Heterogametic Sex Hypothesis

In mammals, including humans, sex is generally determined by the X and Y chromosomes. If a baby has a pair of X chromosomes, she’s a girl. If the baby inherits an X chromosome and a Y chromosome, he’s a boy.

In birds, however, the situation is reversed. Female birds have a pair of unlike sex chromosomes while males have the like pair. (In this sort of sex-determination system, scientists use the letters W and Z, and talk about a ZW pair for female birds and a ZZ pair for males.)

The “heterogametic sex hypothesis” holds that if something goes haywire with a gene on one of a woman’s X chromosomes, her cells have a spare to rely on. But men, with only a single X chromosome, have no such reinforcements. The same sort of problem may happen with a male’s unpaired Y chromosome.

In men, “if there’s any deleterious mutations or any mutations that will reduce the lifespan, you don’t have a backup,” said Fernando Colchero, who is also with the Max Planck Institute.

For their study, Colchero, Staerk and their colleagues collected data on the lifespans of 528 mammal species and 648 bird species kept in zoos. The team found that most other mammals are like humans, with the females of nearly three-fourths of mammal species outliving their male counterparts.

But in birds, 68 percent of species studied showed a bias toward male longevity, as expected from their chromosomal makeup.