Menopause: Adaptation or epiphenomenon?

Menopause is a nonfacultative and irreversible cessation of fertility that occurs in all female conspecifics well before the senescence of other somatic systems and the end of the average adult life span (Fig. 1).(1-3) So defined, menopause occurs only in humans and one species of toothed whales.(1-4) According to evolutionary theories of senescence, there should be no selection for postreproductive individuals.(5) Thus, evolutionary biologists and anthropologists have long been interested in why human females have menopause. Many have suggested that menopause is a hominine adaptation, the result of selection for a postreproductive life span that permitted increased maternal investment in existing offspring.(3.6-9) Others are persuaded that premature reproductive senescence is an epiphenomenon, either the result of a physiological trade-off favoring efficient reproduction early in the fertile part of life or simply the by-product of increases in life span or life expectancies.(10-17) Menopause poses two separate questions: why it originated and what is maintaining it?