Senescence and the stress axis in male ground squirrels

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 mea..., Study areas and field procedures 
We trapped prebreeding (within a few days of first emerging from hibernation) and postbreeding males (2–3 weeks later, same site). For the 3 western species (AGS, CGS, and RGS) logistics (i.e. ~2500 km between the AGS and RGS sites), asynchronous timing of emergence and reproduction, and a narrow window of reproduction within a species permitted us to trap only them in 2007 and the 2 eastern species (FGS, TLGS) in 2008.  In 2007, we trapped RGS at Kinsella, Alberta (53°N 111°33’W) on March 25–27 and April 15–16, AGS at the Pelly River Ranch, Yukon (62°50’N 137°18’W) on April 9–12 and April 27–30, and CGS at Kananaskis, Alberta (51°2’N 115°2’W) on April 22–24 and May 7–9, respectively. In 2008, we trapped TLGS on April 15–19 and May 5–11 and FGS on May 6–11 and May 28–30 near Portage La Prairie, Manitoba (TLGS at 50°7’N 98°23’W, FGS at 50°12’N 98°14’W). 
Animals were trapped by placing burrow traps or cage traps (Tomahawk #102, Tomahawk Live Trap Company..., Statistical analysis
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 ...