Perhaps the most important initial observation to be made about tree shrews is that they are neither shrews nor do they spend most of their time in the trees. Instead, these small, brown, diurnal, squirrel-like denizens of Southeast Asian rainforests are a mostly terrestrial family and do not seem to be closely related to any extant taxon. They have a generalized and in some respects primitive morphology, which allows them to be proficient at running, jumping, and climbing on many different substrates [1]. In part because of their sparse fossil record, the phylogenetic placement of tree shrews is a hotly debated topic in mammalian evolution. Tree shrews were initially placed in the insectivorous group Menotyphla. However, detailed analysis of anatomy convinced most scientists that these animals were close enough to extant Primates to be a part of the order [2]. Subsequent analysis showed that tree shrews do not share many convincing shared derived features with Primates [3]. Thus they have been recognized as a close relative to Primates in their own order, Scandentia. Much of the attention given to this obscure group of Southeast Asian mammals is due to their proposed relationship to Primates. In addition, arguments have been particularly caustic because of the equivocal nature of several key pieces of information such as: a lack of a good fossil record; their mix of primitive and derived features; and the long time since the split of their lineage from any living sister taxon. Tree shrews have been traditionally grouped with bats, flying lemurs, and primates in the superorder Archonta. This arrangement is often supported by comparing isolated systems from extant species only. Recently Eric Sargis analyzed the axial skeleton of 14 tupaiids, and found that Ptilocercus shared many features with dermopterans and chiropterans. He concluded that Ptilocercus is the best model for the axial skeleton of early archontans [4]. However, studies including both fossil and extant species fail to find support for Archonta [5]. Similarly, the majority of available molecular work also denies Archonta as a natural group. For instance a preliminary study of the entire mitochondrial genome of Tupaia belangeri shows that under a variety of assumptions Tupaia is not closely related to primates. Instead, tree shrews, rabbits, ferungulates, and armadillos are more closely related to each other than any other eutherians. While the exact relationships among these taxa are not yet resolvable, it currently appears that Tupaia is most closely related to Lagomorpha [6].

Tree shrews are divided into two subfamilies: Ptilocercinae, containing the genus Ptilocercus, and Tupaiinae, comprised of Anathana, Dendrogale, Tupaia, and Urogale. The specimen available for dissection is the laboratory species, Tupaia belangeri. The laboratory stock was brought over from Asia, probably Thailand, and bred from individuals from the closely related T. glis, T. belangeri, and T. chinensis [1]. While it is not known for certain which species were originally used, the current lab animals seem to most closely resemble T. belangeri. Many lab studies of T. belangeri and a few field studies of T. glis and the closely related T. longipes have helped illuminate the behavior and ecology of this enigmatic mammal.

While T. belangeri is capable of some degree of arboreality, it is most often found on or near ground level for most of the year. During dry months, when leaf litter desiccation reduces the amount of prey available on the ground, they are found more frequently in the trees. Insects, mainly coleopterans, were found in almost all of the stomachs and feces of captured animals. Fruit and fibrous material were both present, but in greatly reduced proportions relative to insect remains [7].
While usually a very timid animal, T. belangeri are aggressively territorial, and will even kill cage-mates in captivity [8]. Chasing, vocalizing, posturing, and biting are all common during tree shrew fights [9]. Territories are marked by scent glands located in the abdomen and chin [10]. For the most part, male and female territories overlap to the point that they can be considered monogamous pairs, as females keep other females away and males chase off other males. Although males and females do not interact often, they will share a burrow at night. While some tree shrews use burrows dug by other species, there are some reports of digging [1].
Young are produced shortly after the mild south-west monsoons, during the time of greatest insect abundance. Females usually have a litter size of two and commonly breed once a year. The gestation period is 40-52 days at the end of which young are secured in a nest isolated from the parental nest. Nests are composed of an inner layer of fibrous material surrounded by a layer of crudely overlapped large leaves. Mothers are unique among extant mammals in that they do not groom their young, clean the nest, or rescue their young [1]. They visit their offspring once every 48 hours to nurse for about 5 minutes. The young are fed a rich diet of 10% protein and 25% fat allowing them to grow from 10g at birth to 60-80g at weaning [10]. Small social groups composed of parents and their offspring are seen until the young reach sexual maturity.

Mortality seems to be most common for adults during the north-east monsoon season. It appears that high rain and reduced food supplies, not predation, have the greatest impact on survival. Animals in their first year seem to disappear most frequently during the six-month breeding period. It is not clear whether this is due to territorial displacement by adults or simply related to the recently attained independence from parents [7].

References

1 Emmons, L.H. 2000. Tupai: A Field Study of Bornean Treeshrews.
2 Le Gros Clark, W.E.1925. On the skull of Tupaia. Proceedings of the Zoological Society of London 1925: 559-567.
3 Luckett, W.P., ed. 1980. Comparative biology and evolutionary relationships of tree shrews. New York: Plenum Press.
4 Sargis, E.J. 2000. A preliminary qualitative analysis of the axial skeleton of tupaiids (Mammalia, Scandentia): functional morphology and phylogenetic implications. J. Zool., Lond 253: 473-483.
5 Kay, R.F., R.W. Thorington, P. Houde. 1990. Eocene plesiadapiform shows affinities with flying lemurs not primates. Nature 345:342-344.
6 Schmitz, J., M. Ohme, and H. Zischer. 2000. The Complete Mitochondrial Genome of Tupaia belangeri and the Phylogenetic Affiliation of Scandentia to Other Eutherian Orders. Mol. Biol. Evol. 17(9):1334-1343.
7 Langham, N.P.E. 1982. The ecology of the common tree shrew, Tupaia glis, in peninsular Malaysia. J. Zool., Lond 197:323-44.
8 Hendrickson, J.R. 1954. Breeding of the tree shrew. Nature, Lond 174: 794-795.
9 Sorenson, M.W. 1974. A review of the aggressive behavior in the tree shrews. In Primate aggression, territoriality and xenophobia: a comparative perspective: 13-29. Holloway, R. ed. New York: Academic Press.
10 Martin, R.D. 1968. Reproduction and ontogeny of tree shrews (Tupaia belangeri), with reference to their general beaviour and taxonomic relationships. Z. Terpsychol. 25:409-532.