A schematic showing the development of adult fat-tailed dunnarts and newborn mouse mice on the day they were born. Credit: Alan Henderson Minibeast wildlife/Supplied
Some of the most famous mammals in the world are found in Australia. A kangaroo or koala is easily recognized by most people.
However, it's highly unlikely that you have seen one of 19 species of Australia’s most mysterious marsupials, scurrying through grass at night.
The nocturnal marsupialsmammals Dunnarts, who often refer to themselves as "marsupial mouse" because they carry their young in a pouch. In fact, they are so similar to mice that, in cold weather, dunnarts can keep warm by sharing their nests and common house mouse.
This is however the end of their closeness, as they have not shared an ancestor since approximately 160 million years ago.
One species, the fat-tailed dinnart (Sminthopsis.crassicaudata), was first described in English in 1844 by John Gould, an ornithologist. It is a semi-arid, extreme-environment-tolerant species that has been a great deal of interest to scientists since the mid-1900s.
The fat-tailed dunnart is part of the Dasyuridae Family, which also includes Tasmanian quolls and Tasmanian devils. It is a small, light animal that weighs 10-20g and has a length head and body of 60-90mm. The tail measures 45-70cm.
It is one of the smallest carnivorous marsupials. It comes out at night to eat small mammals, reptiles, and spiders. It has a long, carrot-shaped tail that is almost as long as its body. This tail stores fat reserves to help it when food is scarce.
They can live in open woodlands or arid shrublands.
The fat-tailed dunart has been emerging as a key model species to expand our knowledge of mammalian evolution, retina development in nocturnal animals and as a tool for marsupial conservation.
The fat-tailed dunnart itself is not endangered, but other dunart species such as the Kangaroo island dunnart or Sandhill dunnart are.
It is also a Dasuyrid marsupial and belongs to the same order with the endangered Tasmanian Devil, the numbat, and the extinct Tasmanian tiger (Tasmanian Tiger).
Because marsupial laboratory model resources are very limited (compared to traditional mammalian models such as the laboratory mouse), the fat-tailed dinnart offers an excellent opportunity for us to create a laboratory marsupial modeling system. This will allow us to study the biology of the unique Australian fauna, and provide vital conservation tools.
Our research team created a comprehensive resource to address the gap in our knowledge of marsupial biology. This can be used as a reference for other research projects.
Amazingly, almost all of the development in the dunnart happens in the pouch. This means that we can watch the joeys develop in real time without needing to perform any invasive surgery.
To attach to the teat, suckle, and breathe simultaneously, fat-tailed dunnarts require a well-developed nose and mouth. Images of the secondary jaw joints from microCT scans of fat-tailed pouch young dunnarts. (a) A view of the secondary jaw joint, with an inset white box showing the entire skull. Close-up of the secondary jaw joint. Abbreviations for bones: c = condyle, et = epitotympanic rings (purple), in = incus(yellow), in = incus [yellow], ml = malleus and sq = squaremosal (dark). Scale bar = 1 mm. Credit: University of Melbourne
Fat-tailed dunnarts have 13.5 days pregnancies, which is one of the longest of any mammal. The joeys grow to be smaller than a grain or rice and have the heart, lungs, bladder, and brain visible through translucent skin.
The majority of brain development in laboratory mice occurs during pregnancy.
The development of bones in the skull and forelimbs was the focus of our latest research.
These structures were of particular interest to us because marsupials exhibit a shift in when forelimbs or the oral region appear, compared with placental mammals like mice and humans.
Joeys must have strong forelimbs in order to reach the pouch, and a well-developed nose and mouth to attach to the teat and suckle simultaneously.
Our research team raised a colony debonairs and collected joeys, from birth to weaning. Micro-computed Tomography (microCT) was used to examine the young. This non-invasive imaging tool provides 3D (3D) images with high resolution of internal structures.
This technique was used to determine when bones and soft tissue first appear.
Surprisingly, on the day of birth, the joeys did not form any mineralised bone tissue (ossification), despite crawling to their pouches and attaching themselves to the mother’s teat.
MicroCT scanning showed that the nasal, forelimbs and oral cavities were all well developed. However, there was no ossified bone in these areas.
It was somewhat surprising, considering that we know from other marsupial species such as kangaroos or tammar wallabies that there has been an acceleration in the development of the forelimbs and oral bones. These bones were present either before birth or after.
The first bones appeared in the jaw and forelimbs of the dunnartjoeys approximately 24 hours after the birth.
The CT scans series allows us to pinpoint the first appearance of every bone in the dunart. This will make it a very useful resource for studying the evolution and development of the jaw and forelimbs.
Information about the life history and ecology of marsupials is crucial to our understanding of these rare mammals. Unfortunately, one third of Australia's marsupials face extinction.
This research improves the utility and efficiency of the dunart model, as well as assisting in conservation strategies in an ever-changing environment.
These data, along with a detailed series of stagings, are freely available.
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