (introduced individuals excluded) were unchanged, except that juvenile mortality was pos- itively related to density; (3) emigration rates remained low and did not seem to be influ- enced by density, while immigration rates were negatively influenced by density; (4) the proportion of young reproductive females was negatively influenced by density; (5) body length and growth rate were negatively influenced by density only in yearlings, and at least in females, no catchup on growth is apparently possible; no difference in reserve storage was found; and (6) female reproduction was modified by the manipulation through clutch size and hatching success, which were negatively influenced by density; however, no dif- ference was detected for hatching date, number of abortive (clear) eggs, prenatal mortality, reproductive investment, and body mass of live neonates. Rapid density readjustments were mostly explained by immigration into the site where density was decreased, and by high mortality of introduced individuals and of native juveniles where density was increased. Other parameters that were influenced by the ma- nipulation could only have induced delayed effects on population density. Different age and sex classes showed different responses to density. These responses indicate that density dependence plays a key role in shaping the demography of this lizard species.

I used null hypotheses about basking behavior and body temperatures (Tb) to evaluate the extent and effectiveness of behavioral temperature regulation in Anolis cristatellus and A. gundlachi. I sampled these species in January and August near the extremes of their altitudinal distributions in Puerto Rico. Hollow electroformed copper lizard models, randomly positioned within habitats occupied by the lizards, served as operative temperature (Te) thermometers, providing baseline predictions about basking rates and Tb of nonthermoregulating lizards. Comparison of the basking behavior and Tb of lizards to the basking rates and tem- peratures of the models provided unambiguous evidence of nonrandom thermoregulatory activity in some of the lizard populations. Although basking opportunities were similar in the habitats occupied by both species, basking rates differed markedly: A. cristatellus basked more frequently than did randomly positioned models, and its basking rates were higher at high elevation and in January; A. gundlachi used basking sites at random at both elevations in both months. In addition, A. cristatellus limited its activity, particularly at high elevation, to times when operative temperatures were relatively high or basking sites were available (i.e., sunny weather). As a result of its opportunistic basking activity, mean Tb of A. cristatellus was often elevated above mean Te, and A. cristatellus populations experienced only limited variation of mean Tb with elevation and season. In contrast, A. gundlachi mean Tb approximated mean Te, and the Tb of A. gundlachi varied more with elevation and season. Mean Tb in A. cristatellus was always higher than the mean Tb of A. gundlachi at every elevation and month combination. The data also suggest that the low Tb experienced by A. gundlachi at high elevation might dramatically reduce sprinting ability, whereas the Tb experienced by A. cristatellus allows a majority of lizards at high elevation to sprint quickly.

The diet of the predominantly herbivorous Bonaire Island whiptail lizard (Teiidae: Cnemidophorus murinus) was examined to assess three models of diet selection by generalist herbivores. These models were: single-nutrient maximizing, toxin avoiding, and nutritional wisdom. At three sites we gathered data on the diet of the lizard over a full year (both wet and dry seasons), on the relative abundance of all plant food types available to the lizards during that period, and on the nutritional composition of each plant type. Thirteen nutritional variables were measured, including content of energy, protein, minerals, water, and potentially toxic plant secondary compounds, and digestibility of protein. The lizards were generalist feeders, consuming a wide variety of flowers, fruits, leaves, nectar, and some animal material. Most stomachs contained more than one food type, suggesting C. murinus typically samples several kinds of plant materials each day. The lizards were selective with regard to foods eaten; most foods were not taken in proportion to their availability in the environment. Potential plant foods varied in nutritional quality, but no one nutrient was correlated with dietary preferences. Multivariate analysis revealed that preference or avoidance of a potential food type could be predicted by a combination of nutritional properties, but these differed among sites. Despite differing plant assemblages at each site, annual intake of nutrients by the lizards was similar among sites. The results best support the nutritional wisdom hypothesis; the Bonaire whiptail lizard may assemble its diet to obtain the proper balance of required nutrients, while avoiding dangerous levels of plant secondary compounds.

