Introduction
Treeshrews are small squirrel-like mammals native to the tropical regions of Southeast Asia (Figure 1). Due to its close genetic relationship to primates, treeshrews have been used as an alternative medical experimental animal to nonhuman primates, and have since received extensive attentions.

Figure 1 Photos of treeshrews

1 Characteristic Description
Treeshrew is a slender mammal having a long tail and grayish to reddish-brown soft fur. The body size of terrestrial species seems larger than that of the arboreal species; the former has larger claws for digging up insects. The treeshrews were omnivorous animals feeding on insects, small vertebrates, fruits and seeds. The dental formula of treeshrews in general is with poorly developed canine teeth (small) and the unspecialised molars (Hutterer et al., 1984). The lateral neck stripes of treeshrews are one of the hallmarks distinguishing from other species.

Arboreal treeshrews are binocular so they may have good visual sense; most treeshrews are diurnal except the pen-tailed treeshrew, which is nocturnal.

Female treeshrews reach sexual maturity about four months after birth, most treeshrew species have no definite breeding season, but breeding mostly occurs from February to July. The gestation period lasts about 45-50 days, and they can give birth to three or more litters per pregnancy. The newborns blind and hairless and is able to leave the nest after about a month. During lactation, maternal care from the mother treeshrew relatively little, only a few minutes are spent every other day on suckle young (Hutterer et al., 1984).

Treeshrews live together in small family groups, and to defend their territory from outsiders’ invasion, they often use a variety of odor glands or urine to mark their territory.

2 Species Taxonomy
Treeshrews belong in Scandentia order with two families: Tupaiidae and Ptilocercidae; there are five genera know as Anathana, Dendrogale, Tupaia, Urogale and ptilocercus which consist of 20 species (Figure 2). Genus Tupaia having 15 species is the largest genus in the order. The generic name of Tupaia came from Malay word tupai, which means of squirrel (Nowak and Paradiso, 1999). Northern treeshrew (Tupaia belangeri) and common treeshrew (Tupaia glis) are two of most famous species in the Tupaia genus.

Figure 2 Species of treeshrews in Order Scandentia (Pettigrew et al., 1989; Janecka et al., 2007)

Northern Treeshrew (Tupaia belangeri), also known as the Burmese treeshrew, can be found in countries in Southeast Asia. In mainland China, Northern treeshrew can be found in Guang Xi, Hai Nan, Gui Zhou, Yun Nan, Si Chuan, Tibet and other regions, living mainly near tropical and sub-tropical forests, bushes, and villages. Type locality for this species is close to Rangoon, Myanmar. There have been 8 Northern treeshrew subspecies named, among which Tupaia belangeri chinensis, also known as the Chinese treeshrew was named by Anderson in 1879. Chinese treeshrews are found in Si Chuan (Southwestern region), Yun Nan and other parts of mainland China. Type locality for Chinese treeshrew is Bang Xi and Sang Da Valley of Yun Nan Province. Northern treeshrew has a body shape similar to squirrels, with body length of 190 mm to 200 mm, and tail length of around 160 mm. Its tail region is rich in furs, which disperse toward each side. Adult Northern treeshrew has body weights ranging from 120 g to 150 g. Front and hind feet both contain 5 digits with well-developed and sharp claws. It also has pointy and long mouth region, and relatively short ears. The eye sockets of its cranium protrude well toward the back forming boney eye balls and a relatively large cranial cavity. Fur colour of type subspecies is reddish brown, with light grey coloured furs in the region below its jaw and in the abdominal region.

Tupaia glis, also known as common treeshrew, is a small mammal in the Tupaiidae family. It originates from Thailand, Malaysia, and Indonesia. Common treeshrew has one of the largest body sizes among all treeshrew species. Common treeshrew has an average body length of 160 mm to 210 mm, and an average body weight of approximately 190 g. It has different coloured furs, with reddish brown, grey, or black colour for back region, and white for abdominal region. Its long furry tail appears to be greyish brown colour, and can reach a full body length. The paws are not covered by furs and are armed with sharp nails. A region right above its long nose is bare without any fur. Male and female have similar physiology. According to data gathered from 21 common treeshrew samples, the body lengths range from 170 mm to 235 mm; tail lengths range from 170 mm to 242 mm, and hind foot lengths range from 45 mm to 56 mm.

