Humans are not the only animals to undergo reproductive cessation. There are several species that go through menopause including non-human primates, whales, rodents, dogs, rabbits, elephants and domestic livestock. We are unique in that we undergo menopause at a relatively early age compared with other species. A long post-reproductive lifespan presents an evolutionary puzzle, however, because theory suggests that there should be no selection for genes which promote survival past the end of reproduction. So, why do humans go through menopause so early?
One argument, called the “Grandmother Hypothesis” suggests that the presence of menopause at such a young age has to do with the benefit of extended familial groups in which the continued presence of the older female contributes to the survival of young even though they themselves can no longer reproduce. (See Hawkes et al. 1998 or this article in the New York Times for a basic discussion of the theory).
In hunter-gatherer cultures today, said Dr. Hawkes, “women are strong and economically productive into their 60s….Women are not being helped along by others. The flow of help is going into the other direction.”
Research on the Grandmother Hypothesis was bolstered by a 2004 study published in Nature by Lahdenpera et al. which identified that children are 12% more likely to survive to adulthood when they have a grandmother’s support than when they don’t. Recently, Cant and Johnstone (2007) extended the life of the “Grandmother Hypothesis” by proposing that the timing of reproductive cessation in humans is best understood as an evolutionary adaptation to reduce reproductive competition between generations of females in the same family unit. Shanley et al. (2007) suggest that it is necessary to meet multiple conditions in order to significantly enhance fitness.
For menopause to be adaptive there must be a combination of factors, and we have shown that the most important are a dramatic increase in maternal mortality with age and a benefit to grandchildren provided by the maternal grandmother. (p. 2949).
However, earlier menopause has also been observed among animals in which females provide very little or no reproductive care, such as this case in female guppies, leading many to debate whether this hypothesis is accurate for understanding the early cessation of reproduction among humans. In 1998, Packer et al. argued against the Grandmother Hypothesis using data from Tanzanian lions and baboons. More recently, Holmes (2008) argues that midlife fertility loss is not unique to humans, but represents a canonical reproductive aging pattern shared by other female vertebrates when a high proportion of healthy, aged individuals are released from selection for continued reproduction. Others like Marlowe (2000) have argued that maybe it’s not about mothers at all, and that once males became capable of maintaining high status and reproductive access beyond their peak physical condition, selection favored the extension of maximum life span in males and early menopause on females is a byproduct of the loss of oocytes combined with increased lifespan.
The debate is complicated by the fact that researchers aren’t always arguing over the same issue. Some researchers are trying to understand why menopause is only present among certain species, while others like Hawkes et al., Shanley et al., Marlowe, and Holmes are trying to understand why menopause occurs at such a young age among humans. Clearly more work is necessary…
