Western Swamp Tortoise breeding facility at the Perth Zoo
On April 21, a group of 17 members and friends met at Perth Zoo for a ‘Behind the Scenes’ tour of the Western Swamp Tortoise (Pseudemydura umbrina) captive breeding facility. Head keeper Bradie Durell met us inside the zoo gate and led us up into the ‘backblocks’ of the zoo, adjacent to the Onslow Street boundary. Amid understandably tight security, we were ushered into the enclosed area where our first port of call was the ponds where the hatchlings spend their first months of life. We were lucky enough to see some brand new young ones that were about two to seven days old and who were only the size of a ten-cent coin! Bradie showed us inside the hut where the eggs are taken for incubation in a drier-than-usual vermiculite substrate, soon after they are laid.
The adult animals spend the wetter months (technically called the hydro-period) feeding and breeding in a network of ponds. New format ponds are planned once funding comes through, apparently. During the drier time of the year, keepers move the animals to a separate, shaded aestivation area. Here they mimic wild behaviour by entering a torpid state under Banksia leaf litter and within artificial tunnels, emerging only periodically to drink from shallow water dishes provided. Presumably the latter behaviour would happen in the wild if a summer storm created puddles of water close to the tortoise’s aestivation place.
Bradie explained how carefully all the animals are monitored for correct breeding to give the best genetic diversity, though it seems that some females thwart the human attempts at control by retaining sperm from a previous mating and therefore the paternity of some hatchlings is uncertain. Early in the captive breeding program the keepers tried to boost the reproductive rate of the tortoises by providing year-round food and not placing the animals into aestivation. This resulted in lower fertility rates! It seems that they need to take that ‘time out’ in torpor to keep their metabolism correctly balanced. Now the keepers are sometimes able to get females to ‘double clutch’ and this has boosted numbers that can be released into the wild. In the most successful years there have been 60-70 new animals — which are usually 2-5 years old — released at reserves such as Twin Swamps or Ellen Brook. Lately, however, fertility rates in the captive animals have halved and the keepers and Recovery Team scientists are trying to determine what has caused this worrying drop.
After farewelling Bradie, the group made its way to the numbat enclosure, where their keeper had promised us a talk as part of her scheduled program. One of Western Australia’s rarest mammals and its faunal emblem, the endearing little numbat (Myrmecobius fasciatus) was as popular as ever, running over various logs and feeding from her dish while the keeper spoke. The numbat also has an intensive captive breeding program at the zoo and many young numbats have been released into the wild to boost dwindling numbers. Things don’t always go smoothly here either, as the keeper pointed out that the display female had recently lost her newborn babies for unknown reasons.
After a reviving coffee, the final port of call for the naturalists was the nocturnal house, where we viewed the tiny Narbarlek or pygmy rock wallaby (Petrogale concinna monastria), who has recently taken up residence at the zoo after being rescued from his dead mother’s pouch by a Broome wildlife carer. He could not be released to the wild because he lacked the survival skills needed, due unfortunately to being hand-reared. The positive side of this is that he can teach us about his kind at close quarters, which is an unusual opportunity. We also saw many other Australian nocturnal animals, including the Woylie, Chuditch, Rakali (Water Rat), Western Ringtail Possum, Bilby, Quokka and Ghost Bat.
Tanya Marwood