Sex Question Friday: Why Does Menopause Exist And Is It Unique To Humans?

Every Friday on the blog, I answer people’s questions about sex, love, and relationships. This week’s question comes from a reader who wanted to know more about the topic of menopause:

“Are humans the only species in which the females experience menopause? Why does menopause exist?”

Thanks for these great questions! As it turns out, human females are not unique in having what some scientists term a “post-reproductive lifespan” (or PRLS for short). In fact, studies have found that so many primate and non-primate species show evidence of a PRLS that it can be considered "a general mammalian trait" [1]. The thing that is unique about humans is the length of women’s PRLS. In terms of both the absolute number of years spent in menopause and the percentage of the lifespan this accounts for, humans are almost unparalleled. Indeed, women can spend upwards of one-third of their lives in this post-reproductive stage. There are certain types of whales (e.g., the short-finned pilot whale) that have a rather lengthy PRLS too, but there are few other species that even come close to us.

So why does menopause account for such a large proportion of women’s lives? Scientists do not know the answer for sure, but they have offered a few theories suggesting that menopause might have evolved for a reason in humans.

One theory stems from the fact that a woman’s risk of dying from childbirth complications increases with age [2]. As a result, menopause might have evolved as a means of protecting women from those health risks and also protecting any young, dependent children they might have from losing a major caregiver. The thought is that, at some age, the increased health and mortality risks associated with pregnancy and childbirth begin to outweigh the benefits of continued reproduction.

An alternative theory is that menopause might have evolved because grandmothers who are past their reproductive age will redirect their caregiving efforts to their grandchildren, thus helping to ensure survival of their offspring [2]. The basic idea is that because human infants are completely dependent upon their caregivers for so many years, having other family around to share childcare responsibilities would be quite advantageous. This idea has been referred to as “the grandmother hypothesis.”

Of course, it is possible that both of these explanations may be viable (indeed, some research has found support for both [2]), but others are certainly possible too. However, the reality is that this is one of those questions about human sexuality that will likely always be somewhat of a mystery.

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[1] Cohen, A. A. (2004). Female post-reproductive lifespan: A general mammalian traitBiological Reviews79, 733-750.

[2] Shanley, D. P., Sear, R., Mace, R., & Kirkwood, T. B. (2007). Testing evolutionary theories of menopauseProceedings of the Royal Society B: Biological Sciences274, 2943-2949.

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