Panther power on the hour from the rebel to you
Why do humans menstruate, when most animals don’t? When you shake the tree of life, you find that only a handful of mammals aside from us – primates, a small number of bat species, and the elephant shrew – have opted for the monthly bleed.
Evolution is often viewed in terms of a cost-benefit ledger: if something is costly, it must have some benefit. Women lose over half a standard glass of wine’s worth in iron-rich blood and tissue – about 90 millilitres – each time they menstruate, so the process does seem quite costly. And in the predator-filled environs of our early ancestors, leaving a trail of blood was presumably not advantageous.
So how did menstruation arise? Over recent decades, evolutionary biologists have come up with three key theories to explain human menstruation.
