Carlos Solis
Throughout human history, men’s lives have been shorter than women’s. On average, women live 5.4 years longer than their male counterparts. To uncover the reasons behind this pattern, a team of researchers from the Max Planck Institute for Evolutionary Anthropology in Germany turned to our animal relatives for clues.
During their study, the scientists examined more than 1,000 animal species to identify common evolutionary traits among species where one sex lives longer than the other.
Humans are not the only species where females tend to outlive males. In 72 percent of the species studied, females live on average 12 percent longer than males. For instance, female moose can live more than twice as long as males.
Meanwhile, in birds, males generally outlive females: in about 68 percent of bird species, males live on average 5 percent longer.
Key Factors: Genetics, Polygamy, and Child Rearing
One key explanation the team identified is chromosomes — a fundamental difference between the sexes. In mammals, females have two X chromosomes, while males have one X and one Y chromosome.
Lead researcher Johanna Sterck told BBC’s Science Focus that because men have only one X chromosome, harmful mutations on that chromosome can have larger effects. A second X could compensate for those impacts. Women are therefore better protected from some harmful genetic mutations thanks to their second X chromosome.
Dr. Sterck also said there is a ‘toxic Y chromosome’ hypothesis: the Y chromosome may carry genetic elements that negatively affect health.
In birds, the situation is reversed: females have two different sex chromosomes (ZW), while males have two identical ones (ZZ). However, not all bird species follow the pattern of males living longer.
“We found some species that behaved contrary to what we expected. For example, in many raptors, females are larger and live longer than males. Thus, sex chromosomes may only be part of the story,” Ms. Sterck pointed out.
Reproductive strategies are another important factor. Sterck said males often pay a high price for reproduction: “To find a mate, they expend energy fighting rivals, which increases the risk of injury and shortens lifespan.”
The team found that male birds are generally more monogamous, while only a small percentage of male mammals settle with a single partner.
While the strongest or most attractive males may produce more offspring, the biological investment in traits like large muscles or antlers can shorten lifespan.
In monogamous species, competition is less fierce, and the difference in lifespan between males and females is smaller.
The next factor influencing lifespan for both sexes is child-rearing. One might expect that the sex responsible for raising children would pay for it with a shorter life. But the team found the opposite.
“We found the opposite: individuals who care for offspring often live longer. This is especially true for primates, whose young remain dependent on their mothers for a long time,” Ms. Sterck explained.
For example, primate infants can stay with their mothers for up to 10 years. Therefore, the longer a mother lives, the longer she can care for her offspring, increasing the chances she’ll see her child reach maturity.
The study’s findings were published in the journal Science Advances.
Photo: Unsplash
