Kalutas live fast and die young — or, at least, the males do. Male kalutas, small mouselike marsupials found in the arid regions of Northwestern Australia, are semelparous, meaning that shortly after they mate, they drop dead.
This extreme reproductive strategy is rare in the animal kingdom. Only a few dozen species are known to reproduce in this fashion, and most of them are invertebrates. Kalutas are dasyurids, the only group of mammals known to contain semelparous species. Only around a fifth of the species in this group of carnivorous marsupials — which includes Tasmanian devils, quolls and pouched mice — are semelparous and, until recently, scientists were not sure if kalutas were among them.
Now there is no doubt that, for male kalutas, sex is suicide.
In a study, published in April in the Journal of Zoology, researchers from the University of Western Australia and the University of Queensland confirmed that kalutas exhibit what is known as obligate male semelparity.
“We found that males only mate during one highly synchronized breeding season and then they all die,” said Genevieve Hayes, a vertebrate ecologist and the lead author of the study.
Dr. Hayes and her colleagues monitored the breeding habits of a population of kalutas in Millstream Chichester National Park in Western Australia during the 2013 and 2014 breeding seasons. In both seasons, the researchers observed a complete die-off of males. Although other scientists have observed male kalutas exhibiting semelparity in captivity, this was the first time it had been seen in the wild.
Kalutas evolved independently of other semelparous dasyurids, so the confirmation that male kalutas die after mating suggests that this unorthodox reproductive strategy has evolved not once, but twice in dasyurids.
“It’s really interesting that it would evolve twice in dasyurids because it’s such an extreme mating system,” Dr. Hayes said.
Extreme indeed. Kalutas, which reach sexual maturity at just 10 months, have only one two-week window in early September during which resources in their environment are abundant enough to support reproduction. During these brief, frenzied breeding seasons, male kalutas mate with several females — for up to 14 hours at a time — until they succumb to exhaustion and die.
“It’s an inevitable death from chronic stress,” said Christopher Dickman, a professor of terrestrial ecology at the University of Sydney, who was not involved with the study.
One to two months before the onset of the mating season, male kalutas stop producing sperm and start producing large amounts of testosterone and corticosteroids. Although this influx of hormones drives them to mate, it also suppresses their immune system and puts immense stress on their internal organs.
“The precise cause of death is usually ulceration of the gut track,” Dr. Dickman said. “They’ll be leaking blood into their body and begin to suffer organ collapse.”
As gruesome as the kaluta’s reproductive strategy is, it is not without merit. Like pouched mice, wambengers and other dasyurids, kalutas are polyandrous, which means they mate with multiple partners. And scientists suspect that, like their polyandrous cousins, female kalutas are capable of internally stockpiling sperm for up to two weeks before fertilization. Doing this allows females to produce offspring — six to eight per litter — using sperm from multiple partners. Dr. Hayes and her colleagues performed paternity tests on eight litters born after the first breeding season and found that all but one had been sired by multiple males.
“The fact that females can store sperm is probably one of the ultimate factors that led to the evolution of male die-off,” Dr. Dickman said. “Males simply wouldn’t have any guarantee of fathering young if they were to mate with one female over the two-week period. The only chance that they’ve got to guarantee paternity is to mate with as many females as possible.”
For male kalutas, mating enough times, with enough partners, to ensure that their genes are passed on takes a lot of energy — and a lot of sperm. Mustering all that energy and ejaculate ultimately costs them their lives — but for an animal that weighs less than a light bulb, this may not be such a bad strategy.
“Mammals this small generally don’t live for more than a year, so if there’s only one opportunity to successfully rear a litter each year, then it makes sense to invest as much energy as you can into reproduction,” Dr. Dickman said. “Forgive the pun, but in this case, it’s wise to put all your eggs in the one basket.”