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Antechinus go out with a bang

Australia's little marsupial mice live fast and die young in a frenzy of winter mating known as big bang reproduction.

At this time of year, the undergrowth is a hive of activity as mouse-sized marsupial carnivores called antechinus come together for a once-in-a-lifetime mating ritual — literally.

Australia is home to ten species of antechinus, also known as marsupial mice. Despite the name, these insect-eating marsupials (dasyurids) bear only a superficial resemblance to rodents, says conservation biologist Dr Diana Fisher from the University of Queensland.

"Their faces and behaviour are different. They can be active during the day as well as night and unlike mice, which are herbivores, most antechinus are primarily insectivourous, and boldly hunt spiders, cockroaches and other insects. Some also eat fruit and flowers, and a few will even eat vertebrates such as frogs and lizards.

"Few people living in cities have encountered antechinus because the presence of cats and clearing of understorey vegetation has wiped out populations in urban areas. Where they are found, they are usually confused with mice," she adds.

"However, in country areas they're quite well known, with some like the yellow-footed antechinus (A. flavipes), making themselves at home inside people's houses, especially in the kitchen and other areas where insects congregate."

They happily build their spherical nests inside armchairs, seldom-used handbags, clothes drawers and old box-like television sets.

Away from human civilisation most live in treehollows, coming down to ground level to forage in leaf litter, except for the dusky (A. swainsonii) and swamp antechinuses (A. minimus), found in Tasmania and Victoria, which make burrows in the ground and nest in fallen trees and ground-level vegetation.

"Dusky antechinus are the largest species and their range overlaps with that of the agile antechinus (A. agilis), which they will eat if they can catch them," says Fisher.

"Where the two species are found in the same area researchers have noticed that the much smaller agile antechinus' behaviour changes: they use trees more, and tend to forage on the ground less."

Also found in alpine areas, the two species remain active throughout the winter, using well-worn pathways under the snow to forage for food.

Fast Facts When: June to September Where: There are ten different species of antechinus. Species such as the tropical (A. adustus), Atherton (A. godmani), cinnamon (A. leo), and sub-tropical antechinus (A. subtropicus) live in very small, isolated pockets of woodland in tropical Queensland. The yellow-footed antechinus (A. flavipes) has a wide range. It lives in forested areas of eastern Australia and south west Western Australia. The brown antechinus (A. Stuartii) lives in woodland along the south east NSW coast. The agile antechinus (A. agilis), which looks very similar to the brown antechinus, also lives in woodlands in NSW and Victoria. As does the dusky antechinus (A. swainsonii) which also lives in Victoria and parts of Tasmania. The swamp antechinus (A. minimus) lives in wet heath in coastal areas of the mainland and in button grass sedgelands from the coast to sub-alpine areas of Tasmania. The fawn antechinus (A. bellus) lives in isolated pockets across the Top End.

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Love kills

"Antechinus look dull, but everything else about them is extreme," says Dr Fisher, adding that their most striking feature is their breeding.

Males live for exactly eleven-and-a-half months, dying from stress-induced immune system breakdown about two weeks after mating. Females, especially from larger species, may live longer, with around 30 to 50 per cent raising two litters, while only ten per cent of females from smaller species live long enough to breed again.

All animals from a population breed at the same time, with 70 per cent giving birth on the same day in some populations, although the timing varies with latitude and altitude, says Fisher.

"Animals from higher altitudes in north Queensland can breed as early as June, with the season becoming later the further south a population lives.

"Mating is triggered by the change in day length, which of course varies with latitude, and is timed so that the young are born in time for the insect explosion in summer," says Fisher.

All females come into oestrus at the time, triggering a mating frenzy among males. Copulation is a violent affair with males biting the backs of the females' necks during their brief encounter before each moves on to other partners. A fortnight later, every male is dead, overwhelmed by the stress-related corticosteroids produced during the frenzy of mating.

"Basically, they escalate the amount of competition until it kills them," says Fisher, adding that they suffer internal bleeding, gangrene and other infections until it overwhelms them.

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Competitive edge

Dr Fisher studied the mating habits and paternity of brown antechinus (A. stuartii), whose females usually only breed once, and found that sperm competition is the driving force behind their extreme breeding synchronisation and mating effort.

"Although each mating typically takes about twelve hours, females aren't particularly fussy and will have sex with multiple partners if they can, avoiding only those males that are picked on by other males, and perceived to be weak," she says.

"Fertilisation doesn't take place immediately, with females storing sperm for up to two weeks in their oviducts. Sperm from only the strongest males then go on to fertilise her eggs.

"When the babies are born a month later, they may have up to four different fathers, and that the offspring sired by fathers whose sperm competed with others were stronger, and had a better start in life than those who didn't."

Competition continues after birth. Like all marsupials, babies are born at an embryonic stage. Antechinus have the biggest litters of any Australian marsupial, averaging eight to ten babies, although yellow-footed antechinus can have 14. There is no pouch, with babies attaching to a teat on the mother's underside, then travelling slung under her belly in an open pouch, and clinging on for dear life.

"Strangely, they have about 20 per cent more babies than nipples," says Fisher, meaning that several babies die at birth. "But they're pretty cheap to produce, at least in the early stages. By the time of weaning at about 100 days old, the litter weighs four times as much as the mother, who exhausts herself and loses weight trying to feed her growing brood."

Because antechinus rely so heavily on insect population peaks coinciding with their breeding season, they are extremely vulnerable to climate change.

"For these animals change is only possible on an evolutionary scale. "If the climate uncouples the food supply from the breeding cycle, they are in big trouble," she says.

"It may already be already happening to the three species found in Kakadu in years when the monsoon is late, [which has happened] over the past few decades."

Live fast, die youngDying after only a single reproductive event is known as semelparity, and is found in insects like mayflies, in some spiders and in molluscs such as octopus. However, it is much rarer in vertebrates, occurring only in a few fish species like salmon and some species of trout, and one lizard, the Labord's chameleon. Among mammals, it is only known to occur in insect-eating marsupials, and until fairly recently was thought to exist only in antechinus.



It has now been found that several other dasyurid genera including Phascogales, Dasykalutas and some populations of dibblers (Parantechinus) have physiological die-off, that is, they have the immune collapse mechanism where all males die at once. The red-tailed phascogale is very rare now, but once lived in the Western Desert, while the little red kaluta (Dasykaluta rosamondae) lives in the spinifex grasslands of the Pilbara, throwing into doubt a theory that semelparity in marsupials is only found in forest dwellers.



Most dibblers (Parantechinus apicalis) live in heathland in southwest WA, however they have an island population on the west Australian coast in which the males die off while the main population doesn't, while savanna populations of northern quolls (Dasyurus hallucatus) have zero male survival to breed again but no apparent immune collapse that triggers the die-off.



"Several South American mouse opossums are also known to have zero male survival following breeding, including Gracilinanus microtarsus, Marmosops incanus, Marmosops paulensis, and Monodelphis dimidiata," says Dr Fisher. "[Like northern quolls], they don't have immune collapse, and females also have very low survival to the second year."



"It's fairly obvious there's more to this issue than just food availability."

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