Wild sex leads to ‘reproductive suicide’

By AAP with Wes Judd 9 October 2013
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A new study has revealed why tiny Australian marsupials mate frantically until they die.

SEX IS A MATTER of life or death for this Australian marsupial, and a new study says sperm competition is the reason.

Australian researchers examined ‘reproductive suicide’ in species of antechinus, a mouse-like marsupial, in which males die after extensive mating bouts.

“The males use up all their energy mating, causing their immune systems to collapse in what we call ‘die-off’,” says Dr Diana Fisher, lead researcher and biologist at The University of Queensland in Brisbane.

Why male antechinus die after mating

The males of all 13 species of the genus Antechinus – an insect-eating marsupial – suffer from post-coital death. Earlier studies have speculated that this behaviour evolved to ensure there was enough food for the mother and the offspring.

However the new findings, published this week in the journal PNAS, show that self-sacrifice also takes place when food is abundant.

Diana and her team suggest that what drives the lethal orgy is the brevity of the female breeding cycle – sometimes as short as one day – and the need for the female to be impregnated by as many males as possible.

Faced with the promiscuity of females intent on copulating with as many fertile males as possible in a short time-frame has driven sperm competition. Males store up large amounts of sperm which are expelled in an epic bout of mating that ends in death.

“Their testosterone goes through the roof and their stress levels rise to such a point that mating is lethal to them,” says Diana.

Reproductive suicide among animals

The research also revealed post-mating death to be more common among antechinus living at higher latitudes. In these environments, the availability of food cycles predictably each year and females have adapted their reproduction so young are born into a period of maximum food abundance. This eventually led to in an extremely short breeding cycle, and higher sperm competition among males.

“The most interesting part of this study is the latitudinal correlation between these deaths and the resources,” says Dr Andrew Baker, an evolutionary biologist at Queensland University of Technology. “They’ve been driven down this evolutionary pathway that they can’t help.”

Suicidal reproduction, known as semelparity, probably affects less than 5 per cent of Australian animals, according to Andrew, however it is not uncommon in the greater animal kingdom.

A praying mantis rips off her mate’s head after sex, while a black widow spider eats her mate. Honey bee drones also die after mating with the queen, suffering from an explosive ejaculation that tears the penis off. Pacific salmon, too, are known to swim upstream to spawn and die.

Learn about a newly discovered species of antechinus in the Nov/Dec print issue of Australian Geographic. Out early November.