In the rainforests of Borneo, whispers echo of a legendary vampire so fierce it can take down prey many times its size. It has saw-like teeth, consumes the blood and organs of its victims, and yet is rarely seen. What is this fearsome creature, you ask? Er, it’s a squirrel.

The Bornean tufted ground squirrel (Rheithrosciurus macrotis) got its menacing reputation back in 2014 after an article on the elusive critter in Science called them “vampire squirrels”. According to Science, local legends told tales of the squirrels perching on low branches to jump on unsuspecting deer, slashing their jugular veins, disemboweling them, and eating their organs.

Now, scientists have carried out the first-ever study on the feeding behavior of tufted ground squirrels (TGS) and revealed its dietary habits. R. macrotis does indeed have a highly specialized diet; it appears to feed almost exclusively on the hardest seeds found in the rainforest.

Researchers led by Andrew Marshall of the University of Michigan studied the elusive squirrels in their natural rainforest habitat in Gunung Palung National Park, Borneo, over a number of years, observing them feeding 79 times. Though the team never witnessed the squirrels attacking any muntjacs (an Asiatic deer about the size of a small dog), they were surprised to discover they had a highly specialized diet, feeding mainly on the seeds of C. decumanum and M. leptopoda, two plants known for having the hardest seeds in the forest. Compared to the multiple other animals whose feeding they observed, the researchers argue their findings suggest when it comes to feeding, the ground squirrel is the most specialized taxon in the forest.

"We’re talking seeds so hard that a strong human with a hammer would have to work pretty hard to get into one," Marshall told IFLScience. 

"We have close to the largest and smallest squirrel species in the world at our site, and a total diversity of at least a dozen species. This would presumably lead to intense competition and the need to specialize to carve out their own niche. The very hard seeds they consume are inaccessible to virtually all other species, meaning the competition for them would be more limited than it would be feeding on easier-to-process seeds and fruits." 

OK, so not a vampire, but the tufted ground squirrel is a truly weird creature in many other ways. "Their biogeographic history is an enigma," Marshall told IFLScience. "They are most closely related to South American squirrels and seem to have split from them over 8 million years ago. Presumably they would have had to colonize Southeast Asia across land, yet they left no fossil (or living) ancestors in North America or mainland Asia. It’s a genuine puzzle." 

It also has unusual teeth; long incisors in the upper and lower jaw that have carved ridges like a saw edge. "The sawlike teeth of TGS seem unlike anything recorded in other mammals," Marshall said. "[I]t, therefore, stands to reason that they are specialized adaptations to their unusually specialized diets."

Best of all, however, is the fact the tufted ground squirrel has the world's fluffiest tail. In fact, its tail is 30 percent more voluminous than the rest of its body, according to a 2014 study. 

The tufted squirrel is twice the size of regular tree squirrels, around 35 centimeters (13.8 inches) long, with its bushy, silvery tail adding a further 30 centimeters (11.8 inches). When moving, the tail obscures the body, making it appear much larger than it is, possibly to confuse predators like clouded leopards. It could also be to prevent predators from getting a good grasp, the long fluffy hair fooling them into thinking there was something solid to hold onto.

There are still so many things we don't know about this mysterious, elusive creature and time could be running out, the researchers warn. 

"With forests in Indonesia and across the tropics being lost at alarming rates we run the risk of losing species before we can collect even the most basic information about their ecology," Marshall and colleagues wrote in their study.

"R. macrotis possesses several highly unusual features that make it a legitimate subject of serious research attention. In this context, it is truly remarkable how little we know about the species – although our evidence suggests that 'assassin squirrel' would be a better moniker than 'vampire squirrel'." 

I don't know about you, but I am all for finding out more about the elusive Bornean assassin squirrel.