Alex R. Gunderson

Plasticity in thermal tolerance has limited potential to buffer ectotherms from global warming

Global warming is increasing the overheating risk for many organisms, though the potential for plasticity in thermal tolerance to mitigate this risk is largely unknown. In part, this shortcoming stems from a lack of knowledge about global and taxonomic patterns of variation in tolerance plasticity. To address this critical issue, we test leading hypotheses for broad-scale variation in ectotherm tolerance plasticity using a dataset that includes vertebrate and invertebrate taxa from terrestrial, freshwater and marine habitats. Contrary to expectation, plasticity in heat tolerance was unrelated to latitude or thermal seasonality. However, plasticity in cold tolerance is associated with thermal seasonality in some habitat types. In addition, aquatic taxa have approximately twice the plasticity of terrestrial taxa. Based on the observed patterns of variation in tolerance plasticity, we propose that limited potential for behavioural plasticity (i.e. behavioural thermoregulation) favours the evolution of greater plasticity in physiological traits, consistent with the ‘Bogert effect’. Finally, we find that all ectotherms have relatively low acclimation in thermal tolerance and demonstrate that overheating risk will be minimally reduced by acclimation in even the most plastic groups. Our analysis indicates that behavioural and evolutionary mechanisms will be critical in allowing ectotherms to buffer themselves from extreme temperatures.

Feather-Degrading Bacteria: A New Frontier in Avian and Host–Parasite Research?

Birds are important models for the study of host–parasite interactions (Loye and Zuk 1991, Clayton and Moore 1997). Much of this research has focused on arthropod ectoparasites that feed on feathers (e.g., Clayton et al. 2003, Proctor 2003), because feathers are so important to avian life-history traits. Feathers function in thermoregulation (Stettenheim 2000), communication (Andersson 1994, Shuster and Wade 2003), and flight (Rayner 1988). Damaged feathers have reduced abilities to perform these functions (Booth et al. 1993, Swaddle and Witter 1997, Ferns and Lang 2003, Williams and Swaddle 2003), so there are likely fitness consequences for individuals possessing damaged feathers. A subset of plumage bacteria that can degrade feathers has garnered interest, because it may impose significant evolutionary selection pressures on birds, as arthropod ectoparasites do. Aspects of avian morphology, behavior, and life history may be influenced by a coevolutionary battle between birds and feather-degrading bacteria (FDB) that damage their plumage. Research on FDB and birds is in its nascent stages; however, a substantial body of literature has attempted to understand how birds and these microbes interact. Here, I synthesize what we currently know, highlight important gaps in our knowledge, and suggest next steps for the field, while focusing on three fundamental questions: What are FDB and how do they degrade feathers? How prevalent are FDB on birds? And finally, how can FDB and birds influence one another?

Rapid change in the thermal tolerance of a tropical lizard.

The predominant view is that the thermal physiology of tropical ectotherms, including lizards, is not labile over ecological timescales. We used the recent introduction (∼35 years ago) of the Puerto Rican lizard Anolis cristatellus to Miami, Florida, to test this thermal rigidity hypothesis. We measured lower (critical thermal minimum [CTmin]) and upper (critical thermal maximum [CTmax]) thermal tolerances and found that the introduced population tolerates significantly colder temperatures (by ∼3°C) than does the Puerto Rican source population; however, CTmax did not differ. These results mirror the thermal regimes experienced by each population: Miami reaches colder ambient temperatures than Puerto Rico, but maximum ambient temperatures are similar. The differences in CTmin were observed even though lizards from both sites experienced nearly identical conditions for 49 days before CTmin measurement. Our results demonstrate that changes in thermal tolerance occurred relatively rapidly (∼35 generations), which strongly suggests that the thermal physiology of tropical lizards is more labile than previously proposed.

