Though called 'treeshrews', and despite having previously been classified inInsectivora, they are not trueshrews, and not all specieslive in trees. They areomnivores; among other things, treeshrews eat fruit. As fellow members ofEuarchonta, treeshrews are closely related toprimates, and have been used as an alternative to primates in experimental studies ofmyopia, psychosocial stress, andhepatitis.[5]
Treeshrews are slender animals with long tails and soft, greyish to reddish-brown fur. The terrestrial species tend to be larger than the arboreal forms, and to have larger claws, which they use for digging up insect prey. They have poorly developed canine teeth and unspecialised molars, with an overalldental formula of2.1.3.33.1.3.3[6] They have a higherbrain to body mass ratio than any other mammal, includinghumans,[7] but high ratios are not uncommon for animals weighing less than 1 kg (2 lb).
Treeshrews have goodvision, which isbinocular in the case of the more arboreal species.
Female treeshrews have a gestation period of 45–50 days and give birth to up to three young in nests lined with dry leaves inside tree hollows. The young are born blind and hairless, but are able to leave the nest after about a month. During this period, the mother provides relatively little maternal care, visiting her young only for a few minutes every other day to suckle them.
Treeshrews reach sexual maturity after around four months, and breed for much of the year, with no clear breeding season in most species.[6]
Treeshrews areomnivorous, feeding on insects, small vertebrates, fruit, and seeds. Among other things, treeshrews eatRafflesia fruit.
The pen-tailed treeshrew in Malaysia is able to consume large amounts of naturally fermented nectar from flower buds of the bertam palmEugeissona tristis (with up to 3.8% alcohol content) the entire year without it having any effects on behaviour.[8][9]
Treeshrews have also been observed intentionally eating foods high incapsaicin, a behavior unique among mammals other than humans. A singleTRPV1mutation reduces their pain response to capsaicinoids, which scientists believe is anevolutionary adaptation to be able to consume spicy foods in their natural habitats.[10]
Pitcher plants like theNepenthes lowii, supplements its carnivorous diet with tree shrew droppings.[11][12][13]
Treeshrews were moved from the orderInsectivora into the orderPrimates because of certain internal similarities to primates (for example, similarities in thebrainanatomy, highlighted by SirWilfrid Le Gros Clark), and classified as a "primitiveprosimian", however they were soon split from the primates and moved into their ownorder. Taxonomists continue to refine the treeshrews' relations to primates and to other closely related clades.
Molecular phylogenetic studies have suggested that the treeshrews, with the primates and theflying lemurs (colugos), belong to thegrandorderEuarchonta. According to this classification, the Euarchonta are sister to theGlires (lagomorphs androdents), and the two groups are combined into thesuperorderEuarchontoglires.[14] However, the alternative placement of treeshrews as sister to both Glires andPrimatomorpha cannot be ruled out.[15] Some studies place Scandentia as sister of the Glires, which would invalidate Euarchonta: It is this organization that is shown in the tree diagram below.[16][15]
Several other arrangements of these orders have been proposed in the past, and the above tree is only a well-favored proposal.[17] Although it is known that Scandentia is one of the mostbasal euarchontoglire clades, the exact phylogenetic position is not yet considered resolved: It may be a sister of Glires, Primatomorpha,[18] orDermoptera, or separate from and sister to all other Euarchontoglires.[19][20] Sharedshort interspersed nuclear elements (SINEs) offer strong evidence for Scandentia belonging to the Euarchonta group:[21]
The 23 species are placed in fourgenera, which are divided into two families. The majority are in the "ordinary" treeshrew family,Tupaiidae, but one species, thepen-tailed treeshrew, is different enough to warrant placement in its own family,Ptilocercidae; the two families are thought to have separated 60 million years ago.[22] The former Tupaiidae genusUrogale was disbanded in 2011 when the Mindanao treeshrew was moved toTupaia based on a molecular phylogeny.[22]
The fossil record of treeshrews is poor. The oldest putative treeshrew,Eodendrogale parva, is from theMiddle Eocene ofHenan, China, but the identity of this animal is uncertain. Other fossils have come from theMiocene of Thailand, Pakistan, India, andYunnan, China, as well as thePliocene of India. Most belong to the family Tupaiidae; one fossil species described from the Oligocene of Yunnan is thought to be closer to the pen-tailed treeshrew.[23]
^Chin L, Moran JA, Clarke C (April 2010). "Trap geometry in three giant montane pitcher plant species from Borneo is a function of tree shrew body size".The New Phytologist.186 (2):461–70.doi:10.1111/j.1469-8137.2009.03166.x.PMID20100203.
^Janecka, Jan E.; Miller, Webb; Pringle, Thomas H.; Wiens, Frank; Zitzmann, Annette; Helgen, Kristofer M.; Springer, Mark S.; Murphy, William J. (2 November 2007). "Molecular and genomic data identify the closest living relatives of the Primates".Science.318 (5851):792–794.Bibcode:2007Sci...318..792J.doi:10.1126/science.1147555.PMID17975064.S2CID12251814.
^abZhou, Xuming; Sun, Fengming; Xu, Shixia; Yang, Guang; Li, Ming (1 March 2015). "The position of tree shrews in the mammalian tree: Comparing multi-gene analyses with phylogenomic results leaves monophyly of Euarchonta doubtful".Integrative Zoology.10 (2):186–198.doi:10.1111/1749-4877.12116.ISSN1749-4877.PMID25311886.
^Meredith, Robert W.; Janečka, Jan E.; Gatesy, John; Ryder, Oliver A.; Fisher, Colleen A.; Teeling, Emma C.; Goodbla, Alisha; Eizirik, Eduardo; Simão, Taiz L. L. (28 October 2011). "Impacts of the Cretaceous terrestrial revolution and KPg extinction on mammal diversification".Science.334 (6055):521–524.Bibcode:2011Sci...334..521M.doi:10.1126/science.1211028.ISSN0036-8075.PMID21940861.S2CID38120449.
^Lin, J.; Chen, G.; Gu, L.; Shen, Y.; Zheng, M.; Zheng, W.; Hu, X.; Zhang, X.; Qiu, Y.; Liu, X.; Jiang, C. (2014). "Phylogenetic affinity of tree shrews to Glires is attributed to fast evolution rate".Molecular Phylogenetics and Evolution.71:193–200.doi:10.1016/j.ympev.2013.12.001.PMID24333622.
^abRoberts, T.E.; Lanier, H.C.; Sargis, E.J.; Olson, L.E. (2011). "Molecular phylogeny of treeshrews (Mammalia: Scandentia) and the timescale of diversification in Southeast Asia".Molecular Phylogenetics and Evolution.60 (3):358–372.doi:10.1016/j.ympev.2011.04.021.PMID21565274.
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