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 somat ic systems and the end of the average adult life span (Fig. 1).(1-3) So def ined, menopause occurs only in humans and one species of toothed whales.(1- 4) According to evolutionary theories of senescence, there should be no sel ection for postreproductive individuals.(5) Thus, evolutionary biologists a nd anthropologists have long been interested in why human females have meno pause. Many have suggested that menopause is a hominine adaptation, the res ult 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 o f a physiological trade-off favoring efficient reproduction early in the fe rtile part of life or simply the by-product of increases in life span or li fe expectancies.(10-17) Menopause poses two separate questions: why it orig inated and what is maintaining it?