Tree-shrews as Primates


There exists a great controversy as to whether tree-shrews (family Tupaiidae) should be placed in an order with primates or whether they are insectivores. These creatures are mammals mostly found in south-east Asia. The table 1 shows the extant tree-shrews. There are two subfamilies, Ptilocercinae and Tupaiinae, with the subfamily Ptilocercinae as being primitive because of being nocturnal (Gould, 1978; cited in Martin, 1990). All tree-shrews share some common characteristics: relatively small body mass, generally omnivorous (eating arthropods and fruit), the skeleton has an unspecialized placental mammalian pattern, all digits have claws, and the hands and feet are not prehensile (Martin, 1990). Not all of the genera of tree-shrews are arboreal, the genera Lyonogale and Urogale are mostly terrestrial, and the rest of the genera are probably best described as being semi-arboreal (Martin, 1990). Most tree-shrews share many morphological and behavioral characteristics with squirrels, so much so that the Malay word tupai is used for both tree-shrews and squirrels (Martin, 1990).

One view is to place the tree-shrews in the order Insectivora. The order Insectivora is made up of elephant-shrews, shrews, hedgehogs, tenrecs, moles, and possibly tree-shrews. Now it was suggested that the order Insectivora be divided into the suborders Menotyphla (elephant-shrews and tree-shrews) and Lipotyphla (shrews, hedgehogs, tenrecs, and moles) (Haeckel, 1866; Weber, 1928; cited in Martin, 1990). Tree-shrews have been thought of both being primitive and advanced as compared to other insectivores (Martin, 1990). It has been noted that tree-shrews have very primitive placental mammalian characteristics as compared to other members of the order Insectivora (Carlsson, 1922; cited in Martin 1990). Given the differences or rather the primitiveness of the tree-shrews it has also been proposed to place the tree-shrews in the separate order Scandentia (Butler, 1972; cited in Martin, 1990).

Tree-shrews are also included in the same order with primates by some. Now the characteristics that are shared between tree-shrews and primates have been noted as to being primitive amongst placental mammals (Martin, 1990). Table 2 shows the characteristics which are thought as to be shared between tree-shrews and primates and also an analysis of these characteristics. All of them can be explained as either being primitive to placental mammals or as to being incorrect (Martin, 1990). The problem with placing tree-shrews into the order Primata is that tree-shrews and primates do not share any derived characteristics, ones that are not found in any other mammals (Martin, 1990). There are also characteristics shared amongst primates that are not found in tree-shrews (Martin, 1990); some of these are found in table 3. Until more evidence can be found the status of where to place the tree-shrew will remain unresolved, so it can not be definitively said that tree-shrews are primates (Martin, 1990).
Table 1. This table shows all of the extant tree-shrews (Lyon, 1913; Napier and Napier, 1967; cited in Martin, 1990).
Scientific Name Common Name
Subfamily Ptilocercinae
Ptilocercus lowii Pen-tailed tree-shrew
Subfamily Tupaiinae
Tupaia belangeri Belanger's tree-shrew
Tupaia glis Common tree-shrew
Tupaia longipes Long-footed tree-shrew
Tupaia montana Montane tree-shrew
Tupaia nicobarica Nicobar tree-shrew
Tupaia picta Painted tree-shrew
Tupaia palawanensis Palawan tree-shrew
Tupaia splendidula Rufous-tailed tree-shrew
Tupaia minor Pygmy tree-shrew
Tupaia javanica Indonesian tree-shrew
Tupaia gracilis Slender tree-shrew
Anathana ellioti Indian tree-shrew
Lyonogale tana Terrestrial tree-shrew
Lyonogale dorsalis Striped tree-shrew
Urogale everetti Philippine tree-shrew
Dendrogale melanura Southern smooth-tailed tree-shrew
Dendrogale murina Northern smooth-tailed tree-shrew


