The New Holland mouse (Pseudomys novaehollandiae) is one of Victoria’s threatened native rodents. The charismatic little species has only been recorded from 3 areas across the state in the past 15 years (blue dots in map below) whereas historically it was recorded from 10, including metropolitan Melbourne (red dots in map below). That’s why I’ve embarked on a PhD to determine the status of NHMs across Victoria, and help protect this species from further decline.
One of the greatest challenges for studying the status and conservation of New Holland mice (and many native Australian rodents) is that they can be very difficult to find; you can’t just see them in your binoculars or hear them calling in the bush. Adding to the challenge – many native rodents, including NHMs, go through natural periods where they persist in such low numbers that traditional survey efforts such as live trapping fail to detect them. New Holland Mice are also particularly fickle about their habitat preferences and may only persist locally for a few years before moving on, making their populations even more difficult to track.
That’s why I’ve been trialling the use of cameras to detect the New Holland mouse. Traditional live trapping can be a great method for detecting a species in an area, and it’s critical if I want to know about health and reproduction, estimate abundance, or get DNA samples. However, sometimes when a species is at low densities it takes a huge amount of effort to be reasonably confident that the species isn’t there, which in a world of limited time and funding drastically reduces the area you can survey; a real challenge when a species moves in the landscape. This is where cameras can come in handy – you set them once and rather than having to come back every morning and afternoon to check each trap, you can just leave them in place for weeks at a time. The animals are attracted to a tasty lure (I like to use peanut butter, oats, golden syrup and vanilla essence) and while they investigate, the camera senses the heat and motion and snaps a photo.
Cameras allow you to survey much wider areas, for longer periods of time with a fraction of the effort of live trapping – at least until you have to sift through the images and identify the animals. Once I know that NHMs are present in an area from the camera trapping, I can target those areas for live trapping to collect the rest of my data. My challenge, and the reason I did a camera trial, rather than just jumping straight into using cameras as a survey method, was identifying New Holland mice in the images.
Rodents tend to look very similar on camera, particularly if the images are in black and white. It doesn’t help that New Holland mice are about the same size as the non-native house mice (Mus musculus); they can be hard for some people to tell apart when they are holding them in their hand. Since the house mouse has infiltrated all known New Holland mouse habitat in Victoria I needed to tell them apart from less than perfect images in colour and in black and white.
Tens of thousands of images later, I can happily say that New Holland mice and house mice are distinguishable on both colour and black and white images. In the colour images the species can be distinguished by differences in colouration, but in black and white the distinction is all in the shape of the two rodents. New Holland mice have a much sturdier build, thick set neck and snubby nose, whereas house mice are much more slender with a pointed nose; sort of like the difference between rugby and footy players.
Now that I’ve got the IDs sorted, I’ll be using cameras (and live traps) to survey across Victoria and see where the New Holland mouse is persisting so that we can do our best to halt the species’ further decline.
Can you ID a New Holland mouse? Have a go with my quiz!
Originally published here