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Heteromyidae

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Although they are very different in physical appearance, the closest relatives of the heteromyids are pocket gophers in the family Geomyidae.

Description

There are about fifty-nine members of the family Heteromyidae divided among six genera. They are all small rodents, the largest being the giant kangaroo rat (Dipodomys ingens) with a body length of 15 cm (6 in) and a tail a little longer than this. In many species the tail is tufted and is mainly used for balance. Other adaptations include partially fused vertebrae in the neck, short fore limbs and much enlarged bullae (bubble-shaped bones in the skull). The skulls vary widely across the group but they are all thin and papery and do not have the robust cranial crests and ridges found on the skulls of members of the family Geomyidae. The skull has other peculiarities. There is an extra hole that penetrates the rostrum, distinctive occluded teeth and the masseter muscle, which moves the lower jaw, is set far forward on the snout, an arrangement found in squirrels, beavers, pocket gophers, heteromyids and a few other groups. The dental formula is 1.0.1.31.0.1.3 × 2 = 20 teeth in total. In the kangaroo rats, the teeth continue to grow all the time, being worn away as the animal chews. The molars have two-lobed cusps. The upper incisors are grooved and the enamel on the molars is quickly worn away by chewing leaving the dentine exposed. In the kangaroo rats they are unrooted but in the pocket mice they have roots.

Fur-lined cheek pouches are a feature across the family Heteromyidae. They have openings near the mouth and extend backwards along the sides of the neck. The fur on the animal's body is in general short and fine and often matches in colour the soil of the region in which the animal lives, being some shade of buff, pale brown, reddish-brown or grey. The spiny pocket mice have coarser fur and spiky bristles on the back near the tail.

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