JERBOA, properly the name of an Arabian and North African jumping rodent mammal, Jaculus aegyptius (also known as Jaculus, or Dipus, jaculus) typifying the family Jaculidae (or Dipodidae), but in a wider sense applied to most of the representatives of that family, which are widely distributed over the desert and semi-desert tracts of the Old World, although unknown in Africa south of the Sahara. In all the more typical members of the family the three middle metatarsals of the long hind-legs are fused into a cannon-bone; and in the true jerboas of the genus Jaculus the two lateral toes, with their supporting metatarsals, are lost, although they are present in the alactagas (Alactaga), in which, however, as in certain allied genera, only the three middle toes are functional. As regards the true jerboas, there is a curious resemblance in the structure of their hind-legs to that obtaining among birds. In both groups, for instance, the lower part of the hind-leg is formed by a long, slender cannon-bone, or metatarsus, terminating inferiorly in triple condyles for the three long and sharply clawed toes, the resemblance being increased by the fact that in both cases the small bone of the leg (fibula) is fused with the large one (tibia). It may also be noticed that in mammals and birds which hop on two legs, such as jerboas, kangaroos, thrushes and finches, the proportionate length of the thigh-bone or femur to the tibia and foot (metatarsus and toes) is constant, being 2 to 5; in animals, on the other hand, such as hares, horses and frogs, which use all four feet, the corresponding lengths are 4 to 7. The resemblance between the jerboa's and the bird's skeleton is owing to adaptation to a similar mode of existence. In the young jerboa the proportion of the femur to the rest of the leg is the same as in ordinary running animals. Further, at an early stage of development the fibula is a complete and separate bone, while the three metatarsals, which subsequently fuse together to form the cannon-bone, are likewise separate. In addition to their long hind and short fore limbs, jerboas are mostly characterized by their silky coats - of a fawn colour to harmonize with their desert surroundings - their large eyes, and long tails and ears. As is always the case with large-eared animals, the tympanic bullae of the skull are of unusually large size; the size varying in the different genera according to that of the ears. (For the characteristics of the family and of its more important generic representatives, see RoDENTIA.) In the Egyptian jerboa the length of the body is 8 in., and that of the tail, which is long, cylindrical and covered with short hair terminated by a tuft, 10 in. The five-toed front limbs are extremely short, while the hind pair are six times as long. When about to spring, this jerboa raises its body by means of the hinder extremities, and supports itself at the same time upon its tail, while the fore-feet are so closely pressed to the breast as to be scarcely visible, which doubtless suggested the name Dipus, or twofooted. It then leaps into the air and alights upon its four feet, but instantaneously erecting itself, it makes another spring, and so on in such rapid succession as to appear as if rather flying than running. It is a gregarious animal, living in considerable colonies in burrows, which it excavates with its nails and teeth in the sandy soil of Egypt and Arabia. In these it remains during great part of the day, emerging at night in search of the herbs on which it feeds. It is exceedingly shy, and this, together with its extraordinary agility, renders it difficult to capture. The Arabs, however, succeed by closing up all the exits from the burrows with a single exception, by which the rodents are forced to escape, and over which a net is placed for their capture. When confined, they will gnaw through the hardest wood in order to make their escape. The Persian jerboa (Alactaga indica) is also a nocturnal burrowing animal, feeding chiefly on grain, which it stores up in underground repositories, closing these when full, and only drawing upon them when the supply of food above ground is exhausted (see also Jumping Mouse).
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