Wallaby a tiny climber

Volume 9 Number 4 April 8 - May 12 2013

Researchers have for the first time seen the movements of the Australian tammar wallaby in utero. By Rebecca Scott.

At birth, the tiny tammar wallaby weighs only half a gram and is less than two centimeters in length.

From conception it has spent a mere 29 days in its mother’s uterus, and once delivered, must make the journey from birth canal to its mother’s pouch, where it latches on to a teat and remains linked to a constant milk supply for the next 250 or so days.  Remaining in the pouch and sucking intermittently, the pouch young emerges at 200 days and begins to supplement its milky diet with grass. 

A team of scientists, led by the University of Melbourne and the Leibniz Institute for Zoo and Wildlife Research in Germany, has managed to visualise the developing tammar wallaby, from when it was only 1.5mm in size and have followed it through to birth.

Using high-resolution ultrasound, climbing movements of the tiny wallaby fetus can be seen up to three days before birth. Hence, it practices the climbing motion in utero required to crawl up its mum’s furry tummy to the pouch. The observations were made by first author Dr Barbara Drews in Germany.

Reproductive biologist Professor Marilyn Renfree of the Department of Zoology at the University of Melbourne, the senior author on the study, says it was the first time embryos of an Australian marsupial had been seen via ultrasound.

“Being able to see the developments of the tammar fetus in utero and its first movements that help its survival is an extraordinary thing to watch and provides insights into how mammals survive,” she says.

The findings, published in Nature’s open access journal Scientific Reports, show the tammar wallaby fetus at day 23 of the 26 day pregnancy repeatedly thrusting and grasping its left and right forelimbs in a coordinated effort to prepare for its remarkable journey from the birth canal to the pouch. 

The tammar wallaby is one of the smaller species within the genus Macropus, which includes 14 species of kangaroos and wallabies.

“It is a model species for understanding marsupial reproduction and evolution,” Professor Renfree says.

Dr Brandon Menzies, Centenary Fellow with the Department of Zoology who assisted the studies, says it was assumed previously that the tiny marsupial baby only started crawling instinctively after birth. 

“However, in terms of relative developmental stage, this new footage may represent the earliest coordinated behaviour observed in utero in any mammal and highlights the incredible adaptations of newborn marsupials to navigate their way safely to the pouch,” he says.

Professor Geoff Shaw, zoologist and co-author on the paper said the development of the tammar fetus at three days before birth was equivalent to a four to five week old human fetus. 

“Humans gain the ability to coordinate our limb movements at about week 12 of pregnancy, so the tiny wallaby is doing a lot more much earlier.

“They are doing “the great Australian crawl!” he says.

The study has also documented several other features of wallaby pregnancy that may be shared by all marsupials including a very tight developmental program with very little variation in active pregnancy length.

Other findings include observing coordinated movements of the uterine lining that roll the embryo within the uterus possibly to optimize the exchange of nutrients and gases between the mother and embryo.

Marsupials have a reproductive pattern characterised by particularly immature young at birth, and this new study casts new insights into the remarkable adaptations that allow the newborn to survive the journey from the birth canal to the pouch where most of its development will occur. 

http://www.zoology.unimelb.edu.au

Watch video of the tammar wallaby's intra-uterine growth at: 

http://www.bitly.com/wallabybaby