This story appears in the June 2016 issue of National Geographic magazine.
In a fairy tale by the brothers Grimm, a pied piper’s music lures rats to their doom. What leads the mouselike creature pictured here to its death? Its own libido.
Life is short and sex-centered for the genus Antechinus. Six months after they’re born, the small, carnivorous marsupials reach adulthood. For five more months, they gain weight that they’ll burn off having sex, says mammalogist Andrew Baker of Australia’s Queensland University of Technology. Then the animals enter “a one- to three-week period where they spend all their time mating.” Males fight over females, promiscuous mating ensues, and a single coupling can last 14 hours. Small wonder, as Baker observes, that “both sexes become really stressed.”
As a human does when stressed, an Antechinus produces the hormone cortisol—useful in small amounts but poisonous in large ones. Antechinus males “also have all this testosterone coursing through them from trying to get girls,” Baker says, and the testosterone keeps cortisol gushing when it should shut off. As the cortisol hits toxic levels, males’ immune and other systems fail, and they drop dead by their first birthday. The Antechinus population has been halved—until the females bear their annual litters of four to 14 jelly-bean-size young, which, six months later, will be adults.
“If you had to sit down and design a reproductive system,” Baker says, “you wouldn’t come up with this one.”