Male marsupial mice just don’t know when to stop. For Antechinus stuartii, their debut breeding season is so frenetic and stressful that they drop dead at the end of it from exhaustion or disease. It may be the females of the species that are driving this self-destructive behaviour. Suicidal breeding, known as semelparity, is seen in several marsupials. This is likely linked to short breeding seasons and the fact that the marsupial mice only breed once a year. It is not clear why this is, but it may be that females can only breed when the population of their insect prey reaches its peak. A year is a long and dangerous time for a small animal, so under these circumstances males might do best to pump all their resources into a single breeding season. To test this idea, Diana Fisher of the University of Queensland in St Lucia, Australia, and her colleagues tracked how insect abundance changed with the seasons in the marsupials’ home forests. Sure enough, they found that the marsupials’ breeding seasons were shortest where insect abundance followed a predictable annual pattern. But the insects are not the whole explanation. It turns out that females do sometimes survive the year and breed again. So why do the males always die?
The key factor is that the females are highly promiscuous, says Fisher. Coupled with the short breeding season, this leads to intense competition between males. “Males that exert extreme effort in this short time are at an advantage.” In other animals, the males might fight, but these marsupials are peaceful. To monopolise a female, the males can copulate for up to 12 hours at a time. But the real contest is played out between their sperm. Rather than growing fighting-fit bodies, the males pour everything they have into fighting-fit sperm. This leaves them with nothing in reserve to fend off disease afterwards. Sperm competition is probably the explanation for suicidal breeding, agrees Ken Kraaijeveld of the Free University in Amsterdam, the Netherlands. However, he thinks high mortality rates in the species are a factor, making it too risky to breed with only a single female. “Males might just monopolise one female, guard her against other males and thus ensure paternity,” he says. “I suggest that they don’t because female mortality is high and males thus have to hedge their bets.” Fisher says the fact that the mice are marsupials probably also contributes to the males’ vulnerability. Marsupials give birth to underdeveloped young and must devote weeks or months to their care, whereas placental mammals give birth to relatively advanced young that grow up fast. As a result, small placental mammals can breed several times each year, taking the pressure off males to perform in any given breeding period. Small marsupial males are not afforded the same luxury. “I do feel sorry for the poor males of species with die-off,” adds Fisher. They are bright and inquisitive, she says, but “you can’t get too attached”.
October 29, 2013
Original web page at New Scientist