Habitat constrains and shapes successful ecological and physiological strategies and thus provides the context for the evolution of life-history traits. The stress axis plays a vital role in the endocrine system and is a critical component adapting mammals to particular habitat pressures. It is subject to both individual activational and organizational plasticity as well as to evolutionary modification. To illustrate, I examine the suite of traits of the stress axis associated with breeding frequency in male mammals, which varies in a continuum from semelparity to iteroparity. During the breeding season, males in species at the semelparous end of the continuum exhibit high concentrations of free corticosteroids, low concentrations of glucocorticoid-binding protein, a failure of the negative feedback system, a gonadal axis that is not inhibited by high corticosteroid concentrations, and immunosuppression. Iteroparous species exhibit the opposite traits. The evolutionary constraints selecting for the former may partially be related to phylogeny (in marsupials) as well as to an interaction of the restrictions imposed by the environment on female reproduction, the mating system, the high costs of reproduction, and the low adult survival during the nonbreeding season.