On the evolution, life history, and proximate mechanisms of human male reproductive senescence

Abstract

Reproductive senescence is a central and defining life-history characteristic of every known mammal. Within the scope of human senescence research, attention has been mainly focused on females, particularly in reference to the uniqueness of menopause. However, consideration of the evolution of human male reproductive senescence has been minimal, primarily due to the assumption that male fertility, as compared to that of females, is relatively invariant with age. Moreover, theoretical development of our understanding of human male reproductive senescence has not been extensive despite increasing awareness of the importance of life-history trade-offs in association with aging. Emerging research now illustrates important aspects of male reproductive senescence, exhibit significant variation and phenotypic plasticity, while others are less malleable. Changes in hormone-modulated somatic integrity with age also show important population differences, most likely as the result of reaction norms in response to environmental variation. Coupled with emerging ideas about the energetics of life-history trade-offs in human males, a new perspective is beginning to emerge. It suggests that human males exhibit potentially adaptive shifts in reproductive function in association with age.