Paul E. Hertz

Why tropical forest lizards are vulnerable to climate warming

Biological impacts of climate warming are predicted to increase with latitude, paralleling increases in warming. However, the magnitude of impacts depends not only on the degree of warming but also on the number of species at risk, their physiological sensitivity to warming and their options for behavioural and physiological compensation. Lizards are useful for evaluating risks of warming because their thermal biology is well studied. We conducted macrophysiological analyses of diurnal lizards from diverse latitudes plus focal species analyses of Puerto Rican Anolis and Sphaerodactyus. Although tropical lowland lizards live in environments that are warm all year, macrophysiological analyses indicate that some tropical lineages (thermoconformers that live in forests) are active at low body temperature and are intolerant of warm temperatures. Focal species analyses show that some tropical forest lizards were already experiencing stressful body temperatures in summer when studied several decades ago. Simulations suggest that warming will not only further depress their physiological performance in summer, but will also enable warm-adapted, open-habitat competitors and predators to invade forests. Forest lizards are key components of tropical ecosystems, but appear vulnerable to the cascading physiological and ecological effects of climate warming, even though rates of tropical warming may be relatively low.

Behavioral Drive versus Behavioral Inertia in Evolution: A Null Model Approach

Some biologists embrace the classical view that changes in behavior inevitably initiate or drive evolutionary changes in other traits, yet others note that behavior sometimes inhibits evolutionary changes. Here we develop a null model that quantifies the impact of regulatory behaviors (specifically, thermoregulatory behaviors) on body temperature and on performance of ectotherms. We apply the model to data on a lizard (Anolis cristatellus) and show that thermoregulatory behaviors likely inhibit selection for evolutionary shifts in thermal physiology with altitude. Because behavioral adjustments are commonly used by ectotherms to regulate physiological performance, regulatory behaviors should generally constrain rather than drive evolution, a phenomenon we call the “Bogert effect.” We briefly review a few other examples that contradict the classical view of behavior as the inevitable driving force in evolution. Overall, our analysis and brief review challenge the classical view that behavior is invariably the driving force in evolution, and instead our work supports the alternative view that behavior has diverse—and sometimes conflicting—effects on the directions and rates at which other traits evolve.

Niche lability in the evolution of a Caribbean lizard community

Niche conservatism—the tendency for closely related species to be ecologically similar—is widespread. However, most studies compare closely related taxa that occur in allopatry; in sympatry, the stabilizing forces that promote niche conservatism, and thus inhibit niche shifts, may be countered by natural selection favouring ecological divergence to minimize the intensity of interspecific interactions. Consequently, the relative importance of niche conservatism versus niche divergence in determining community structure has received little attention. Here, we examine a tropical lizard community in which species have a long evolutionary history of ecological interaction. We find that evolutionary divergence overcomes niche conservatism: closely related species are no more ecologically similar than expected by random divergence and some distantly related species are ecologically similar, leading to a community in which the relationship between ecological similarity and phylogenetic relatedness is very weak. Despite this lack of niche conservatism, the ecological structuring of the community has a phylogenetic component: niche complementarity only occurs among distantly related species, which suggests that the strength of ecological interactions among species may be related to phylogeny, but it is not necessarily the most closely related species that interact most strongly.

FIGHT VERSUS FLIGHT: BODY TEMPERATURE INFLUENCES DEFENSIVE RESPONSES OF LIZARDS

Abstract In laboratory studies we determined that the defensive responses used by two agamid lizards, Agama savignyi and A. pallida, change as a function of body temperature. At high body temperatures, these lizards flee rapidly from predators. At lower body temperatures, which reduce sprint speed, the lizards rarely run but instead hold their ground and attack aggressively. This temperature-dependent switch in defensive behaviour may have evolved because cold lizards that live in open habitats would have little chance of outrunning predators. Defensive behaviours of animals may in general be sensitive to physiological variables that influence locomotor performance.

TEMPERATURE REGULATION IN PUERTO RICAN ANOLIS LIZARDS: A FIELD TEST USING NULL HYPOTHESES'

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.

COMPENSATION FOR ALTITUDINAL CHANGES IN THE THERMAL ENVIRONMENT BY SOME ANOLIS LIZARDS ON HISPANIOLA

Populations of the Anolis cybotes species group on Hispaniola behaviorally compen- sate for changes in the thermal environment associated with altitude. Lizards occupy more open habitats, increase basking frequency, and restrict times of activity at high elevation: a result is that the change in mean cloacal temperature with elevation is very small relative to that known for other anoles. In contrast to this large behavioral variation among populations, physiological differences appear to be minor. In this regard, the evbotes group has several parallels with the well-studied cristatellus group of Anolis on Puerto Rico. However, the cybotes group has no equivalent to Anolis gundlcahi, which employs physiological adaptations to compensate for different thermal environ- ments. Kev ivords: altitudinal variation; Anolis lizards; basking; behavioral thermoregulation; body temperature; critical thermal maximum; habitat shift; Hispaniola.

Asynchronous evolution of physiology and morphology in Anolis lizards.

Species‐rich adaptive radiations typically diversify along several distinct ecological axes, each characterized by morphological, physiological, and behavioral adaptations. We test here whether different types of adaptive traits share similar patterns of evolution within a radiation by investigating patterns of evolution of morphological traits associated with microhabitat specialization and of physiological traits associated with thermal biology in Anolis lizards. Previous studies of anoles suggest that close relatives share the same “structural niche” (i.e., use the same types of perches) and are similar in body size and shape, but live in different “climatic niches” (i.e., use habitats with different insolation and temperature profiles). Because morphology is closely tied to structural niche and field active body temperatures are tied to climatic niches in Anolis, we expected phylogenetic analyses to show that morphology is more evolutionarily conservative than thermal physiology. In support of this hypothesis, we find (1) that thermal biology exhibits more divergence among recently diverged Anolis taxa than does morphology; and (2) diversification of thermal biology among all species often follows diversification in morphology. These conclusions are remarkably consistent with predictions made by anole biologists in the 1960s and 1970s.

The Anoles of Soroa: Aspects of Their Ecological Relationships

Abstract Most lizard communities are characterized by having one or two dominant species and a handful of other species that occur at relatively low densities. However, Soroa, a site in the Sierra del Rosario of western Cuba, is home to 11 sympatric species of Anolis, of which nine are found in high abundance. In this study, we evaluate how interspecific differences in structural niche, thermal niche, body size, and behavior might allow the extraordinarily high anole species diversity at this site. We found that all pairs of species differ in at least one of the following niche axes: vegetation types occupied, substrates used, perch height, irradiance at occupied perch sites, and body temperature. Interspecific differences across these axes might serve to reduce competition, allowing the 11 species to live sympatrically within a relatively small geographic area.

Principles and Practices Fostering Inclusive Excellence: Lessons from the Howard Hughes Medical Institute’s Capstone Institutions

This paper describes common elements and assessments of persistence programs at 11 Capstone institutions designated by the Howard Hughes Medical Institute and complements the resources on our companion website. Together, the paper and website provide detail and synthesize principles informed by our experiences in pursuit of inclusive excellence.