Because temperature affects the growth, development, and survival of embryos, oviparous mothers should discriminate carefully among available nesting sites. We combined a radiotelemetric study of animal movements with a spatial mapping of environmental temperatures to test predictions about the nesting behavior of the eastern fence lizard (Sceloporus undulatus). Females made large excursions from their typical home ranges to construct nests in exposed substrates. These excursions appeared to be related solely to nesting because all females returned to forested habitat immediately afterward. On average

We document variation in the degree of arboreality among four populations of the western fence lizard (Sceloporus occidentalis), and investigate how this variation related to locomotor performance capability in an arboreal context (e.g., artificial branches). We raced lizards on four different rod diameters, measured maximum sprint speed, and also scored agility on the rods. Lizards from more terrestrial populations ran faster on thick rods, but slower on thin rods than did lizards from more arboreal populations. The terrestrial lizards experienced a greater difference in sprint speed than did the more arboreal lizards over the range of rod sizes we used. Arboreal lizards were more adept at running on all surfaces in that they stumbled and fell less often, particularly from the thinnest rods. Thus, specialization for maximizing sprinting performance on arboreal surfaces of a particular diameter entails a performance trade—off that is reflected in both speed and "sure—footedness." We investigated whether the population differences in arboreal sprinting are correlated with the significant differences in hind—limb span found among populations. We found that lizards with relatively longer limbs sprinted faster. By using juveniles reared in a common laboratory environment, we demonstrated that these among—population differences in arboreal sprint performance are probably genetically based.

Ecologists increasingly recognize the ability of certain species to influence ecological processes by engineering the physical environment, but efforts to develop a predictive understanding of this phenomenon are in their early stages. While many believe that the landscape-scale effects of ecosystem engineers will be to increase habitat diversity and therefore the abundance and richness of other species, few generalities exist about the effects of engineering at the scale of the engineered patch. According to one hypothesis, activities that increase structural habitat complexity within engineered patches will have positive effects on the abundance or diversity of other organisms. Here I show that, by damaging trees and increasing their structural complexity, browsing elephants create refuges used by a common arboreal lizard. Observational surveys and a lizard transplant experiment revealed that lizards preferentially occupy trees with real or simulated elephant damage. A second experiment showed that lizards vacate trees when elephant-engineered refuges are removed. Furthermore, local lizard densities increased with (and may be constrained by) local densities of elephant-damaged trees. This facilitative effect of elephants upon lizards via patch-scale habitat modification runs contrary to previously documented negative effects of the entire ungulate guild on lizards at the landscape scale, suggesting that net indirect effects of large herbivores comprise opposing trophic and engineering interactions operating at different spatial scales. Such powerful megaherbivore-initiated interactions suggest that anthropogenic changes in large-mammal densities will have important cascading consequences for ecological communities.

Average body temperatures (Tb) of summer-active Sceloporus merriami were lower in the morning (33.30C) than in the evening (37.0?) at an arid locality in Big Bend National Park, Texas. During midafternoon, lizards were inactive and retreated to deep crevices and caves. According to a multivariate analysis, overall rates of lizard activities (movements, feeding, and social displays) were similar between morning and evening activity, indicating that lizards were behaving similarly during the different activity periods. However, at 370, estimated rates of metabolism and water loss were greater and sprint speed was lower relative to these rates at 330, suggesting that substantial resource and performance costs may have been imposed by the observed higher evening activity Tb. To investigate this unusual pattern of body temperature variation, I compared the morning and evening distributions of available operative environmental temperatures (Te) as estimated by an array of randomly deployed and oriented copper models of S. merriami placed on rock and ground surfaces within lizard home ranges. In the morning, lizards had access to a wide range of Te (300-600), and a large fraction of the Te distribution was below 360 at the time of peak morning activity. During the morning, a hypothetical increase in Tb to within the interval 36?-38? would have allowed lizards to occupy 900% depending on the exact time), and microclimates below 360 were unavailable until just before dark. The available Te distributions in the evening forced lizards to accept a higher activity Tb, and concomitantly higher metabolic and water loss rates, and lower locomotor performance, or remain inactive in crevices and caves. Thus in the evening, and not in the morning, the gain in usable home range due to accepting a higher Tb was great, and the lizards exploited this fraction at an increased Tb only in the evening. These data suggest that the distribution of available Te can directly influence the cost/benefit economics of lizard body temperature selection.

