Senescence and the stress axis in male ground squirrels
Description
A critical time in the life of a male occurs at reproduction when his behavior, physiology, and resources must be brought to bear for the central purpose of his life. We ask whether reproduction results in dysfunction of the stress axis, is linked to life history, and causes senescence. We assessed if deterioration in the axis underlies variation in reproductive lifespan in males of 5 species of North American ground squirrels whose life history varies from near semelparity to iteroparity. The most stressful and energy-demanding time occurs in spring during the intense 2–3 week breeding competition just after arousal from hibernation. We compared their stress axis functioning before and after the mating period using a hormonal challenge protocol. We found no evidence of stress axis dysfunction nor was there a relationship between reproductive lifespan and stress axis functional deterioration. Moreover, there was no consistent relationship between free cortisol levels and downstream measures. Thus, stress axis function was not traded off to promote reproduction and conclude that it is a pre-requisite for life. Hence, it functions as a constraint and does not undergo senescence.
Notes
All data were analyzed using SAS 9.2 (SAS Institute Inc., Cary, NC, USA). For simple comparisons among species, we used one-way ANOVAs and the conservative Tukey-Kramer multiple comparison post-hoc test. For our regressions of postbreeding GC levels against mean reproductive lifespan and probability of surviving more than one breeding season (PROC GLM), we did not test for or control for phylogenetic non-independence. In part, our sample size precluded using phylogenetic contrasts: tests for phylogenetic independence perform poorly on small datasets and if our species were not independent, our sample size would be reduced by one in order to do the contrast. Our decision is further justified by the fact that we selected very closely related species, and because previous comparative studies of GC levels have found that much more diverse species groupings are nonetheless phylogenetically independent.
For the prebreeding and postbreeding comparisons, data were first examined for normality. Where significantly non-normal data could not be normalized with a transformation, we used the Wilcoxon-Mann-Whitney non-parametric two-sample test using the EXACT option to generate a Monte Carlo-based exact P-value. Because we released prebreeding Franklin's ground squirrels and some of our postbreeding samples came from recaptured individuals, we analyzed differences between pre- and postbreeding periods using a mixed model in SAS PROC MIXED, with subjects as a repeated measure. All other species comparisons used PROC TTEST; where pre- and postbreeding data had non-homogeneous variances based on the folded-F test, we use Satterthwaite-adjusted degrees of freedom for the t-tests (which can result in degrees of freedom that are not whole numbers). Unless otherwise specified, we present means with 95% confidence intervals.
Funding provided by: Natural Sciences and Engineering Research Council of Canada
Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100000038
Award Number: RGPIN 6169
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