3 Classification Controversy
The Scandentia order was once placed in the Insectivora and Primates order. The members of Scandentia have similar habits and appearance to Insectivora, but their skull characteristics are similar to those found in some primitive prosimian, so there has been a great controversy as to whether treeshrews should be placed under Insectivora or Primates. The Insectivora order was later found to be a polyphyletic group, its members have become independent or been placed in orders one after another (Bartolomucci et al., 2002), instead, the Scandentia order was eatablished and was placed under the clade Euarchonta the same rank (order) as the primates according to molecular studies, closely related to Dermoptera and Primates (Figure 3).

Figure 3 Polyphyletic tree of Scandentia

4 As an Experimental Animals
In view of the closely relationship between treeshrews and primates, and treesthew’s unique characteristics suitable for laboratory animal, such as relatively small body mass, high brain-to-body mass ratio, short reproductive cycle and life span, treeshrews have been proposed to be used as an experimental animal alternative to nonhuman primates. For the past decades, treeshrews as an alternative animal model has been widely applied in biomedical research and medicine safety testing (Cao et al., 2003).

So far, many progresses have been archive on establishing treeshrews as an animal modle for medical research. Two human virus infection models, hepatitis C virus (HCV) and hepatitis B virus (HBV), have been established (Yan et al., 1996, Zhao et al., 2002,); myopia research model (Norton et al., 2006), social stress and depression research model (Fuchs, 2005; Kampen et al., 2002) and the models of aging and learning behaviors (Bartolomucci et al., 2002) have been reported as well. Obviously, with the advances in modern molecular biology, especially in the development of next-generation genome sequencing technology, the deciphering of the genetic code of treeshrews will make treeshrews become an important alternative for medical research and drug trials alternative to nonhuman primates.

References
Bartolomucci A., Biurrun G. D., Czeh B., Kampen M. V., and Fuchs E., 2002, Selective enhancement of spatial learning under chronic psychosocial stress, Eur. J. Neurosci., 15(11): 1863-1866
http://dx.doi.org/10.1046/j.1460-9568.2002.02043.x

Cao J., Yang E. B., Su J. J., Li Y., and Chow P., 2003, The tree shrews: adjuncts and alternatives to primates as models for biomedical research, J. Med. Primatol., 32(3): 123-130
http://dx.doi.org/10.1034/j.1600-0684.2003.00022.x

Fuchs E., 2005, Social stress in tree shrews as an animal model of depression: an example of a behavioral model of a CNS disorder, CNS Spectr., 10(3): 182-190
PMid:15744220

Hutterer R., Wilson D. E., and Reeder D. M., 1984, Mammal Species of the World, 3rd edition, pp. 440–445

Janecka J. E., Miller W., Pringle T. H., Wiens F., Zitzmann A., Helgen K. M., Springer M. S., Murphy W. J., 2007, Molecular and Genomic Data Identify The Closest Living Relatives of Primates, Science, 318(5851): 792-794
http://dx.doi.org/10.1126/science.1147555

Norton T. T., Amedo A. O., and Siegwart Jr J. T., 2006, Darkness causes myopia in visually experienced tree shrews, Invest. Ophthalmol. Vis. Sci., 47(11): 4700–4707
http://dx.doi.org/10.1167/iovs.05-1641

Nowak R. M.Paradiso J. L., 1999, Walker's Mammals of the World, Johns Hopkins University, pp.245

Pettigrew J. D.,F. R. S., Jamieson B. G. M., Robson S. K., Hall L. S., McAnally K. I., and Cooper H. M., 1989, Phylogenetic relations between microbats, megabats and primates (Mammalia: Chiroptera and Primates), Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences, 325(1229): 489-559
http://dx.doi.org/10.1098/rstb.1989.0102

Kampen M. V., Kramer M., Hiemke C., Flugge G., and Fuchs E., 2002, The chronic psychosocial stress paradigm in male tree shrews: evaluation of a novel animal model for depressive disorders, Stress, 5(1): 37-46
http://dx.doi.org/10.1080/102538902900012396

Yan R. Q., Su J. J., Huang D. R., Gan Y. C., Yang C., and Huang G. H., 1996, Human hepatitis B virus and hepatocellular carcinoma. I. Experimental infection of tree shrews with hepatitis B virus, J. Cancer Res. Clin. Oncol, 122(5): 283-288
http://dx.doi.org/10.1007/BF01261404

Zhao X. P., Tang Z. Y., Klumpp B., Wolff-Vorbeck G., Barth H., Levy S., Weizsäcker F. V., Blum H. E., and Baumert T. F., 2002, Primary hepatocytes of Tupaiabelangeri as a potential model forhepatitis C virus infection, J. Clin. Invest., 109(2): 221-232,
http://dx.doi.org/10.1172/JCI13011