The tufted ground squirrel or groove-toothed squirrel (Rheithrosciurus macrotis ) is a species of rodent in the family Sciuridae. It is the sole species in the genus Rheithrosciurus. It is found only on the island of Borneo. Confirmed elements of its diet include nuts, seeds and insects, for which it has been filmed foraging on the forest floor.
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. It weighs about 1–2 kg (2.2–4.4 lb). Its incisors have 7-10 distinctive longitudinal grooves. Its skull is also distinctive, being longer and flatter than most squirrels. The animal's dorsum is predominantly brown with a reddish tone, and it has unusually hairy ears with large red to dark brown tufts. A longitudinal stripe of a white to buff to yellow colour, sometimes accompanied by a dark brown stripe, runs along the flank.
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. (Compare 90% for the red squirrel.) It is unclear why the squirrel has such a large tail but scientists have suggested that it may have evolved to distract predators or to prevent them getting a firm grasp when attacking. It may alternatively have a function in communicating with other squirrels or in courtship. Other possible explanations, such as being used to keep the animal warm or for balance, seem unlikely as the squirrel lives on the ground in a warm region. The tail has a grizzled charcoal colour with white frosting and rises in a plume, with the longest hairs at the tip.
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). It has occasionally been seen in orchards and secondary forests but sightings are rare. Due to deforestation it is considered to be vulnerable and is totally protected in Sarawak, one of the two Malaysian states on Borneo. Hunting with a licence is legal in Sabah, the island's other Malaysian state. Some natives use the squirrel's tail to decorate the hilt of their parang knives.
The Bornean tufted ground squirrel (Rheithrosciurus macrotis) got quite a 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!