Men who make it to adulthood without succumbing to the male habit of dying in accidents shouldn't congratulate themselves too soon: their life expectancy still doesn't match a woman's. In industrialized countries, women at every age out-survive men. And it's not just humans. Males that die before females have been observed throughout the animal kingdom. It's even true of the lowly fruit fly, and it looks like harmful mutations in mothers' genes are to blame.
This idea, which has been put forward before, is called the Mother's Curse. It has to do with a little loop of DNA that's passed down in humans—and in most other animals—exclusively through the mother. This DNA hides inside the mitochondria, which are the cell's batteries, and doesn't get packaged up with the rest of the genetic material when sperm are made. Those tiny sperm will rendezvous (if they're lucky, of course) with an egg that supplies all its own mitochondria, along with their DNA.
The Mother's Curse theory says that since fathers don't get any say in the makeup of mitochondrial DNA, it could carry mutations that harm men without being weeded out by natural selection. These anti-male mutations might be the reason for males' shorter lifespans.
Researchers led by Florencia Camus at Monash University in Australia examined this question in fruit flies, an animal whose genes are well understood and easily fooled with. By crossbreeding different fly types, they created 13 lines of fruit flies that were identical except for their mitochondrial DNA. They watched these flies for differences in male and female lifespan. Then they sequenced the mitochondrial DNA itself to see what was driving those differences.
In a study published in Current Biology, the team found that males died sooner across all the fly types. They also saw wide variation in male longevity, while female lifespans were more consistent. Females in most of the lines lived for around 60 days; males were variable but never lived much longer than 50 days. Since their mitochondrial DNA was the only thing that differed between the flies, something in that DNA must have been responsible.
Looking at the actual letter-by-letter differences in the mitochondrial DNA, the researchers found that fly types that were farther from each other genetically also differed more in longevity. In other words, more mutations in the mitochondrial DNA led to more variability in lifespan. Together, these findings support the idea that mitochondrial mutations cause males to die early—in fruit flies, anyway.
Mutations that somehow harm males, but not females, are free to pile up in the DNA of mitochondria. Since females pass down this DNA on their own, evolution is essentially blind to its effect on males. It remains to be seen whether the same mechanism is at work in animals that aren't fruit flies, including humans. If so, men will be able to blame their mothers for their shorter life expectancies. They might want to find a more positive way, though, to fill their abbreviated time on Earth.
M. Florencia Camus, David J. Clancy, & Damian K. Dowling (2012). Mitochondria, Maternal Inheritance, and Male Aging. Current Biology DOI: 10.1016/j.cub.2012.07.018