My own squirrel babies (actually juveniles now) go by the name of “Tufty” because of their fluffy tails. (There was once a male named Tufty E., but now there are three siblings, including two females that I call Tuftina and Tuftette.) But their tails are as nothing compared to those of a ground squirrel from Borneo described in a new paper in Taprobanica by Emily Mae Meijaard et al. (reference and free download below; see also report in Science).
The squirrel has been known for a while: it’s the largest squirrel in Borneo, lives mostly on the ground, and its Latin name is Rheithoscirus macrotis. The researchers, from Australia and Jarkarta, note that its closest living relative is in South America, bespeaking either an unknown extinct ancestor in Asia (likely) or a long-distance migration event (unlikely).
What’s remarkable about this rodent is that it has the fluffiest tail of any squirrel—in fact, the fluffiest tail (measured by relative volume) of any animal in the world.
It’s not easy to find, and over many years the authors got exactly 7 camera-trap photos of the best. I’ve put three below. The first picture shows the critter looking almost straight on, with its tail plumed out. Look at that monstrous tuft!!!
Jebus! Using measurements from the photos, the authors calculate that the squirrel has a volume 130% that of the rest of its body. That’s the biggest tail volume/body volume of any mammal—by far.
The table below gives its competitors in relative tail volume, along with the putative function of the voluminous tail. The red squirrel is the closest, but it’s lame: the tail is only 90% the volume of its body.
Of course we wonder why it has this huge plume, especially for a frugivore (fruit eater) which lives on the ground. The authors examine various hypotheses, including warmth (not viable since it’s not hot where it lives), and signalling (perhaps). Reader Malcolm, who called this article to my attention, suggested sexual selection. That’s a viable theory, but would be more viable if we knew that males had larger tails than females, and there’s no evidence of that.
The most viable hypothesis, according to the authors, is defense: it may confuse predators in pursuit (there are several species of felids on Borneo, including the clouded leopard and marbled cat), or make it easier for the squirrel to escape if it’s grabbed by the tail. The authors admit, however, that this hypothesis is speculative.
An interesting sidelight is that there are some hints that this animal may be partly carnivorous. The end of the paper gives some anecdotal evidence which has led to this animal being nicknamed “the vampire squirrel”:
It will not be easy to study this uncommon and elusive species on the ground in dense rainforests where direct observation would be much hampered. Some insights may be obtained from talking to local hunters who may have more frequently observed the species, either at rest or in flight when chased by a hunter or his dogs. In fact, there is a relatively rich animal folklore regarding Rheithrosciurus. People of Borneo have traditionally hunted the species for meat and ornamental use of its tail, among others to adorn machetes (Banks, 1931). Remarkably for a squirrel, forest-dwelling people consider this squirrel to be quite fierce (J. Payne, pers. comm.). One of us (RD) was in fact told a story by a local hunter in northern Central Kalimantan (Indonesian Borneo) of Rheithrosciurus attacking and killing deer to eat the ruminant’s stomach contents, liver and heart. On asking how a squirrel could kill a large animal such as a muntjak, Muntiacus muntjak, the response was that the squirrel waits on a low branch for a deer to pass below, jumps on its back and bites the jugular vein, whereon the deer bleeds to death. Once dead the squirrel proceeds to disembowel the deer and eat the stomach contents, heart and liver. Dayak hunters sometimes find these disembowelled deer in the forest, none of the flesh eaten, which to them is a clear sign of a squirrel kill. In villages close to the forest edge there were also accounts of the squirrel killing domestic chickens and eating the heart and liver only. Although, the existence of carnivorous squirrels might be a bit hard to believe, the above might fit the description of Banks (1949), who notes that Rheithrosciurus is known as being “wary, difficult to observe and biting fiercely”.
I have my doubts about this thing biting jugular veins.
h/t: Malcolm via Doubtful News
Meijaard, E. M. R. A. Dennis, and E. Meijaard. 2014. Tall tales of a tropical squirrel. Taprobanica 6:27-31