The Bornean tufted ground squirrel (Rheithrosciurus macrotis), also known as the groove-toothed squirrel, is a fascinating and elusive rodent native to the island of Borneo. Sporting a voluminous, club-shaped tail, the tufted ground squirrel carries the largest tail-to-body size ratio of any mammal. This striking animal has captured the attention of scientists and locals alike, not only for its unique physical characteristics but also for its mysterious behavior and the myths surrounding it.
Physical Characteristics
The tufted ground squirrel is a medium-sized squirrel with a head-and-body length of around 34 cm, with the tail almost as long, bringing their total length to nearly 70 cm! Weighing in at around 1-2 kg (2.2-4.4 lb), their bodies are overshadowed by a fluffy, charcoal-and-white frosted tail that’s 130% the volume of their body. Rheithrosciurus is noted for having the largest known tail to body size ratio of any mammal, with the volume of its tail including the air included in the fluff being 130% of the volume of its body, compared to 90% for the red squirrel. Its dorsum is predominantly brown with a reddish tone, and they sport dramatic tufts of dark fur on their ears, giving them a striking appearance. The squirrel's head and body measure about 335-352 mm (13.2-13.9 in) long, with the tail measuring a further 299-342 mm (11.8-13.5 in) long. Its incisors have 7-10 distinctive longitudinal grooves. Its skull is also distinctive, being longer and flatter than most squirrels.
Scientists believe the tail could serve to confuse predators or attract mates. 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. It may alternatively have a function in communicating with other squirrels or in courtship.
Their sharp incisors feature 7-10 saw-like grooves that are perfect for cracking hard nuts. "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."
Habitat and Distribution
Glimpsing Tufted Ground Squirrels in their natural home is exceedingly rate. The species is known to live only on hillsides in lowland primary forest on the island of Borneo, at altitudes of under 1,100 metres (3,600 ft). They are found only on the island of Borneo and prefer dense lowland primary forests up to 1,100 meters in elevation. However, they will sometimes venture into secondary forests, orchards or smallholdings. They prefer hillsides in lowland primary forests, orchards, and secondary forests. The tufted ground squirrel is medium-sized, with soft, grayish-brown fur and a white underside. This elusive creature is primarily found in lowland and montane forests, often near rivers or in areas with thick vegetation.
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Diet and Feeding Behavior
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. The tufted ground squirrel’s diet includes mostly hard seeds, particularly those of the Canarium tree. They also eat fruits, nuts, and insects, adapting to the available food sources in their forest habitat. Despite their sharp teeth, tales of their carnivorous nature have led to them being dubbed “vampire squirrels”, however these claims were subsequently discovered to be lacking in evidence.
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."
The "Vampire Squirrel" Myth
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. 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.
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”. The squirrel is locally reputed to be a carnivore and has been dubbed the "vampire squirrel". According to forest-dwelling Dayak hunters, it attacks and kills deer to eat their stomach contents, liver and heart. The squirrel is said to achieve this by waiting on low branches, jumping on passing deer and biting their jugular veins to make them bleed to death. Once the deer dies, the squirrel eats the deer's internal organs. The hunters report occasionally finding deer disembowelled in this way and attribute them to "squirrel kills". It is also said to kill domestic chickens and eat their hearts and livers.
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Despite their sharp teeth, tales of their carnivorous nature have led to them being dubbed “vampire squirrels”, however these claims were subsequently discovered to be lacking in evidence. “Vampire Squirrel” myth is patently untrue: Though local folklore suggests that these squirrels attack deer from above to feast on their organs, researchers have yet to find scientific proof.
Conservation Status and Threats
Due to habitat destruction at the hands of the agricultural and logging industries; hunting for their meat; and trapping for their fur, these cool squirrels are currently listed as Vulnerable by the IUCN. They are listed as Vulnerable on the IUCN Red List and are threatened by deforestation, hunting, and forest conversion to palm oil. Due to deforestation it is considered to be vulnerable and is totally protected in Sarawak, one of the two Malaysian states on Borneo. This animal has no protections in place.
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.
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Other Interesting Facts
It also has unusual teeth; long incisors in the upper and lower jaw that have carved ridges like a saw edge.
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"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."
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).
Rheithrosciurus is believed to forage on the ground and the lower canopy, holding its tail high over its head as it does so, and also climbs high trees to seek food.