Science & Environment

Sex is matter of life, then death for male marsupials

Image caption Sex is so frenetic for some small marsupial species that it results in the death of the males

A new study suggests that some species of marsupials mate with such vigour and intensity that it quite literally kills them.

The scientists say that males die in large numbers after mating with as many partners as possible in sex sessions lasting up to 14 hours at a time.

A key factor in this costly coitus is the promiscuous behaviour of females who all breed at the same time of year.

The study is published in the journal PNAS.

Suicidal reproduction or semelparity is well known in many species of plants and fish but is rare in mammals.

This new study looks at the mating behaviour of 52 different species of small, insect eating marsupials in Australia, South America and Papua New Guinea.

They found that in some of these animals, such as the antechinus, the phascogale and the dasykaluta, male attempts to father offspring cost them their lives.

Lust for life?

This "dying-off" trait is more likely to be found in species living in regions where food was plentiful in one period of the year.

This makes the females of the species more likely to shorten their mating seasons so they only give birth when there is plenty to eat.

In these marsupials, the females have also synchronised their reproductive cycles.

They are also highly promiscuous, as this promotes sperm competition among the males, explained Dr Diana Fisher from the University of Queensland, the lead author of the study.

"The females that mate with more males, get to weed out the poor quality males because of this sperm competition," she said.

"The ones that succeeded in fertilisation were the ones with better offspring survival."

While this ensures sturdy descendents, the process is fatal for the fathers.

The males attempt to mate with as many females as possible in long, laborious bouts, driven by high levels of hormones including testosterone.

Image caption Other species including the red back spider engage in suicidal reproduction

These chemicals in turn elevate their levels of stress hormones and their systems are unable to cope, says Dr Fisher.

"If we humans get huge stress, we have a feedback system and we bring it down.

"But the marsupials just keep ramping it up more and more and are driven to spend all their time mating competitively.

She added: "And its the stress hormone that does that."

Dr Fisher says that this is sexual selection driven by females. It is a different strategy to other mammals where the males sometimes fight for the right to mate or are selected by the females on the basis of looks or strength.

"In this case, it is having the one that's best at sperm competition," she said.

The research team says that the strategy of losing half the species in the act of reproduction can work well as an evolutionary strategy if the population is dense.

"Every other animal has a cost of reproduction," said Dr Fisher.

"It's just they've got it all at once."

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