Biological Impacts of Thermal Extremes: Mechanisms and Costs of Functional Responses Matter

Thermal performance curves enable physiological constraints to be incorporated in predictions of biological responses to shifts in mean temperature. But do thermal performance curves adequately capture the biological impacts of thermal extremes? Organisms incur physiological damage during exposure to extremes, and also mount active compensatory responses leading to acclimatization, both of which alter thermal performance curves and determine the impact that current and future extremes have on organismal performance and fitness. Thus, these sub-lethal responses to extreme temperatures potentially shape evolution of thermal performance curves. We applied a quantitative genetic model and found that beneficial acclimatization and cumulative damage alter the extent to which thermal performance curves evolve in response to thermal extremes. The impacts of extremes on the evolution of thermal performance curves are reduced if extremes cause substantial mortality or otherwise reduce fitness differences among individuals. Further empirical research will be required to understand how responses to extremes aggregate through time and vary across life stages and processes. Such research will enable incorporating passive and active responses to sub-lethal stress when predicting the impacts of thermal extremes.

Patterns of Thermal Constraint on Ectotherm Activity

Thermal activity constraints play a major role in many aspects of ectotherm ecology, including vulnerability to climate change. Therefore, there is strong interest in developing general models of the temperature dependence of activity. Several models have been put forth (explicitly or implicitly) to describe such constraints; nonetheless, tests of the predictive abilities of these models are lacking. In addition, most models consider activity as a threshold trait instead of considering continuous changes in the vigor of activity among individuals. Using field data for a tropical lizard (Anolis cristatellus) and simulations parameterized by our observations, we determine how well various threshold and continuous-activity models match observed activity patterns. No models accurately predicted activity under all of the thermal conditions that we considered. In addition, simulations showed that the performance of threshold models decreased as temperatures increased, which is a troubling finding given the threat of global climate change. We also find that activity rates are more sensitive to temperature than are the physiological traits often used as a proxy for fitness. We present a model of thermal constraint on activity that integrates aspects of both the threshold model and the continuous-activity model, the general features of which are supported by activity data from other species. Overall, our results demonstrate that greater attention should be given to fine-scale patterns of thermal constraint on activity.

Tests of the contribution of acclimation to geographic variation in water loss rates of the West Indian lizard Anolis cristatellus

Phenotypic plasticity can contribute to the process of adaptive radiation by facilitating population persistence in novel environments. West Indian Anolis lizards provide a classic example of an adaptive radiation, in which divergence has occurred along two primary ecological axes: structural microhabitat and climate. Adaptive plasticity in limb morphology is hypothesized to have facilitated divergence along the structural niche axis in Anolis, but very little work has explored plasticity in physiological traits. Here, we experimentally ask whether Puerto Rican Anolis cristatellus from mesic and xeric habitats differ in desiccation rates, and whether these lizards exhibit an acclimation response to changes in relative humidity. We first present microclimatic data collected at lizard perch sites that demonstrate that abiotic conditions experienced by lizards differ between mesic and xeric habitat types. In Experiment 1, we measured desiccation rates of lizards from both habitats maintained under identical laboratory conditions. This experiment demonstrated that desiccation rates differ between populations; xeric lizards lose water more slowly than mesic lizards. In Experiment 2, lizards from each habitat were either maintained under the conditions of Experiment 1, or under extremely low relative humidity. Desiccation rates did not differ between lizards from the same habitat maintained under different treatments and xeric lizards maintained lower desiccation rates than mesic lizards within each treatment. Our results demonstrate that A. cristatellus does not exhibit an acclimation response to abrupt changes of hydric conditions, and suggest that tropical Anolis lizards might be unable to exhibit physiological plasticity in desiccation rates in response to varying climatic conditions.

