Agile antechinus: adorable acrobats with unusual sex lives

Name(s): Agile antechinus (Antechinus agilis), also called the agile marsupial mouse

Size: Length 80-116 mm, plus a tail of 75-102 mm; weight 16-44g

Diet: Carnivores – mainly insects and spiders with the occasional flowers or fungi

Habitat: Found at lower levels of the Great Dividing Range and into coastal forests of Victoria and southern NSW

Conservation status: Not endangered

Superpower: “Agile” is a very good description. If you hold the agile antechinus at the base of a tree and let it go, it always runs up the trunk – very fast. They are like Cirque de Soleil gymnasts; no gymnastic feat seems beyond them, and they do it with such verve!

The agile antechinus is an adorable marsupial that resembles a mouse or shrew, but has the personality of a lion. Like many other dasyurids, agile antechinus are carnivores with interesting reproduction, short pregnancies and a long lactation period. These critters live in communal nests in tree hollows. They are mostly nocturnal, but can occasionally be found sunbathing on fallen tree trunks or rocks to warm up in the morning.

Agile antechinus eat mainly insects and spiders. They are also happy to snack on a mouse, some pollens or, like any gourmet, a native truffle in the autumn – a bit gritty but delicious. Those humans living in coastal forest areas know they are co-habiting with an antechinus when they begin to find piles of little bones. Their antechinus houseguests are helping them by eating their pest mice: a true symbiotic relationship.

The agile antechinus have highly synchronised sex lives, getting together during a frantic period of 1-3 weeks in July or August. When the day length starts to increase following the winter solstice in June, a response is triggered in the antechinus to start their breeding season. This synchronisation ensures that young agile antechinus are born when there are plenty of insects around to eat. The timing of the breeding season is further fine-tuned by population density: the denser the population, the shorter the season.

During this breeding season, the females gather where they are visited by the males.  Foreplay is lengthy and copulation is long.  Not only is this fun, but it keeps the other males away. The females can store sperm for up to 21 days after mating. All the males die after the mating frenzy, in an event known as “the die-off”.

The nearest human equivalents to agile antechinus males may be chief executives of large companies. Like them, agile antechinus live eventful and rich lives, are always active, and have full-on sex lives. They both have high stress levels and develop gastric ulcers, which are often fatal in antechinus. Dasyurids are now known to have their own species of Helicobacter, the bacterium that causes gastric bleeds, or they can be infected by the human version. This highly synchronised life and high stress level leads to the die-off of male antechinus each year. A warning to human CEOs – avoid too much synchrony.

Back to the females – half of the 27 days of pregnancy in agile antechinus are taken up by developmental arrests, which are pauses in foetal development that allow the antechinus to maximise their reproductive performance. Together with the synchronisation of breeding, these pauses ensure that no female gives birth until all the males are dead, giving them a stress-free lactation period to look after their young for 70 days. The females have between six and 10 teats, and they usually raise a number of young equal to or slightly less than this number, depending on the season.

Surely you can agree that these minuscule marsupials deserve your vote in Australian Mammal of the Year!