The tufted ground squirrel, Rheithrosciurus macrotis, is an elusive species living in the dense rainforests of Borneo. Few scientists have set eyes on this rare squirrel, currently listed as vulnerable, though according to folklore, the fierce squirrels have attacked and killed tusked deer called muntjacs to eat their stomach contents, liver, and heart. The squirrel waits on a low branch, then jumps on to the back of a passing muntjac and bites its jugular, forcing the larger animal to bleed out. A disemboweled deer or domestic chicken with none of its flesh eaten is a sure sign of a squirrel kill.

Tall tales aside, the tropical squirrel is also remarkable in many ways. Even though it’s only found on one Southeast Asian island (having colonized Borneo as early as 36 million years ago), its nearest living relatives are an ocean away in South America. Its penis bone (or baculum) and incisors, oddly enough, are like those of the genus Sciurus, common to Eurasia and the Americas, and unlike other Southeast Asian squirrels.

And then there’s the spectacularly large, club-shaped tail, first described in 1867. From head to body, the squirrel is about 35 centimeters long (and twice as large as most tree squirrels), yet with its tail, it can look as big as a silvered leaf monkey that’s over twice its size. The tail, which consists mostly of long fur, is sometimes used to adorn machetes of local hunters.

Over several years, a family of researchers -- Erik Meijaard of People and Nature Consulting International in Jakarta, remote sensing scientist Rona Dennis, and their daughter Emily Mae Meijaard of the British International School-Jakarta -- have collected seven camera trap photos of Rheithrosciurus. After measuring the tails and bodies of the individuals in the pictures from motion-activated cameras, the trio calculated an approximate tail volume of 130 percent relative to body volume. That means the volume of the tail is 30 percent larger than the volume of the body, suggesting that the tufted ground squirrel has the most voluminous tail relative to body size of all mammals. Their work was published in Taprobanica, a journal of Asian biodiversity, in June.

Here are some camera trap photos, one from Sarawak, Malaysia, and another from East Kalimantan, Indonesia, on the island of Borneo.

As a bushy-tailed comparison, the tail volume to body volume ratios are 100 percent in ring-tailed cats whose tails allow them to do cartwheels, common stripped possums with prehensile tails, and squirrel gliders who use them as rudders when gliding: Their tails all occupy the same volume as their bodies. Red squirrels are only 90 percent, and the red panda is just 40 percent.

Maintaining something so bushy must require some energetic sacrifices, so surely there must be some evolutionary reason to justify the expense. Researchers have previously proposed various functions for the tail: ranging from balance and temperature regulation to courtship and warning signals.

According to this latest work, the most logical explanation for the extravagant tail is defense against terrestrial feline predators like the Sunda clouded leopard, bay cat, or marbled cat. In flight, the big gray tail obscures the actual body. This confuses the predator in pursuit, and if that doesn’t work and the wild cats pounces, the tail would likely be the target of the strike, leaving the predator with a limited hold on the squirrel.

Updated July 15, 2014: Here are two more camera trap photos of the elusive squirrel.

[h/t Science]

Images: Joseph Wolf, Proceedings of the Zoological Society of London 1856 via Wikimedia (top) & Meijaard et al., 2014 (middle) & Integrated Conservation​ (bottom two)