The tree-shrews are a small group of about 20 species of typically tree-dwelling small mammals in the families Tupaiidae and Ptilocercidae, found in south-east Asia from southern India and China to the islands of Indonesia and the Philippines. They are superficially squirrel-like in appearance, and the tree-shrew genus name Tupaia comes from “tupai”, the Malay word for squirrel. However, their evolutionary relationships to other mammals have been the subject of considerable debate for over a century, with different studies suggesting that they are either most closely related to primates or to the colugos or “flying lemurs”.
Despite their relatively low species diversity, tree-shrews are among the most remarkable and interesting small mammals. They have the highest brain to body mass ratio of any mammals, and display several other intriguing biological characteristics; for example, pen-tailed tree shrews (Ptilocercus lowii) have been used to investigate alcohol tolerance in humans, as they consume large amounts of fermented nectar yet do not exhibit signs of intoxication. Several tree-shrews have also developed symbiotic relationships with pitcher plants, feeding on nectar from the plants and in return defecating into them to supply them with much or all of their required nitrogen. It is highly likely that many more surprises await researchers investigating these enigmatic and understudied mammals.