Death by Sex

No, I’m not talking death by ice pick. This is death by sex itself.

Northern Brown Antechinus image from the Australian Museum

This is about the Antechinus

Antechinuses are small, carnivorous, shrew-like marsupials with bristly fur that primarily prey on invertebrates such as spiders, beetles (including larvae), and weevils. The majority of Antechinus species occur in Australia and only two species (currently with a putative third) have been described in New Guinea. Members of this species have been called broad-footed marsupial mice, pouched mice, or antechinus shrews.

What is most interesting about Antechinus is that the males literally screw themselves to death.

It’s August in Australia, and a small, mouse-like creature called an antechinus is busy killing himself through sex. He was a virgin until now, but for two to three weeks, this little lothario goes at it non-stop. He mates with as many females as he can, in violent, frenetic encounters that can each last up to 14 hours. He does little else.

A month ago, he irreversibly stopped making sperm, so he’s got all that he will ever have. This burst of speed-mating is his one chance to pass his genes on to the next generation, and he will die trying. He exhausts himself so thoroughly that his body starts to fall apart. His blood courses with testosterone and stress hormones. His fur falls off. He bleeds internally. His immune system fails to fight off incoming infections, and he becomes riddled with gangrene.

He’s a complete mess, but he’s still after sex. “By the end of the mating season, physically disintegrating males may run around frantically searching for last mating opportunities,” says Diana Fisher from the University of Queensland. “By that time, females are, not surprisingly, avoiding them.”

And then they die. All of them. Every single year old male exhausts all of his bodily resources and dies.

The technical term for this is semelparity, from the Latin words for “to beget once”. For semelparous animals, from salmon to mayflies, sex is a once-in-a-lifetime affair, and usually a fatal one. This practice is common among many animal groups, but rare among mammals. You only see it in the 12 species of antechinuses and a few close relatives, all of which are small, insect-eating marsupials.

Researchers have known about this phenomenon for decades, but previously believed that death resulted from starvation as the males do not feed during this period. However, Diana Fisher from the University of Queensland has another theory.

These animals feed on insects, and some experience a glut of food once a year but very little at other times. This seasonality increases the further you get from the equator. The species with the most seasonal menus also had shorter breeding seasons, and their males were more likely to die after mating.

Fisher thinks that as the ancestors of antechinuses spread south through Australia and New Guinea, they encountered strong yearly fluctuations in their food supply. The females were better at raising their young if they gave birth just before the annual bonanza, and were well-fed enough to wean their joeys. Their mating seasons shortened and synchronised, collapsing into a tight window of time.

Since Antechinus females suckle their young for up to four months, they wer eonly able to have a single litter per year. Rather than fight for breeding rights like many other species, and  growing antlers or claws, male Antechinuses put their energy into their testes.

Fisher found a clear relationship between suicidal reproduction and testes size. The biggest testes of all, relative to body size, belong to species whose males die en masse, followed by those where a minority survive to mate again, and then by those with several breeding seasons.

The males that put the greatest efforts into sperm competition fathered the most young. It didn’t matter if they burned themselves out in the process, if they metabolised their own muscles to fuel their marathon bouts. These animals are short-lived anyway, so putting all their energy into one frenzied, fatal mating season was the best strategy for them. Living fast and dying young was adaptive.

Biology: it really is all about the sex.

This entry was posted in Biology and tagged , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s