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 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.