Mammals
Brown Antechinus
Antechinus stuartii · also called Stuart's antechinus, marsupial mouse
The brown antechinus looks like an ordinary mouse in an Australian forest. It is a marsupial, a small relative of the Tasmanian devil, and it has one of the strangest life histories of any mammal: the males breed for a single frantic fortnight and then, every one of them, die.
Overview
Antechinus stuartii is a small carnivorous marsupial of the moist forests of eastern Australia, hunting beetles, cockroaches, spiders, and centipedes through the leaf litter. Despite the mouse-like look it is in the quoll and devil family, and the female carries her young in a shallow pouch.
The males die
After a single annual mating season, every male in the population dies within about two weeks, at 11 months old — an all-or-nothing strategy called semelparity that among mammals is almost unique to antechinuses.
Mated to death
Before mating, males stop making sperm and flood their bodies with stress and sex hormones to power a search for as many mates as possible, with copulations lasting 12 to 14 hours. The sustained hormone surge wrecks their immune system and breaks down their tissues, and they die of bleeding, infection, and organ failure. It is thought to pay off because the young must be raised on a brief, precisely timed autumn insect glut.
Status
Least Concern and common in suitable east-coast forest, though reduced by habitat clearing, frequent fire, and feral cats. Several rarer antechinus species are threatened.
Brown Antechinus: common questions
Do the male antechinuses really all die after mating?
Yes. In the brown antechinus and its close relatives, the entire male cohort dies within days to a couple of weeks after the annual mating period, so effectively no wild male lives past its first birthday. This is not a matter of a few males dying; it is total. Field studies that trap antechinuses find plenty of males in late winter, then almost none from spring through the following winter, when the next generation of males matures. The females, which can live to breed a second season, and the newly weaned juveniles carry the population through.
Why does mating kill them?
The proximate cause is a runaway stress response. As the season approaches, males stop producing sperm, so they must mate using the sperm they have already stored, which creates intense competition to mate with as many females as possible in a very short window. To power days of frantic activity and marathon copulations, their bodies pump out testosterone and, crucially, corticosteroid stress hormones, and they switch off the normal feedback that would shut this down. The prolonged flood of stress hormones suppresses the immune system, breaks down muscle and other tissue for energy, and causes ulcers and internal bleeding, and the males die of the resulting infections, haemorrhages, and organ failure, not of the mating itself.
Is the antechinus the only mammal that does this?
Nearly. Suicidal reproduction, or male die-off after a single breeding season, is known in about a dozen species, almost all of them antechinuses and a few very close Australian marsupial relatives such as the phascogales and the kaluta. No placental mammal does it in this complete, obligatory form. It is common enough in insects, fish such as salmon, and some other animals, but among mammals it is essentially an antechinus speciality, which is why the group gets so much attention from biologists studying the evolution of ageing and life history.
How is a brown antechinus different from an introduced house mouse?
The quickest cues are the head and the feet. An antechinus has a longer, more pointed, almost shrew-like snout, larger eyes, thin crinkled ears, and small sharp teeth for eating insects, and it moves in quick darting rushes. A house mouse has a blunter face, a longer bare scaly tail, chisel-like front teeth, and a musty smell. Antechinuses also often live in and around houses in bushland areas, where they are frequently mistaken for mice, but they do not gnaw stored food or wiring the way a mouse does, and a female may be seen dragging a train of clinging young. If it is eating the moths and cockroaches rather than the cereal, it is probably an antechinus.