Sociality, Hierarchy, Health Comparative Biodemography: A Collection of Papers (2014) / Chapter Skim
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13 Biodemography of Ectothermic Tetrapods Provides Insights into the Evolution and Plasticity of Mortality Patterns--David A. W. Miller, Fredric J. Janzen, Gary M. Fellers, Patrick M. Kleeman, and Anne M. Bronikowski
13 Biodemography of Ectothermic Tetrapods Provides Insights into the Evolution and Plasticity of Mortality Patterns--David A. W. Miller, Fredric J. Janzen, Gary M. Fellers, Patrick M. Kleeman, and Anne M. Bronikowski
Pages 295-314
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From page 295... ...
, describing basic patterns and determining how early-life and late-life exposure to poor environmental conditions shape these patterns. Classic evolutionary theory of aging predicts that the declining power of natural selection with advancing age will mold the age-dependent trajectory of mortality for a species, or the rate of aging (Hamilton, 1966; Promislow and Bronikowski, 2006)
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. At the same time, empirically determined mortality trajectories, such as species- pecific s G ompertz curves, and notions that animals do not display significant s enescence in the wild have been challenged by larger datasets and more sophisticated analyses in many species, including humans (Brunet-Rossinni and Austad, 2006; Carey and Vaupel, 2006; Bronikowski et al., 2011; Jones et al., 2014)
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Studying factors related to cohort and annual effects in wild populations provides an opportunity to test the magnitude of environmental influences in shaping age-dependent mortality and thereby clarifies the nuances of evolutionary senescence theory. Natural selection over evolutionary time periods has led to vertebrate species that vary in lifespan by orders of magnitude (< 1yr – 100+ years)
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Then we examine how environmental variation influences age-dependent mortality, by focusing on how shared cohort and annual environmental conditions act and interact on mortality patterns. Our goal is to expand the comparative perspective into aging in nonmodel species.
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Our painted turtle data are from long-term studies of a nesting population of painted turtles on the Mississippi River between Iowa and Illinois (Whiteside County, Illinois)
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. We first estimated the age-specific mortality schedule from all individuals pooled within each population (i.e., one population of yellowlegged frogs, three populations of garter snakes, one population of painted turtles)
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We again were unable to examine sex-related differences in the painted turtles and garter snakes. Due to data constraints, we also did not consider sex-specific differences for yellow-legged frogs.
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. In addition, whereas long-lived garter snakes and yellowlegged frogs showed strong evidence of cohort and annual effects on mortality, painted turtles were more resilient and were seemingly buffered against years of poor cohort quality and environmental variance (see Figure 13-2)
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Lifespan values are calculated from the Gompertz parameters except in the case of short-lived snakes, for which we were unable to fit a Gompertz model and instead estimated mean lifespan from the known-age mark-recapture model. For population 1 of the long-lived snakes, a Makeham term improved the fit: 303 c = 0.23 with CI (0.15, 0.32)
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. The model with the strongest support for both painted turtles and shortlived garter snakes only included effects of age (Table 13-2, Figure 13-2)
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Unlike the long-lived g arter snakes, whether cohorts were of poor or good quality did not influence this pattern. Interestingly, the painted turtles had higher mortality in the first 2 years after maturation, decreasing in the next 2 years before starting to increase again in later life, suggestive of a signature of senescence even within these broad age-classes.
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Age (yrs.) FIGURE 13-2 Tests for effect of early life experience and environmental variation on late life mortality for short-lived and long-lived ecotypes of the western terrestrial garter snakes (Thamnophis elegans)
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. In the case of painted turtles, longterm data were only available for females, while for garter snakes, sex was unknown for the youngest age-classes and could only be determined for individuals recaptured at older ages.
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. The apparent buffering of female painted turtles against detrimental effects of early stress may be due to the consistency of food availability that characterizes their habitats and their resilience to extreme environmental perturbations such as flooding (Jergenson et al., 2014)
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ACKNOWLEDGMENTS The authors acknowledge the states of California and Illinois and Y osemite National Park for research permits during the course of this study. We also thank the various field crews who worked with us, the National Science Foundation for funding the painted turtle and garter snake research, and the USGS Amphibian Research and Monitoring Initiative for funding the Sierra Nevada yellow-legged frog research.
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. Survivorship and population densities of painted turtles (Chrysemys picta)
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. Slow mortality rate accelerations during aging in some animals approximate that of humans.
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. Stochastic population dynamics in populations of western terrestrial garter snakes with divergent life histories.
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. Pleiotropy, natural selection, and the evolution of senescence.
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