Comparative studies of aging are a promising approach to identifying general properties of and processes leading to aging. While to date, many comparative studies of aging in animals have focused on relatively narrow species groups, methodological innovations now allow for studies that include evolutionary distant species. However, comparative studies of aging across a wide range of species that have distinct life histories introduce additional challenges in experimental design. Here, we discuss these challenges, highlight the most pressing problems that need to be solved, and provide suggestions based on current approaches to successfully carry out comparative aging studies across the animal kingdom.
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Abstract Temperature affects nearly every aspect of how organisms interact with and are constrained by their environment. Measures of organismal energetics, such as metabolic rate, are highly temperature‐dependent and governed through temperature effects on rates of biochemical reactions. Characterizing the relationships among levels of biological organization can lend insight into how temperature affects whole‐organism function. We tested the temperature dependence of cellular oxygen consumption and its relationship to whole‐animal metabolic rate in garter snakes (
Thamnophis elegans ). Additionally, we tested whether thermal responses were linked to shifts in the fuel source oxidized to support metabolism with the use of carbon stable isotopes. Our results demonstrate temperature dependence of metabolic rates across levels of biological organization. Cellular (basal, adenosine triphosphate‐linked) and whole‐animal rates of respiration increased with temperature but were not correlated within or among individuals, suggesting that variation in whole‐animal metabolic rates is not due simply to variation at the cellular level, but rather other interacting factors across scales of biological organization. Counter to trends observed during fasting, elevated temperature did not alter fuel selection (i.e., natural‐abundance stable carbon isotope composition in breath, δ13Cbreath). This consistency suggests the maintenance and oxidation of a single fuel source supporting metabolism across a broad range of metabolic demands. -
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Abstract An understudied aspect of vertebrate ecoimmunology has been the relative contributions of environmental factors (E), genetic background (G) and their interaction (G × E) in shaping immune development and function. Environmental temperature is known to affect many aspects of immune function and alterations in temperature regimes have been implicated in emergent disease outbreaks, making it a critical environmental factor to study in the context of immune phenotype determinants of wild animals.
We assessed the relative influences of environmental temperature, genetic background and their interaction on first‐year development of innate and adaptive immune defences of captive‐born garter snakes
Thamnophis elegans using a reciprocal transplant laboratory experiment. We used a full‐factorial design with snakes from two divergent life‐history ecotypes, which are known to differ in immune function in their native habitats, raised under conditions mimicking the natural thermal regime—that is, warmer and cooler—of each habitat.Genetic background (ecotype) and thermal regime influenced innate and adaptive immune parameters of snakes, but in an immune‐component specific manner. We found some evidence of G × E interactions but no indication of adaptive plasticity with respect to thermal environment. At the individual level, the effects of thermal environment on resource allocation decisions varied between the fast‐ and the slow‐paced life‐history ecotypes. Under warmer conditions, which increased food consumption of individuals in both ecotypes, the former invested mostly in growth, whereas the latter invested more evenly between growth and immune development.
Overall, immune parameters were highly flexible, but results suggest that other environmental factors are likely more important than temperature per se in driving the ecotype differences in immunity previously documented in the snakes under field conditions. Our results also add to the understanding of investment in immune development and growth during early postnatal life under different thermal environments. Our finding of immune‐component specific patterns strongly cautions against oversimplification of the highly complex immune system in ecoimmunological studies. In conjunction, these results deepen our understanding of the degree of immunological flexibility wild animals present, information that is ever more vital in the context of rapid global environmental change.
