Tupaia is a treeshrew genus in the Tupaiidae family that was first described by Thomas Stamford Raffles in 1821. The name of this genus derives from the Malay word tupai meaning squirrels or small animals that resemble squirrels.
Raffles described the genus as having an elongated snout, 8 to 10 incisors, well developed limbs, five-toed naked feet, and the sole furnished with projecting pads and sharp claws, with a habit and tail of a squirrel.
Marcus Ward Lyon published a revision of the genus in 1913, and also noted the squirrel-like appearance of Tupaia species, which only lack the long black whiskers and have smaller ears. They donât have any markings on the face, the naked area of the nose is finely reticulated, an oblique stripe on the shoulder is more or less distinct, and the tail is haired but not tufted. The braincase is about as wide as the maxillary tooth row is long. The temporal fossa is smaller than the orbit. The dental formula is 22.214.171.124.1.3.3. The first pair of upper incisors is longer than the second, while the second pair of lower incisors is slightly larger than the first and third pairs. The lower canines are better developed than the upper ones and stand high above the adjacent premolars. The size of head, body and tail varies between species.
One outstanding characteristic of Tupaia species is their color vision. They have rod and cone visual receptors similar to primates and humans.
Classification and taxonomic history
When Diard and Duvaucel described the first specimen of the common treeshrew Tupaia glis in 1820, they considered it a species of Sorex.
Subsequent to Raffles description of the genus, the following additional Tupaia species were described:
- Large treeshrew Tupaia tana â" by Raffles in 1821
- Horsfield's treeshrew Tupaia javanica â" by Horsfield in 1821
- Northern treeshrew Tupaia belangeri â" by Wagner in 1841
- Striped treeshrew Tupaia dorsalis â" by Schlegel in 1857
- Ruddy treeshrew Tupaia splendidula â" by Gray in 1865
- Nicobar treeshrew Tupaia nicobarica â" by Zelebor in 1868
- Pygmy treeshrew Tupaia minor â" by GÃ¼nther in 1876
- Mountain treeshrew Tupaia montana â" by Thomas in 1892
- Painted treeshrew Tupaia picta â" by Thomas in 1892
- Long-footed treeshrew Tupaia longipes â" by Thomas in 1893
- Slender treeshrew Tupaia gracilis â" by Thomas in 1893
- Palawan treeshrew Tupaia palawanensis â" by Thomas in 1894
- Calamian treeshrew Tupaia mÃ¶llendorffi â" by Matschie in 1898
- Golden-bellied treeshrew Tupaia chrysogaster â" by Miller in 1903
- â Tupaia miocenica â" by Mein and Ginsburg in 1997
In the past, various authors proposed to place treeshrews in the ordinal rank Insectivora, or considered them close relatives of primates. Since 1972, the treeshrew families Tupaiidae and Ptilocercidae are grouped in the order Scandentia.
Distribution and habitat
Tupaia species range from northeastern India, Burma, and Nicobar Islands eastward to members of the Philippine Islands, and from central China south to Java, Borneo and Sumatra, including islands on the southwest coast. They do not occur on Celebes, nor on islands to the east of Java, with the possible exception of Bali. They inhabit the dense undergrowth of tropical forests. With the exception of Tupaia minor, they are primarily terrestrial and forage on the forest floor, usually below 1.5Â m (4.9Â ft). Since they are rarely seen crossing wide roads, it is likely that populations are negatively affected by fragmentation of forests caused by logging operations.
Ecology and behaviour
Early naturalists described wild-caught captive Tupaia specimens as restless, nervous and rapidly reacting to sounds and movements. Their auditory sensitivity is highly developed as the broad frequency range of their hearing reaches far into the ultrasonic.
The shape of the cheek-teeth of Tupaia species indicate that they are foremost insectivores. Captive specimens were reported to hunt ants, flies, crickets, grasshoppers, cockroaches and small beetles. They hold their food between the forelegs while sitting on their haunches. After feeding they smooth the head and face with both forepaws, and lick the lips and palms. They are also fond of water, both to drink and to bathe in. They fortify their diet with soft fruits that are mostly dispersed by birds. They swallow the pulp but reject fibrous components, which they cannot digest due to their long and small intestine and rudimentary cecum.
The well developed olfactory perception of treeshrews enables them to easily detect food among the leaf litter on the forest floor. Their sensitivity for odours coupled with scent-marking of their territories is important in their interaction with conspecifics.
Observations of Tupaia species in their natural habitat suggest that they usually form monogamous pairs. Social behaviour differs between species and the available food resources in their territories. Where food is adequate and sufficient, they tolerate conspecifics without engaging in territorial disputes. Common treeshrews observed in the Bukit Timah Nature Reserve defended diminishing food resources by chasing away residents of adjacent areas.
Birds of prey, snakes, and small carnivores are known to hunt treeshrews. Humans have no interest in killing them for food because of their unpleasant taste, and they are rarely seen as pests.
Reproduction and Development
Treeshrews share more similarity with rodents and squirrels than with primates in regards to their reproduction and development. In contrast to primates, who produce one baby with longer gestation periods, treeshrews generally have litters of 2-3 young and are only in utero for about 45 days. Female treeshrews give birth in nests made of many dry leaves, and are known to leave the young unattended while returning occasionally to give them milk. Parental care of Tupaia is relatively limited. The young remain in the nest for 33 days on average, developing gradually before they exit the nest. There are ten identified embryonic developmental stages in Tupaia. Baby tree shrews are sexually immature until about 90 days after birth.
Their morphological similarity to primates makes them an important model organism in human medical research. A study investigating the effects of the Borna disease virus on tree shrews has given new insight into neurological disease. Since Tupaia share so many similarities with primates yet are more abundant and have more plentiful progeny than them, there is increasing interest in using them as an alternative model for use as a model in human medical research. Successful psychosocial studies were carried out, and it was found that dramatic behavioral, neuroendocrinal, and physiological changes occurred in subordinate male Tupaia, similar to depressed human patients. Their susceptibility to viruses has also piqued interest in using them to study immune responses to infections such as hepatitis B. Tupaia have been used to overcome the limitations of using rodent models in the study of human biology and disease mechanisms, as well as the development of new drugs and diagnostic tools. Recent studies have utilized tree shrews to study infectious, metabolic, neurological, and psychiatric diseases as well as cancers.
In 2013, the Virology Journal published an article that documents the use of the Tupaia belangeri as medical models for the H1N1 influenza virus. This was advantageous because other possible candidates such as guinea pigs, rats, mice, and other rodents leave gaps in the information especially regarding clinical symptoms and transmission. Tupaia, on the other hand, display moderate systemic and respiratory symptoms as well as pathological changes in the respiratory tract, supporting its use as a beneficial model in H1N1 research.
As knowledge increases regarding the use of treeshrews as medical research models, so do the ethical implications regarding the care of these animals for research purposes.