PURPLE-NECKED ROCK-WALLABIES
Purple-necked Rock-wallabies (PNRW; Petrogale purpureicollis) are a medium to large sized rock-wallaby found in the North West Highlands of Queensland. It appears to be the world's only mammal that has purple fur.
PNRW are light brown in colour with a characteristic light purple to rich-red colour on their head, sides of face and neck. For a long time after PNRW were first described by Europeans, people assumed the rock-wallaby had just rubbed against a coloured rock. It's now known that the purple colour is actually secreted from the skin. The adaptive function of the colour is unknown, but given that it's generally more obvious in males than females, it may have something to do with mate choice.
PNRW inhabit granite, sandstone and limestone rock piles, cliffs, crevices and scree slopes. They are extremely comfortable and confident in their rocky habitat. We have witnessed them navigating an almost vertical cliff face with ease. They eat a variety of plants, some of which include the fruits and leaves of Terminalia aridicola, Triodia sp, legumes such as Rynchosia minima, Portulaca oleracea, Ficus sp and various other grasses.
Papp, I., 2023. Yalarrnga cross-cultural investigation of diet, management history and threats to Petrogale purpureicollis (Doctoral dissertation, Macquarie University).
PNRW on the limestone cliffs at Caves Creek, Chidna Station.
PNRW on the limestone cliffs at Caves Creek, Chidna Station.
Setting a Thomas trap to catch PNRW.
Left: A GPS collar being placed on a PNRW during a field trip in 2024.
Right: Taking the tail measurement of a PNRW. Photo credit Braydon Moloney.
Left: PNRW jumping away after being fitted with a GPS Collar.
Right: Rachel releasing a PNRW after processing.
Collared PNRW on Chidna Station.
The data points from our GPS tracking of the seven collared PNRW on Chidna.
GPS point locations for a female PNRW before (green) and after (orange) a fire.
Left: Rachel installing a camera to capture tagged PNRW.
Right: An ear tagged PNRW caught on camera.
Surprisingly, there is very little known about the ecology of PNRW. Our surveys show that Chidna Station supports a large and extensive population, and should be considered the species' stronghold. Station owner Brussie Spreadborough strongly believes the homes of the PNRW on Chidna Station should be protected.
Our aim at Chidna is to manage the landscape so that PNRWs continue to thrive. To do this, we need to learn more about how they use the landscape, how they respond to fires and their overall survivorship.
The main threat to PNRW is habitat loss from mining, and this applies in a big way to Chidna. Large fires are also a threat, mainly because they may force individuals to forage out in the open, which exposes them to high levels of predation. However, fire can also be a good thing, because it can rejuvenate feeding areas by promoting fire loving plants.
GPS TRACKING
We are building our knowledge about PNRW by tracking them with GPS collars. In order to attach GPS collars, we must first capture a wallaby, which we do using a "Thomas" trap. These consist of a wire frame with a shade-cloth bag. They have to be securely fastened to the ground or sturdy trees, so as the trap does not move once a wallaby is caught. They are baited with apple, peanut butter, oats and lucerne.
Once captured, the animal is weighed, morphometrics and a genetic sample are taken and a GPS collar or other tag is fitted.
PNRW are processed on site and are released exactly where they were caught. Handling time is kept to a minimum and so the wallaby is safely hopping away after a few minutes.
The LiteTrack60 RF collars that are fitted to the PNRW store location information on board which we can access and transfer to our receiver when we are within range. The collar is powered by a battery, seen under the chin, and has an auto-drop off mechanism so recapture and removal of the collar is unnecessary.
We have fitted GPS collars on 7 wallabies across 4 sites on Chidna. This has provided detailed information on which habitats the PNRW are using, how they react to fire when their habitat is burnt, how far they travel, how they use water, and the area needed for their home range. The collars take GPS points every 10 minutes for a whole night, every 5 days. Each collar works for an entire year, so we are able to gather seasonal information about their movements as well. Currently we have retrieved 4 collars from the field and another three are still out there collecting valuable information. Preliminary analysis shows PNRW to be very site faithful and home ranges appear to be smaller for females compared to males, 11-ha versus 30-ha.
HOT FIRES
In December 2023, large fires impacted many parts of PNRW habitat on Chidna, including two individuals that were wearing GPS collars. This gave us a valuable opportunity to assess the impact of this wildfire. First, neither of the collared wallabies perished in the fire. The map below shows the GPS locations for one female wallaby. The green dots are her pre-fire locations, and the orange dots post-fire. She had 33% of her home range burnt but her overall home range size didn't change significantly. Interestingly, she was using the burnt area within a week of the fire occurring.
INDIVIDUALLY MARKING PNRW
On Chidna we are individually marking animals using PIT ("microchip") and ear tags. Individuals are then monitored using PIT scanners and cameras (for recording ear tags). This allows us to follow the progress of individuals and obtain information about survivorship and population size through time. The more PNRW we can mark, the more we can understand about the ecology of PNRW and therefore more accurately inform our management actions.
While the population of PNRW on Chidna station appears to be very healthy, future pressures such as mining, repeated wildfire and climate change are likely to impact them. Our work at Chidna is creating an invaluable baseline of information in the face of these threats. Stay tuned for more updates about Chidna's PNRW, as we continue to receive data over the next few months.
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