People-caused introducing of placental mammals to Australia had strongly reflected in evolution of local marsupials. Many carnivorous species of marsupials had been exterminated by people or had died out, when people had exterminated large herbivorous marsupials. And small marsupials frequently did not maintain a competition to placental predators. But one group of Australian predatory marsupials had got great advantage from occurrence of placentals in Australia – these ones were small predators, marsupial mice. When epizootias had reduced the number of placental predators occupied Australia, marsupial predators had not missed their chance and had returned in new quality – as clever, furious and easily adapting species. Gradually they had taken a place in new ecosystems of Australia and further in Meganesia.

The descendant of one Holocene species of marsupial mice, tiny (the total body length is no more 10 cm, not including short tail) species of predatory marsupial mammals, had developed the unique habit of life combining predating with original kind of parasitism. It lives in places where many species of large marsupials live, and spends a part of life, parasitizing in their brooding pouch. This animal penetrates into pouches of large marsupial mammals and eats still tiny newborn cub bearing by them. Then adult animal is attached instead of it to the nipple and feeds on milk secreting by the female involuntarily becoming the host of parasite. This animal is named filicide embryotherium. At embryotherium many infantile features are kept in appearance. The skin of this mammal is covered by thin soft wool and is completely naked in some places. In appearance of animal the sharp sexual dimorphism is expressed. Wool of females is thin and sparse; its color is yellowish-white. Males have richer greyish-yellow wool with cross brownish strips.  Embryotherium, however, spends a significant part of life out of host animal pouch. It is connected mainly with search of sexual partner, settling of young animals and search of host animal by them. Embryotherium has well advanced sensitive eyes and large ears – not living in host pouch animal leads nocturnal habit of life and prefers to hide in various shelters in day time. Its sense of smell is very keen – animal easily distinguishes by smell the female bearing the newborn cub on the background of smells of other part of herd. Paws of embryotherium are strong and supplied with well advanced claws. This animal is rather mobile: embryotherium can swarm up trees, run and jump fast. Its mobility is connected to necessity of search of host animal. By the appearance embryotherium resembles a mouse. It has rather large head with the short muzzle; muzzle, lateral parts of head and throat are almost hairless. Teeth of animal are short and sharp; only incisors and canines develop. The reason of it is a fact that till the significant part of life animal eats soft food and has no necessity to chew it. Milk-teeth of embryotherium are advanced better, than second ones – it is connected to necessity to lead an active habit of life till the first weeks of independent life, up to the meeting of host animal. Some more interesting adaptations to existence in pouch of host animal are the features of excretory system. Embryotherium produces very dry dung and concentrated urine. Leaving the nipple of host animal, embryotherium puts out back from its pouch, sharply throws outside dung and urine, and then hides again. Embryotherium is able to hunt and to eat small animals – worms, slugs and soft-bodied insects like cockroaches and young crickets. Usually such diet is characteristic for young individuals. Males spend in pouches of host animals rather small part of life – they grow up, eating milk of the host animal, and at the approach of sexual maturity leave the safe refuge. Adult males search for females by smell and pair with them actively. Sperm keeps viability in sexual ducts of the female till the long time, and it can bring up to three litters after one pairing. It is an adaptation to secretive parasitic habit of life. After pairing the female at which cubs develop, searches for the host animal. It settles in its pouch, kills and eats its cub, sticks to nipple of the host animal, and raises its own posterity in safety and food abundance. In litter of the embryotherium it may be up to 6 – 7 cubs. The pouch of embryotherium looks like two thin and elastic longitudinal plicas surrounding nipples from two sides. Cubs eat parent milk for very long time – it is their only food till about 4 months, and during the entire life they keep ability to digest milk proteins. Young embryotheriums grown up sizable enough, leave host animal’s pouch and lead independent life within several months. Females reach sexual maturity rather early – at the second month of independent life. After pairing embryos in their bodies run to a diapause and do not develop while the female will reach normal physical standard. Free-living males seldom survive over two years, but separate individuals succeeded to find the host animal after rough courtship season, quickly restore forces and can live up to four-year-old age.

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