during one breeding season for species that live 1 yr (e.g.,
voles and mice), or over multiple breeding seasons for long-
lived species (e.g., marmots). We will examine the stress
axis in two wild rodents.
The meadow vole (Microtus pennsylvanicus)is a short-
lived iteroparous small rodent found throughout grass-
lands of central and northern North America. Meadow
voles have a promiscuous mating system, are short-lived
(1 year), and breed continuously (ca. 3 –5 litters per
summer) throughout the summer and often into winter;
young born early in the breeding season can mature and
breed in that season, and males are not territorial (see ref-
erences in Boonstra 1994). Free GC concentrations in breed-
ing males are usually low, with CBG concentrations always
exceeding total plasma corticosterone concentrations by
about 3 – 4 times (Boonstra and Boag 1992). In breeding
males, androgen concentrations are not correlated to CBG
concentrations.
The red squirrel (Tamiasciurus hudsonicus)is an arbo-
real tree squirrel whose distributional range covers the en-The Role of the Stress Axis in Life-History Adaptations 143Figure 12.3 Life history variation in breeding frequency in male squirrels from the Yukon and the associated
glucocorticoid plasma changes. The hormonal challenge protocol was used in both squirrels: Base levels indicate
initial values, Dex indicates values 2 h after the dexamethasone injection, and P30, P60, and P120 indicate values
30, 60, and 120 min after the ACTH (adrenocorticotropic hormone) injection. In arctic ground squirrels, breeding
(scrotal) males were trapped in May and nonbreeding (abdominal) males were adults trapped in August. In red
squirrels, breeding males were trapped in May. Means (but not standard errors — see original publications) are
presented. The data for arctic ground squirrels are from Boonstra et al. (2001) and for the red squirrel are from
Boonstra and McColl (2000). Corticosteroid binding globulin measured as maximum corticosteroid binding ca-
pacity (MCBC).