We examine environmental constraints on life history characters among three elevationally distinct populations of the desert lizard Sceloporus merriami in west Texas. We show how environmental gradients in temperature and food abundance interact to constrain body temperatures, daily activity times, growth rates, and age-specific body size. We suggest that these differences resulted in opposite responses from males and females with respect to their size and age at first reproduction. The highest elevation site, Maple Canyon (1609 m), has more rainfall, higher food availability, and cooler temperatures compared with the lower elevation sites, Grapevine ( 1036 m) and Boquillas (560 m). Lizards were active throughout the day at Maple Canyon, but were restricted to midday inactivity at the lower sites. Body temperatures (Tb) were higher during inactivity and late-afternoon activity at the lower sites. We suggest that low food availability, fewer hours to forage, and higher Tb constrained the energy budgets of lizards at Boquillas. Consequently, these lizards had low activity rates and low daily growth rates. Paradoxically, lizards at Maple Canyon, the most food-rich site, also exhibited low daily growth rates. This may have resulted from high daily activity expenditure (longer activity period) and lower food processing rate (low inactive Tb). Grapevine lizards had the highest individual activity and growth rates. On a seasonal basis, the pattern of activity time was reversed, and Maple Canyon lizards had a shorter annual active season by 1 mo compared with Grapevine and 2 mo compared with Boquillas. Despite low growth rates at Boquillas, a 2-mo longer activity season proxi- mally resulted in the largest yearlings by May-June. Maple Canyon yearlings grew at a similar rate, but over a shorter time period and were the smallest. At Grapevine, high summer growth rates resulted in large yearlings by late summer and the largest adults, whereas low summer growth rates at Boquillas and Maple Canyon led to smaller adults. The resulting difference in body size between yearling and older age classes was great at Maple Canyon and at Grapevine, which forced yearling males to delay reproduction until they had grown to a size to compete successfully for breeding territories. However, Boquillas yearlings and age > 1 yr lizards were much more similar in size, and many yearlings were able to acquire breeding territories. This suggests that male age of first reproduction was lowest at the low elevation site. In contrast, more yearling females reproduced earlier at the highest elevation site than at other sites. At Grapevine and Boquillas, females similar in size to breeding females at Maple Canyon did not breed in May-June and instead grew and were reproductively active in July-August. We hypothesize that the brief growing season at Maple Canyon may have constrained females to breed early to allow sufficient time for themselves, as well as for their offspring, to amass winter stores. Also, this would increase the likelihood that their offspring were large enough to breed as yearlings. Results suggest an interaction between resource levels and biophysical constraints that may greatly influence differences among populations in important life history character- istics. Although these responses are hypothesized to be proximally induced by environ- mental constraints, the resulting life history differences in age-specific resource allocation to growth, storage, and reproduction may significantly affect fitness.

