Summary

Human females (Homo sapiens) exhibit a dramatic form of reproductive skew in which half the age classes of adults contain only nonbreeders. Among other mammals, only pilot (Globicephala spp.) and killer whales (Orcinus orca) exhibit a similar pattern. The “grandmother” hypothesis suggests that selection can favor post-reproductive survival because older females help their offspring to reproduce. But the indirect fitness gains of helping appear insufficient to outweigh the potential benefits of continued direct reproduction, so this hypothesis cannot explain why women cease reproducing in the first place. Here we present some background on menopause and describe new research which helps to understand both the strange taxonomic distribution of menopause and the timing of reproductive cessation in humans. Specifically, recent models have explored the potential reproductive conflicts that may have arisen in ancestral human families, and the influence of demography on the resolution of such conflicts. These studies suggest that an integrated model which takes into account the potential costs of reproductive competition, as well as the benefits of helping, offers a fuller understanding of the evolution of menopause.

Reproductive skew in human societies

Human societies are characterized by a dramatic and puzzling pattern of reproductive skew. In populations exposed to natural schedules of mortality and fertility (i.e. without access to modern medicine and technology), almost half the age classes of adult human females contain only nonbreeders (Figure 2.1).