Fluffiest mammal tail
tufted ground squirrel Rheithrosciurus macrotis
130 percentage
Indonesia ()

The mammal species with the fluffiest tail is the tufted ground squirrel (Rheithrosciurus macrotis), native to the island of Borneo. Its exceptionally bushy, fluffy tail is estimated to be 130% the volume of the rest of its body, giving this species the largest tail-to-body ratio of any known mammal. Its closest competitors for this record include Australia's striped possums and squirrel gliders, as well as the New World's raccoon-related cacomistles or ring-tailed cats, but in all of these cases the volume of their tail is only equal to that of the rest of their body, rather than exceeding it.

The purpose of this squirrel's exceptionally fluffy tail is not clear. However, researchers suspect that it may serve to make the squirrel look bigger (and therefore potentially more threatening) than it actually is to potential predators like clouded leopards and other carnivores. It may also make the squirrel more difficult to hold on to by such predators than would otherwise be the case if its tail were smaller and less fluffy.

Also of interest but currently unconfirmed scientifically is the startling claim by local hunters in Indonesian Borneo (Kalimantan) that this squirrel species is actually a bloodthirsty carnivore that kills chickens and even small muntjac deer by slashing their throat and then drinking their blood as well as eating their internal organs, before leaving the rest of their carcass to rot.

The study that assessed the size of the tufted ground squirrel's tail – along with several other species – was published in the journal TAPROBANICA on 26 June 2014. The study was led by then-15-year-old Emily Mae Meijaard (Netherlands), assisted by her parents Erik Meijaard and Rona Anne Dennis.