Menopause is a rarity in the animal kingdom. Apart from humans, killer whales and pilot whales are the only other creatures to routinely live decades beyond their child-bearing years. While lifespan may play a role, why do elephants – also long-lived mammals – not experience menopause?

For humans, it boils down to the "grandmother hypothesis,” contends Kristen Hawkes, an anthropologist from the University of Utah, in a statement. The role of grannies is to provide their daughters with a more productive fertile period, helping feed their young once they are weaned in order to allow the mother a faster turnaround time between pregnancies. In doing so, grandmothers have more descendants because their daughters are able to birth more children. This increases the likelihood that the grandmother’s genes, including those for longevity, will be passed on, according to the hypothesis.

Hawkes’ new work builds on her 2012 study that used computer simulations to show that prehistoric “grandmothering” lengthened lifespans, helping humans reach their 70s and 80s –unlike chimpanzee lifespans that are cut short after child-bearing years. 

Now, Hawkes and colleagues have once again run computer simulations, this time showing that grandmothering impacts the uneven ratio of males to females. To reach this conclusion, the researchers ran 60 evolutionary simulations – 30 with grandmothering and 30 without.

The simulations revealed that male-female sex ratios changed over time as a result of grandmothering. In chimpanzee populations – where no grandmothering is present – there is a higher number of fertile females than males. However, in populations with grandmothering, fertile males become more abundant than females. Specifically, the ratio of fertile males to females increases from around 77 males per 100 females to 156 males per 100 females in 30,000 to 300,000 simulated years. The results are published in the Proceedings of the National Academy of Sciences.

This skewed ratio may have led to the development of pair-bonding, suggests Hawkes, where partners form mating pairs as opposed to uncoupled sexual activity.

"It looks like grandmothering was crucial to the development of pair bonds in humans," said Hawkes. “Pair bonds are universal in human societies and distinguish us from our closest living relatives. Our hypothesis is that human pair bonds evolved with increasing payoffs for mate guarding, which resulted from the evolution of our grandmothering life history.” 

According to Hawkes, this favors the male tendency to guard a single female mate and form a pair bond. This may also account for their preference for younger fertile women, as they have longer child-bearing years ahead. 

"This is different than what you see in chimpanzees, where males prefer older females," said Hawkes. "This male bias in sex ratio in the mating ages makes mate-guarding a better strategy for males than trying to seek an additional mate, because there are too many other guys in the competition.”

Her study conclusion differs from the traditional hypothesis in which pair-bonding “resulted from male hunters feeding females and their offspring in exchange for paternity of those kids so the males have descendants and pass on genes.” Most critics of the grandmother hypothesis contend that the evolution of a bigger brain was the largest contributing factor to longer lifespans than apes. 

This recent study is by no means a final say in the great debate. However, whatever the case may be, the robust discussion is sure to spur further scientific inquiry and rigorous study into this evolutionary mystery.