I investigated the relationship between microhabitat use and behavioral thermoregulation in the lizards Sceloporus occidentalis and S. graciosus. I studied two populations of each species along a 1300-m elevational gradient in the San Gabriel Moun- tains of southern California. These species occupy different, but overlapping, elevational ranges, with S. graciosus occurring at higher elevations. Because their ranges differ, the species experience different thermal environments; however, they are nearly identical in several measures of thermal physiology. Body temperatures of active lizards varied little with altitude, although air temperatures decrease substantially over this range. Both species compensated behaviorally for variation in thermal environments by varying basking frequency and microhabitat use. By ther- moregulating, lizards maintained body temperatures that favor high levels of locomotor performance and other physiological functions. Microhabitat use varied markedly with elevation. Sceloporus occidentalis was almost completely arboreal at low elevation; lizards of both species were partly arboreal, partly terrestrial at intermediate elevation; and S. graciosus was mainly terrestrial at high ele- vations. Thus, the species converged in microhabitat use where sympatric. These shifts in microhabitat use paralleled altitudinal changes in (i) the spatial location of thermally suit- able microhabitats and (ii) habitat structure. Behavioral thermoregulation, then, appears to be an important determinant of habitat use in these lizards. Results of this study may have important implications for lizard community ecology. Microhabitats that are otherwise appropriate for lizards can be thermally unsuitable, de- pending on the local thermal environment. Hence, the thermal environment and lizard thermal biology constrain patterns of structural habitat use. As a result, physiological considerations could limit the behavioral and evolutionary options for species experiencing interspecific competition. For example, competitors in sympatry may be unable to evolve microhabitat resource partitioning because only a subset of microhabitats are thermally suitable.

Temperature-dependent sex determination (TSD) occurs in all major reptile lineages, but the selective forces and physiological mechanisms that link sex to incubation temperature may differ among and within those groups. Different models for TSD evolution make different predictions about when offspring sex will respond to environmental cues. Although TSD has evolved in several lizard lineages, there is less detailed information on these taxa than in turtles and crocodilians with TSD. We incubated eggs of an agamid lizard (Amphibolurus muricatus) and a scincid lizard (Bassiana duperreyi), two species with TSD. Rather than manipulate incubation temperature to identify periods of sexual lability (as in most previous studies of this topic), we topically applied the aromatase inhibitor fadrozole to eggs at a variety of times through the incubation period. Fadrozole application sex-reversed the resultant hatchlings if applied from the time of oviposition until at least 60% of the way through incubation. In all of the TSD lizard species studied so far, offspring sex is determined either while the eggs are held inside the mother's body or soon after oviposition, providing substantial maternal control over incubation temperatures at this critical period. Hence, the hypothesis that TSD evolves because it enables offspring sex to be matched to conditions that are unpredictable at the time of laying is less likely to apply to squamates than to turtles, sphenodontians, and (especially) crocodiles, in which the period of sexual lability is delayed until long after oviposition.

Life is defined by a capacity for reproduction, yet the ways in which animals reproduce vary tremendously among species. Reproductive life histories are complex phenomena influenced by a variety of factors, such as physical condition of individuals, food supply, bauplan, and phylogenetic history, which are often correlated. Understanding life histories is crucial in evolutionary ecology because they represent different strategies that evolved to maximize individual fitness. Variation in life history can be attributed to both historical (=phylogenetic) and non-historical (=environmental) causes. We compiled a large data set on lizard life history variables. Our data set consists of life history data for 737 lizard populations, representing 337 species in 33 families from 280 study sites globally. About 64% of these data were collected directly by the authors. Regarding data collected by authors, we sexed lizards by dissection and direct examination of gonads. Females were considered reproductive if vitellogenic follicles or oviductal eggs were present. We regarded the simultaneous presence of enlarged vitellogenic follicles and either oviductal eggs or corpora lutea as evidence for the sequential production of more than one clutch of eggs per year. We considered clutch size as the number of vitellogenic follicles or oviductal eggs in mature females. For each population, we recorded the following variables, if available: adult female mass (g), adult female SVL (mm), female SVL at maturity (based on SVL of smallest reproductive female), offspring SVL (based on hatchling size or smallest individual in population), clutch or litter size (number of offspring per clutch or litter for all reproductive females in the population), total number of clutches or litters per year, clutch frequency (single- or multiple-brooded), relative clutch or litter mass [total volume of eggs or embryos (cc) divided by adult female mass (g)], reproductive mode (oviparous or viviparous), foraging mode (sit-and-wait or active), distribution (tropical or temperate), and preferred habitat type (aquatic, arboreal, bromelicolous, fossorial, psammophilous, saxicolous, semi-arboreal, or terrestrial). These data are valuable to test various life history hypotheses, including phylogenetic design constraints and effects of ecology and climate on reproductive tactics.

