Michael W. Sears

Temperature, Growth Rate, and Body Size in Ectotherms: Fitting Pieces of a Life-History Puzzle

Abstract The majority of ectotherms grow slower but mature at a larger body size in colder environments. This phenomenon has puzzled biologists because classic theories of life-history evolution predict smaller sizes at maturity in environments that retard growth. During the last decade, intensive theoretical and empirical research has generated some plausible explanations based on nonadaptive or adaptive plasticity. Nonadaptive plasticity of body size is hypothesized to result from thermal constraints on cellular growth that cause smaller cells at higher temperatures, but the generality of this theory is poorly supported. Adaptive plasticity is hypothesized to result from greater benefits or lesser costs of delayed maturation in colder environments. These theories seem to apply well to some species but not others. Thus, no single theory has been able to explain the generality of temperature-size relationships in ectotherms. We recommend a multivariate theory that focuses on the coevolution of thermal reaction norms for growth rate and size at maturity. Such a theory should incorporate functional constraints on thermal reaction norms, as well as the natural covariation between temperature and other environmental variables.

Riding the wave: reconciling the roles of disease and climate change in amphibian declines.

We review the evidence for the role of climate change in triggering disease outbreaks of chytridiomycosis, an emerging infectious disease of amphibians. Both climatic anomalies and disease-related extirpations are recent phenomena, and effects of both are especially noticeable at high elevations in tropical areas, making it difficult to determine whether they are operating separately or synergistically. We compiled reports of amphibian declines from Lower Central America and Andean South America to create maps and statistical models to test our hypothesis of spatiotemporal spread of the pathogen Batrachochytrium dendrobatidis (Bd), and to update the elevational patterns of decline in frogs belonging to the genus Atelopus. We evaluated claims of climate change influencing the spread of Bd by including error into estimates of the relationship between air temperature and last year observed. Available data support the hypothesis of multiple introductions of this invasive pathogen into South America and subsequent spread along the primary Andean cordilleras. Additional analyses found no evidence to support the hypothesis that climate change has been driving outbreaks of amphibian chytridiomycosis, as has been posited in the climate-linked epidemic hypothesis. Future studies should increase retrospective surveys of museum specimens from throughout the Andes and should study the landscape genetics of Bd to map fine-scale patterns of geographic spread to identify transmission routes and processes.

Urban Physiology: City Ants Possess High Heat Tolerance

Urbanization has caused regional increases in temperature that exceed those measured on a global scale, leading to urban heat islands as much as 12°C hotter than their surroundings. Optimality models predict ectotherms in urban areas should tolerate heat better and cold worse than ectotherms in rural areas. We tested these predications by measuring heat and cold tolerances of leaf-cutter ants from South Americas largest city (São Paulo, Brazil). Specifically, we compared thermal tolerances of ants from inside and outside of the city. Knock-down resistance and chill-coma recovery were used as indicators of heat and cold tolerances, respectively. Ants from within the city took 20% longer to lose mobility at 42°C than ants from outside the city. Interestingly, greater heat tolerance came at no obvious expense of cold tolerance; hence, our observations only partially support current theory. Our results indicate that thermal tolerances of some organisms can respond to rapid changes in climate. Predictive models should account for acclimatory and evolutionary responses during climate change.

Evolution of Thermal Reaction Norms for Growth Rate and Body Size in Ectotherms: An Introduction to the Symposium

Although the size of an organism is influenced by many features of its environment, the relationship between temperature and body size has captivated generations of biologists. From the renowned observations by Bergmann (1847) to the phylogenetic comparative studies by our contemporaries (e.g., Ashton et al., 2000), evidence of a widespread relationship between temperature and adult body size has been offered repeatedly for various groups of animals. This relationship, which has come to be known as Bergmann’s rule, is an increase in the body size of a species with a decrease in environmental temperature. Bergmann’s rule was inspired initially by endotherms, but numerous species of ectotherms also exhibit Bergmann’s clines in body size (de Queiroz and Ashton, 2004). Furthermore, lab studies have shown that a higher temperature during ontogeny generally results in a smaller size at maturity (Atkinson, 1994, 1995). This thermal plasticity of size could be one of the most taxonomically widespread patterns of phenotypic variation, having been observed in bacteria, protists, plants and animals. Moreover, the correspondence between trends in artificial and natural environments suggests that thermal plasticity causes some of the geographic variation in body size within ectothermic species. Both proximate and ultimate explanations for temperature-size relationships have been proposed, debated, rejected and revised (Berrigan and Charnov, 1994; Sibly and Atkinson, 1994; Perrin, 1995; Sevenster, 1995; Atkinson, 1996; Atkinson and Sibly, 1996; van der Have and de Jong, 1996: Van Voorhies, 1996; Partridge and Coyne, 1997; Angilletta and Dunham, 2003; reviewed by Atkinson and Sibly, 1997). Still, there is little consensus on the key mechanisms that are responsible these relationships. For example, Van Voorhies (1996) proposed that Bergmann’s clines were caused by increasing cell size with decreasing developmental temperature, but Partridge and Coyne (1997) pointed out that larger bodies are not necessarily achieved by having larger cells. Attempts to secure an ultimate explanation for temperature-size relationships have met with similar difficulties; indeed, few models of life-history evolution predict relatively large sizes

MOISTENED SEEDS INCREASE RODENT TRAP SUCCESS

Abstract Seed moisture has been shown to influence the rates of seed cache removal by rodents. Although the precise mechanism is not known, this knowledge might prove useful in field applications. We examined whether moistened bait would increase trap success in desert rodent populations. We placed traps 15 m apart in grids within a 500-ha study area and randomly baited traps with either dry or moistened seeds. We found that traps baited with moistened seeds had 34.9% higher success than traps baited with dry seeds (n = 190, χ2 = 5.389, df = 1, P = 0.020). Our results suggest that application of water to dry seed bait can lead to increased trap success for desert rodents.