A baby rock wallaby peeks out of its mother's pouch.
A baby rock wallaby peeks out of its mother's pouch. (Photo: Paul Fahy/Taronga Zoo)

The unexpected arrival of a newborn brush-tailed rock-wallaby at the Taronga Zoo has keepers celebrating as well as scratching their heads. Why? Because Mica, the mother, hasn't been in the company of a male rock-wallaby for more than a year! That's right — this bundle of joy came completely out of left field.

"We weren’t planning for another joey, so it was quite a shock when we started seeing something moving inside the pouch," said keeper Tony Britt-Lewis.

Here are the facts: Sam, the zoo's only resident male, was moved to a different wildlife park more than a year ago. Before he left, Mica had already been carrying another joey (which we wrote about last August) in her pouch.

So how did Mica manage to become pregnant again after Sam left? Have we witnessed an immaculate conception?

A baby rock wallaby and mother Mica
Who can resist a face like that?! (Photo: Paul Fahy/Taronga Zoo)

It's actually quite simple! The birth is the result of a fascinating biological mechanism known as embryonic diapause. While most mammals have a fairly rigid gestational period, wallabies and other marsupials are capable of postponing gestation in response to unfavorable environmental conditions that could threaten the survival of the mother and her newborn.

"It’s an interesting survival mechanism that allows the mother to delay the development of the embryo in drought conditions or if she already has a joey in the pouch," Britt-Lewis says.

In this case, keepers believe that shortly after Mica gave birth to the other joey last year, she and Sam mated again. However, because the pouch was occupied at the time, the resulting embryo stayed dormant. Once the other joey was old enough to leave Mica's pouch, the embryo then began to develop in the now-vacant pouch.

Fast-forward several months, and flabbergasted keepers were greeted by another fuzzy face peeking out of Mica's pouch.

"Can I sleep in just a little longer, mom?"
"Can I sleep in just a little longer, mom?" (Photo: Paul Fahy/Taronga Zoo)

Although the zoo isn't able to determine the sex of the little bundle of joy yet, Britt-Lewis explains that "Mica is a confident and attentive mum and her joey looks to be very strong. It shouldn’t be long before we start to see it venturing out of the pouch to take its first wobbly steps."

The birth of this tiny creature is more than just an funny story, though. It also represents hope for an endangered species. Brush-tailed rock-wallabies were once found all throughout New South Wales, Australia, but due to habitat loss, invasive predators and hunting, their numbers have been on a decline since the beginning of the 20th century.

It's lunch time for this adorable wallaby family.
It's lunch time for this adorable wallaby family. (Photo: Paul Fahy/Taronga Zoo)

Catie Leary ( @catieleary ) writes about science, travel, animals and the arts.