The lacertid lizard Podarcis hispanica atrata maintained remarkably constant body temperatures (T-b) throughout its daily activity period in early autumn, despite significant variability in available operative temperatures (T-e). Lizard T(b)s were substantially closer to the species' selected temperature range (the target T(b)s for thermoregulation, estimated by the central 80% of the temperatures selected in a laboratory photothermal gradient) than were a random sample of T(e)s, indicating that this population thermoregulates both accurately and effectively. We compared observed behaviors and T(b)s to the predictions of two hypotheses. The ''no thermoregulation'' null hypothesis predicted that lizard activity and microsite use would be unaffected by thermal constraints. The ''only thermoregulation'' hypothesis predicted that lizards would be active only at those times and in those places where T(e)s fell within the selected temperature range. To evaluate the contributions of various behavioral adjustments (activity times, use of microhabitats and sun-shade patches, basking, and shuttling) to temperature regulation, we combined results from direct behavioral observations with information on the T(e)s available at different times of day and in different microsites (combinations of structural microhabitats and sun-shade patches). Lizard activity patterns did not match the predictions of either hypothesis, but the observed pattern of activity produced T(e)s that were, on average, 1.0 degrees C closer to the selected temperature range than were T(b)s predicted by the ''no thermoregulation'' hypothesis. The lizards' use of microsites (combinations of structural microhabitats and sun-shade patches) was more similar to predictions of the ''no thermoregulation'' hypothesis than to those of the ''only thermoregulation'' hypothesis. Nevertheless, the lizards' use of specific microsites produced T(b)s that were, on average, 1.9 degrees C closer to the selected temperature range than were randomly available T(e)s. The relative contributions of different behavioral mechanisms varied with time of day. In the early morning, low movement rates, selection of sunny microsites, and most notably, adoption of a basking posture contributed to temperature regulation, In the late morning and early afternoon, no thermoregulatory behaviors were necessary, as indicated by randomly positioned models that achieved mean T(e)s within the selected temperature range. At midday, frequent movements between sunny and shaded microsites probably reduced heat loads on lizards, preventing T(b)s from exceeding the selected temperature range. In the late afternoon, nonrandom use of microsites and basking behavior appeared to contribute equally to the elevation of T(b)s above randomly sampled T(e)s.

Thermal constraint on energy assimilation is an important source of life history variation in geographically widespread ectotherms such as the eastern fence lizard (Sceloporus undulatus). Fence lizards in southern populations grow faster and produce more offspring per year than do those in northern populations. Biophysical models indicate that this difference in production is the result of thermal constraints on energy assimilation, but they do not exclude intraspecific variation in behavior or physiology. I quantified both thermoregulatory behavior and the thermal sensitivity of metabolizable energy intake (MEI) in lizards from New Jersey (NJ) and South Carolina (SC) populations of Sceloporus undulatus. In the laboratory, I conducted feeding trials to estimate MEI at body temperatures experienced by field-active lizards (20°, 30°, 33°, and 36°C). I also measured preferred body temperature (T p ) of lizards in a thermal gradient. In the field, I estimated the accuracy of thermoregulation by lizards. Both NJ and SC lizards exhibited a maximal MEI at their T p (33°C), but lizards from SC had a significantly higher MEI at this temperature than lizards from NJ. Although lizards in both populations thermoregulated within 2°C of T p , lizards in SC could maintain T p for a longer duration on a daily and annual basis. Therefore, lizards in SC could assimilate more energy because they had a higher maximal MEI during activity and were active for longer durations than lizards in NJ. Geographic variation in the life history of S. undulatus may be caused by differentiation of physiology between populations, as well as by differences in the thermal environments of populations.