1902 Encyclopedia > Squirrel


SQUIRREL. In the article MARMOT (vol. xv. p. 559) an account was given of the three genera forming the Arctomyina, or Marmot sub-family of the large family Sciuridae, and in the present article the members of the other and more typical sub-family, the Sciurina, are noticed. The systematic position of the Sciuridae as a whole and their relations to other rodents are shown in the article MAMMALIA (vol. xv. p. 418); so it is merely with the component genera of the group that we now have to deal.

Of the Sciurina six genera are commonly recognized, the first being the typical one, Sciurus, in which the common English squirrel is included. The characters of the genus are—form slender and agile; tail long and bushy; ears generally well developed, pointed, often tufted; feet adapted for climbing, the anterior pair with four toes and a rudimentary thumb, and the posterior pair with five toes, all the toes having long, curved, and sharp-pointed claws; mammae from four to six in number; skull (see fig. 1) lightly built, very similar in

FIG. 1.—Skull of Sciurus bicolor; natural size.

shape throughout the genus; post-orbital processes long and curved; incisors narrow and compressed; premolars either one or two above and one below; when two are present above, the anterior one is quite minute and very different from the corresponding tooth in the marmots; molars three on each side above and below.

True squirrels are found throughout the greater part of the tropical and temperate regions of both hemispheres, although they are absent both from Madagascar and the Australian region. The species are both largest and most numerous in the tropics, and reach their greatest develop-ment in the Malay parts of the Oriental region.

Squirrels vary in size from animals no larger than a mouse, such as Sciurus soricinus of Borneo, or S. minutus of West Africa, to others as large as a cat, such as the black and yellow S. bicolor of Malaysia (see fig. 1). The very large squirrels, as might be expected from their heavier build, are somewhat less strictly arboreal in their habits than the smaller ones, of which the common English species may be looked upon as typical. The Common Squirrel, S. vulgaris, whose general habits are too well known to need special description, ranges over the whole of the Palaearctic region, from Ireland to Japan, from Lapland to North Italy; but specimens from different parts of this wide range differ so much in colour as to have been often looked upon as different species. Thus, while the common squirrels of north and wesi Europe are of the bright red colour we are accustomed to see in England, those of the mountainous regions of southern Europe are nearly always of a deep blackish grey; those from Siberia again are a clear pale grey colour, with scarcely a tinge of rufous. These last supply the squirrel fur used for lining cloaks. The pairing time of the squirrel is from February to April, and after a period of gestation of about thirty days it brings forth from three to nine young. In addition to all sorts of vegetables and fruits the squirrel is exceedingly fond of animal food, greedily devouring mice, small birds, and eggs.

Although the English squirrel is a most beautiful little animal, it is far surpassed by many of the tropical members of the group, and especially by those of the Malayan region, where nearly all the numerous species are brilliantly marked, and many are ornamented with variously coloured longitudinal stripes along their bodies. One of the commonest and best known of the striped species is the little Indian Palm Squirrel (S. palmarum), which in large numbers runs about every Indian village. Another Oriental species (S. caniceps) presents almost the only known instance among mammals of the temporary assumption during the breeding season of a distinctly ornamental coat, corresponding to the breeding plumage of birds. For the greater part of the year the animal is of a uniform grey colour, but about December its back becomes a brilliant orange-yellow, which lasts until about March, when it is again replaced by grey. The squirrel shown in fig. 2 is a native of Burmah and Tenasserim, and is

FIG. 2.—Burmese squirrel.

closely allied to S. caniceps, but goes through no seasonal change of colour.

The number of species in the genus Sciurus is about 75, of which 3 belong to the Palaearctic, 15 to the Ethiopian, about 40 to the Oriental, and 16 to the combined Nearctic and Neotropical regions.

Genus Rheithrosciurus.

A single very striking species of squirrel, confined to Borneo, and as yet only known from three or four examples, has been separated genetically under the above name. The general shape of its skull is very different from that of other squirrels; but its most peculiar characteristic is the presence of from seven to ten minute parallel vertical grooves running down the front face of its incisors, both above and below, no other squirrel having really grooved incisors at all, and no other member of the wdiole order of rodents incisor grooves resembling these. Its premolars only number 1/1, and its molars are simpler and less ridged than in the other genera. This squirrel (Rh. macrotis) is a magnificent animal, far larger than the English species, with an enormously long bushy tail, long tufted ears, and black and white bands down its sides.

Genus Xerus.

Fur coarse and spiny. Claws long and comparatively straight. Ear-conches minute or entirely absent. Skull with the post-orbital processes short and directed backwards, the bony palate prolonged considerably behind the tooth-row, and the external ridge on the front face of the anterior zygoma-root more developed, and continued much further upwards, than in Sciurus. Premolars 2/1; molars as in Sciurus. This genus contains four well-marked species, known as Spiny Squirrels, all natives of Africa. They are terrestrial in their habits, living in burrows which they dig for themselves. X. getulus, a striped species of North Africa, has much the size and appearance of the Indian palm squirrel; the others are all a little larger than the English squirrel.

Genus Tamias.

The members of this genus are characterized by the possession of internal cheek-pouches, and by their style of coloration, all being ornamented on the back with alternate bands of light and dark colour. Their skulls are slenderer and lighter than those of the true squirrels, from which they differ in several unimportant details. There is only one functional premolar,—the small anterior one usually found in Sciurus being either absent altogether or quite small and functionless. There are four species, all found in North America, one extending also through Siberia into eastern Europe. They are known in America as ''Chipmunks," and are among the commonest and best known of the indigenous rodents. The members of this group seem rather to lead into the genus Spermophilus (see MARMOT) of the sub-family Arctomyina, so that the division of the Sciuridae into two sub-families, although very convenient for classification and description, is rather of an artificial nature, there being no well-defined line of separation between them.

Genera Pteromys and Sciuropterus.

The Flying Squirrels, although they cannot fly in the true sense of the word, can yet float through the air for considerable distances by the aid of an extension of skin connecting their fore and hind limbs, and forming a sort of parachute. This parachute is merely a lateral extension of the ordinary skin of the body, which passes outwards between the limbs and terminates at the wrists and ankles. In addition to the lateral membrane there is a narrow and inconspicuous one passing from the cheek along the front of the shoulder to the front of the wrist, and another—at least in the larger species—stretching across behind the body from ankle to ankle and involving the base of the tail. The flying squirrels are divided into two genera, of which Pteromys contains the larger and Sciuropterus the smaller species. The two differ in certain details of dentition, and in the greater development in the former of the expanded membranes, especially of the "interfemoral" or posterior membrane, which is in the latter almost wholly absent. In Pteromys the tail is cylindrical and comparatively thin, while in Sciuropterus it is broad, flat, and laterally expanded, and evidently compensates for the absence of the interfemoral membrane by acting as a supplementary parachute. In appearance flying squirrels resemble the non-flying forms, although they are even more beautifully coloured than the latter. Their habits, food, &c, are also very similar to those of the true squirrels, except that they are more decidedly nocturnal, and are therefore less often seen by the ordinary observer. Their method of leaping from tree to tree and floating long distances on their extended parachutes is precisely similar to that of the flying phalangers of Australia, a graphic description of which is quoted in PHALANGER (vol. xviii. p. 729). Of each of the two genera there are about thirteen or fourteen species, all natives of the Oriental region, except that one of Sciuropterus is found in North America, and another in Siberia and eastern Europe,—the latter, the Sciurus volans of Linnaeus's Systema Naturae, being the first flying squirrel that was known to European naturalists. (O. T.)

The above article was written by: Oldfield Thomas.

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