Surprised keepers at Taronga Zoo are celebrating the unexpected birth of an endangered Brush-tailed Rock-wallaby – more than a year after its father left the Zoo!

The joey recently started peeking out from mother Mica’s pouch to the surprise of keepers and delight of keen-eyed visitors.

“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.

The birth is the result of a phenomenon known as embryonic diapause, which enables certain mammals to extend their gestation period and time the birth of their young.

The reproductive strategy, which is used by a number of marsupial species including kangaroos, wallabies and wombats, usually occurs when adverse environmental conditions 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,” said Tony.

Experienced mother Mica was carrying another joey in her pouch up until August last year, some five months after the only resident male, Sam, had moved to another wildlife park. Keepers suspect that Mica mated with Sam soon after giving birth to the joey growing in her pouch, and the resulting embryo stayed dormant while her pouch was occupied.

Tony said keepers are yet to determine the sex of the surprise joey, but it appears to be very healthy and about six months of age.

“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,” he said.

Once abundant and widespread across the rocky country of south-eastern Australia, Brush-tailed Rock-wallabies are now listed as an endangered species in NSW. They were hunted extensively for their meat and fur in the early 1900s and today they continue to be threatened by habitat loss and introduced predators, such as foxes and feral cats.

Taronga is part of a coordinated program helping with the recovery of the species.

Visitors may spot the newest joey in Taronga’s Platypus Pools exhibit.