Whether you're a researcher monitoring snow leopards in the Himalayas, their Amur relatives in Russia or their cousins on the African savannahs, the unique spot patterns on a leopard's coat are often the key to telling individual cats apart. Unless you're dealing with a "spotless" cat, that is.

Until recently, that problem that has been standing in the way of a group of researchers studying the leopards on the Malay Peninsula – where almost all of the big cats are jet black. "Not only is this a unique phenomenon for leopards, it is also possibly the only place in the world where an entire animal population is almost completely composed of the melanistic [black] form of a species," the researchers write in a study just published in the Journal of Wildlife Management.

Melanistic Leopard 2015 07 14
Image: James Cook University/Bill Laurance

When it comes to solitary and elusive animals like leopards, camera traps – which take a photo when the camera's sensor recognises movement or body heat – offer researchers an effective, non-invasive and affordable way of monitoring populations. Because each leopard has a unique spot pattern, researchers can ID individual cats from their camera trap photos, helping them estimate the population size of a species.

But camera traps weren't doing much good for the scientists tracking Malaysia's extremely rare black leopards: the cats' dark colouration made their spots virtually invisible in photos. The solution to this spotty dilemma turns out to have been pretty simple: jerry-rigging cameras in order to illuminate the leopards' hidden spots with infrared light.

“Most automatic cameras have an infrared flash, but it’s only activated at night”, explains Dr Gopalasamy Reuben Clements of Australia's James Cook University. “However, by blocking the camera’s light sensor, we can fool the camera into thinking it’s night even during the day, so it always flashes.”

The researchers found that by relying on images where an infrared flash had been used to reveal the leopards' rosette markings, they could accurately identify 94% of the cats in the north east of Peninsular Malaysia. Now, they hope the new method will help them assess leopard populations in other parts of the cats' range.  

Melanistic Leopard1 2015 07 14
Image: James Cook University/Bill Laurance
Melanistic Leopard3 2015 07 14
Image: James Cook University/Bill Laurance
Melanistic Leopard4 2015 07 14
Image: James Cook University/Bill Laurance

The hope is that accurate population data will inform future conservation efforts – and that's crucial for these rare and threatened cats. Not only is their habitat being swallowed up by palm and rubber plantations, but the leopards are also increasingly targeted by poachers.

“Many dead leopards bearing injuries inflicted by wire snares have been discovered in Malaysia,” warns James Cook University's Professor William Laurance. Leopard skins and body parts are also increasingly finding their way to wildlife trading markets in places such as the Myanmar-China border.

"Understanding how leopards are faring in an increasingly human-dominated world is vital," adds Laurie Hedges from the University of Nottingham in Malaysia, who was the lead author of the study. "This new approach gives us a novel tool to help save this unique and endangered animal."