Reproductive Success of Eastern Bluebirds (Siala sialis) on Suburban Golf Courses

ABSTRACT. Understanding the role of green space in urban—suburban landscapes is becoming critical for bird conservation because of rampant habitat loss and conversion. Although not natural habitat, golf courses could play a role in bird conservation if they support breeding populations of some native species, yet scientists remain skeptical. In 2003–2009, we measured reproduction of Eastern Bluebirds (Siala sialis) in Virginia on golf courses and surrounding reference habitats, of the type that would have been present had golf courses not been developed on these sites (e.g., recreational parks, cemeteries, agriculture land, and college campus). We monitored >650 nest boxes and 2,255 nest attempts (n = 1,363 golf course, n = 892 reference site). We used an information-theoretic modeling approach to evaluate whether conditions on golf courses affected timing of breeding, investment, or nest productivity compared with nearby reference sites. We found that Eastern Bluebirds breeding on golf courses reproduced as well as those breeding in other disturbed habitats. Habitat type had no effect on initial reproductive investment, including date of clutch initiation or clutch size ( = 4 eggs). During incubation and hatching, eggs in nests on golf courses had higher hatching rates (80%) and brood sizes ( = 4.0 nestlings brood-1) than nests on reference sites (75% hatching rate; = 3.8 nestlings brood-1). Mortality of older nestlings was also lower on golf courses and, on average, golf course nests produced 0.3 more fledglings than nests on reference sites. Thus, within a matrix of human-dominated habitats, golf courses may support productive populations of some avian species that can tolerate moderate levels of disturbance, like Eastern Bluebirds.

Maltreated Nestlings Exhibit Correlated Maltreatment as Adults: Evidence of a “Cycle of Violence” in Nazca Boobies (Sula granti)

ABSTRACT. The “cycle of violence” hypothesis implicates child abuse as a cause of later violent behavior via social transmission between generations. It has received mixed support from human research and has prompted the study of nonhuman models with comparable abuse behaviors. The underlying biology of child abuse remains a controversial subject, perhaps partly because in nonhuman animals similar behavior occurs relatively rarely in wild populations. The Nazca Booby (Sula granti), a colonial seabird, provides a nonhuman model in which maltreatment of nonfamilial young is widespread under normal living conditions. Essentially all adults show social attraction at some point in their lives to the offspring of other parents, often with a sexual and/or aggressive motivation. Here, we show a correlation between the degree to which a young bird is targeted by such adults and its own infliction of maltreatment later in life. The results provide the first evidence from a nonhuman of socially transmitted maltreatment directed toward unrelated young in the wild.

Estimating the benefits of plasticity in ectotherm heat tolerance under natural thermal variability

Summary Plasticity is a near-ubiquitous feature of the thermal physiology of ectothermic organisms. Understanding the significance of plasticity in evolutionary and ecological contexts requires determining whether observed plasticity is beneficial, and, if so, to what extent plasticity can compensate for environmental change (i.e. is plasticity ‘complete’ or ‘incomplete’). Using site-specific daily temperature records spanning several decades, we test the ‘beneficial acclimation hypothesis’ by calculating the number of days predicted to exceed the heat tolerance limits of 103 terrestrial ectotherm populations when incorporating observed taxon-specific plasticity and when plasticity is ignored. We find that heat tolerance plasticity can reduce the predicted number of overheating events, although the effects are clade specific with plasticity providing greater benefits to amphibians than to reptiles or arthropods. Despite these benefits, heat tolerance plasticity is incomplete; as temperatures rise, plasticity cannot prevent an increase in the risk of overheating. Specifically, we find that a one degree increase in mean maximum temperature leads to approximately one additional day of predicted overheating during the warmest months of the year, on average. We conducted a broad-scale analysis of the degree to which plasticity in heat tolerance reduces the probability that ectotherms experience temperatures beyond their physiological heat limits. Our results indicate that plasticity in heat tolerance benefits ectotherms, but, consistent with previous studies, those benefits are inadequate to provide complete compensation for global climate change.

An affinity for biochemical adaptation to temperature.

![Figure][1] Alex Gunderson and Jonathon Stillman discuss Peter Hochachka and George Someros classic paper ‘The adaptation of enzymes to temperature’, published in Comparative Biochemistry and Physiology in 1968. If you have had the pleasure of diving under sea ice in Antarctica, you