Table 2. This table shows supposed shared characteristics between tree-shrews and primates, from Martin (1990).
Results of Analysis of Characters
Context Shared Similarities
Skull 1. Snout relatively short 1. Snout in fact secondarily elongated in tree-shrews
2. Simplified set of turbinal bones 2. Set of six turbinal bones probably primitive for placental mammals
3. Enlarged, forward-facing orbits 3. Orbits relatively small and laterally facing
4. Postorbital bar present 4. Postorbital bar present as a convergent development in various mammals
5. Pattern of bones in medial orbital wall 5. Palatine/lacrimal contact in medial orbital wall probably primitive
6. Well-developed jugal bone with foramen 6. Well-developed jugal bone with foramen probably primitive
7. Enlarged braincase 7. Braincase has become enlarged convergently in various mammalian groups
8. Inflated auditory bulla containing 'free' ectotympanic ring 8. Auditory bulla formed from entotympanic, not from petrosal; ectotympanic ring is primitively ring-shaped in placental mammals
9. Internal carotid pattern (bony tubes) 9. Enclosure of internal carotid in bony tubes probably primitive
10. 'Advanced' form of auditory ossicles 10. Auditory ossicles do not clearly share derived features with primates
Dentition 1. Tooth-comb present at front of lower jaw, linked with a specialized, serrated sublingua 1. Tooth-comb formed exclusively from incisors as a convergent feature; sublingua present in common ancestor of marsupials and placentals
2. Reduced dental formula 2. Convergent reduction of dental formula in many mammalian groups
3. Similarities in cheek teeth between tree-shrews and certain primates with relatively primitive cheek teeth (e.g. Tarsius) 3. Limited similarities in cheek teeth between tree-shrews and certain primates undoubtedly due to primitive retention
Postcranial Morphology 1. Limbs and digits highly mobile 1. Limbs and digits probably highly mobile in ancestral placental mammals
2. Numerous details of limb musculature 2. Limb musculature shares primitive retentions with prosimians
3. Osteological similarities in both forelimbs and hindlimbs 3. Osteological similarities in forelimbs and hindlimbs attributable to primitive retention from ancestral placental mammals
4. Ridged skin on palms and soles 4. Ridged skin on palms and soles possibly a primitive feature for placental mammals; tree-shrews lack the characteristic Meissner's corpuscles of primates
Brain and Sense Organs 1. Olfactory apparatus reduced 1. Olfactory apparatus not reduced relative to body size in tree-shrews
2. Visual apparatus enhanced 2. Visual apparatus mildly enhanced; numerous primate features lacking
3. Central, avascular area of retina 3. Unusual, spoke-like radiation of retinal vessels; unusual innervation
4. Neocortex expanded; brain size increased 4. Expansion of neocortex and brain size found in many mammals
5. Calcarine sulcus present 5. Calcarine sulcus not present in the brain of tree-shrews
Reproductive Biology 1. Penis pendulous; testes scrotal 1. Pendulous penis and scrotal testes present in many mammals
2. Discoidal placenta, as in tarsiers and simians 2. Discoidal placenta common in mammals; endotheliochorial in tree-shrews
3. Small litter size; small number of teats 3. Small litter size and small number of teats common in mammals; tree-shrew offspring are altricial, not precocial like those of primates
Miscellaneous 1. Caecum present 1. Caecum probably a primitive feature of marsupials and placentals
2. Molecular affinities (e.g. albumins) 2. No convincing molecular affinities between tree-shrews and primates


Table 3. This table shows some features that are shared amongst extant primates but are absent in tree-shrews. Adapted from Martin (1990).
Feature Primate Condition Tree-shrew Condition
hands and feet prehensile not prehensile
brain lateral and calcarine sulcus present lateral and calcarine sulcus absent
scrotum postpenial prepenial
lower incisors two or less on each side of the lower jaw three on each side of the lower jaw
upper incisors arranged transversely * arranged longitudinally
offspring at birth precocial altricial
gestation period relatively long compared to body size relatively short compared to body size

* many archaic primates have longitudinally arranged upper incisors (e.g. Plesiadapis tricuspidens)


REFERENCES:
Butler, P.M. 1972. The Problem of Insectivore Classification. In Studies in Vertebrate Evolution. Eds. K.A. Joysey and T.S. Kemp. Oliver and Boyd: Edinburgh.

Carlsson, A. 1922. Uber die Tupaiidae und Ihre Beziehungen zu den Insectivora und den Prosimiae. Acta Zool, Stockh., Vol. 3, 227-270.

Gould, E. 1978. The Behavior of the Moonrat, Echinosorex gymnurus (Erinaceidae) and the Pentail Tree-shrew, Ptilocercus lowii (Tupaiidae) with Comments on the Behavior of Other Insectivora. Z. Tierpsychol., Vol. 48, 1-27.

Haeckel, E. 1866. Generelle Morphologie der Organismen: Allgemeine Grundzuge der Organischen Formen-Wissenschaft, Mechanisch Begrundet durch die von Charles Darwin Reformierte Descendenz-Theorie (2 vols.). Georg Reimer: Berlin.

Lyon, M.W. 1913. Treeshrews: An Account of the Mammalian Family Tupaiidae. Proc. U.S. Natn. Mus., U.S.A., Vol. 45, 1-188.

Martin, R.D. 1990. Primate Origins and Evolution: A Phylogenetic Reconstruction. Princeton University Press: Princeton, N.J.

Napier, J.R. and Napier, P.H. 1967. A Handbook of Living Primates. Academic Press: London.

Weber, M. 1928. Die Saugetiere: Einfuhrung in die Anatomie und Systematik der Recenten und Fossilen Mammalia, vol. 1: Anatomischer Teil (2nd edition). Gustav Fischer Verlag: Jena.

Last updated: January 5, 2007

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