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Antechinus


Yellow-footed Antechinus

Yellow-footed Antechinus




Antechinus is a genus of dasyurid marsupial that is indigenous to Australia (including Tasmania and some outlying islands) and New Guinea. The majority of Antechinus species occur in Australia and only two species (currently with a putative third) have been described in New Guinea. Members of this species have been called broad-footed marsupial mice, pouched mice, or antechinus shrews. However, these common names are to be considered either regional or archaic and the modern common name for animals of this genus is 'antechinus'.

Antechinuses are small, carnivorous, shrew-like animals that primarily prey on invertebrates such as spiders, beetles (including larvae), and weevils. Some are strictly terrestrial and hunt only at ground level, while others are highly scansorial (climbing) in nature. Most species nest communally in tree-hollows.

Antechinuses have short hair and vary from grey-cinnamon-brown-black in colour, depending on the species. They have prominent whiskers, large ears and a pronounced, pointed snout, giving them an overall shrew-like appearance. Species vary from 80-120mm, nose-to-tail and weigh 16-170g when fully grown. Sexual dimorphism occurs in most species for both weight and skeletal measurements, with males being typically larger and heavier. A. agilis is the smallest known species, and A. swainsonii the largest. Species in Papua New Guinea are not well documented and unknown Papuan species and subspecies likely await discovery.

Their habitat is primarily: temperate rainforest, dry and wet eucalypt forest; temperate rainforest; swamps, wet heaths; disturbed or open woodlands; dry/wet, tropical/subtropical/temperate eucalypt forest/rainforest; tropical rainforest; tropical/subtropical vine-forests; monsoonal open forest.

All Antechinus species except for A. swainsonii are semelparous at least as males – and usually as females too –, meaning that an individual will usually only live long enough to breed once in its lifetime. Breeding occurs in winter (usually August-September) at a time when there is little food available in the environment. The male can spend up to 12 hours mating to ensure breeding success. To accomplish this the males strip their body of vital proteins and also suppress the immune system so as to free up additional metabolic energy. In this way an individual male trades away long-term survival in return for short-term breeding success, and following the breeding season there is a complete die-off of physiologically exhausted males. Breeding is intensely competitive. Males produce large amounts of testosterone and mate-guarding occurs in the form of protracted copulation.

The females can store sperm for up to three days in specialized sperm-storage crypts in the ovary and do not ovulate until the end of the breeding season. Many litters have multiple paternity (i.e., several fathers contribute to a single litter). Females can live for 2-3 years. However, this is unusual, and most females die following the weaning of their first litter. Litter size depends on the number of teats in the pouch. There are as few as 4 teats, usually 8, and in some populations up to 10 can occur. It is currently unknown why teat number varies. However, it is likely that in food-poor environments selection has tended towards fewer teats so that there is a greater parental investment per offspring.

Sperm competition drives suicidal reproduction in antechinus and other marsupials, according to a new study by Australian researchers. The research, which was published in the journal Proceedings of the National Academy of Sciences, overturns previous theories that linked the post-mating mass die-off to altruism or to food availability. The study found that extreme stress, and the failure of the corticosteroid mechanism that controls it, cause immune system collapse, infections and internal bleeding, leading to death.

Males of semelparous species spend most of their lives as immature animals and mature at 11 months, just before the breeding season. They have a few other peculiarities as well: they all shut down their testes before breeding and so rely on sperm stored in the epididymis for mating, which is also lost in their urine. This gives them some urgency to mate. Each mating can take 12 to 14 hours, and they do this over and over again.

The researchers found that females of species with the most extreme male strategy of die-off have even shorter breeding seasons than the availability of food dictates, and this time pressure imposes severe competition on males. In response, semelparous males have large testes relative to body size that allow them to fertilise as many females as possible. Disintegration of their testes before mating means that males devote as much energy to competitive mating as possible. They aren't spending energy on making sperm by this stage, and use their body tissues to fuel the mating frenzy. The males don't engage in physical competition; instead their sperm are left to battle it out in the female's reproductive tract.

Females benefit from sperm competition because the best males sire more of their offspring—antechinus males that excel at sperm competition have better offspring survival. This means it is actually pressure imposed by females via their reproductive timing and encouragement of sperm competition that has selected for semelparity in males. Using all their energy on the one breeding season gives males an advantage in sperm competition, so it's reproductively worthwhile for them even though they die.