Our evolutionary ancestors prioritising sex over survival could be why men don’t live as long as women, according to a new study.
Across the world and throughout history, men have generally lived shorter lives than women. To find out why humans evolved this way, a group of researchers at the Max Planck Institute for Evolutionary Anthropology, in Germany, didn’t look at human history – but instead searched for clues from our animal cousins.
Their study, published in Science Advances, examined over 1,000 animal species to track down which evolutionary traits were common amongst species where one sex tends to outlast another.
They discovered that, though genetics does seem to play a part, monogamy was also a key factor in male longevity. Meanwhile, child-rearing might have helped females expand their lifespan.
In humans, women live an average of 5.4 years longer than men, but we’re not the only species where one sex outlives another. In fact, in 72 per cent of mammals, females outlast their male counterparts by an average of 12 per cent. The female lifespans of one species, the moose, can be over twice as long as males’.
But this isn’t true across the animal kingdom. In birds, it’s the males that tend to live longer – by an average of 5 per cent in the majority (68 per cent) of species.
One of the leading explanations is our chromosomes, given it’s one of the most fundamental differences between sexes. In mammals, females have two X chromosomes, while males have a shortened Y chromosome.
Study lead Dr Johanna Staerk told BBC Science Focus explained that because males have only one copy of the X chromosome, “any harmful mutations on that chromosome are more likely to have an impact, since there isn’t a second copy to compensate.
"The second idea is known as the ‘toxic Y’ hypothesis, which suggests that the Y chromosome itself might carry genetic elements that have deleterious effects on health and ageing.”
Meanwhile for birds – who have ZW chromosomes – it’s the males who have two Zs, while in females one is replaced by a truncated W chromosome. But if this were the only factor, males would live longer across all bird species.
“Some species showed the opposite of the expected pattern,” said Staerk. “For example, in many birds of prey, females are both larger and longer-lived than males. So sex chromosomes can only be part of the story.”
What else is at play? The researchers think it’s not just about sex in the genetic sense, but in the reproductive sense as well. When it comes to mating, birds generally tend to be monogamous, while only a small percentage of mammals stick with one partner.
“Males often pay the price for reproduction,” according to Staerk. “To secure mates, they invest in traits like manes or antlers and spend energy fighting rivals, which increases their risk of injury and shortens their lifespan.”
Though the most attractive, strongest male will have the most offspring, the biological resources needed to grow big muscles or horns has a negative impact on their lifespan. In monogamous species, there isn’t as much competition and so males haven’t had to make that trade-off during evolution. The gap between male and female lifespans in these species is much smaller.
One surprising finding was how child-rearing impacts life expectancy. Ask any parent and you’ll know that bearing and raising children takes a lot of energy – so you might expect the sex responsible for child-rearing to pay the price with a shorter lifespan.
“Instead, we found the opposite – the caregiving sex often lives longer,” said Staerk. “This seems especially true in species like primates, where young remain dependent for a long time.”
Young chimps, for example, can remain with their mothers for up to 10 years. That means the longer the mother lives, the longer she can care for her offspring.
“Living longer may give caregivers a selective advantage by ensuring their offspring reach maturity.”
That said, she explains, it’s hard to say for certain whether this benefit comes from caregiving or from monogamy, since in monogamous species both parents tend to provide care.
So, it seems that being a lifelong partner and a good parent is the key to males having a longer life – at least from evolution’s point of view.
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