Heteromyidae is a family of rodents consisting of kangaroo rats, kangaroo mice, pocket mice and spiny pocket mice. Most heteromyids live in complex burrows within the deserts and grasslands of western North America, though species within the genus Heteromys are also found in forests and their range extends down as far as northern South America. They feed mostly on seeds and other plant parts, which they carry in their fur-lined cheek pouches[1] to their burrows.[2]
Although they are very different in physical appearance, the closest relatives of the heteromyids are pocket gophers in the family Geomyidae.
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.[3] 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.[4] 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.[5]
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.[4]
Heteromyids are endemic to the western United States, Mexico, Central America and northwestern South America. They are creatures of open country, mostly specialising in prairies, arid lands and desert fringes, but the spiny pocket mice are found in both wet and dry tropical forests in Central and northwestern South America. During the Oligocene, the extent of arid lands across the United States was greater than it is today and these animals occurred more widely. Fossils of kangaroo rats are known from the Pliocene.[4]
The kangaroo rats, kangaroo mice and pocket mice are adapted for life in deserts and other arid environments where they largely feed on dry seeds. Kangaroo rats have no need to drink because they are able to extract sufficient water from metabolising their food, obtaining half a gram of water from each gram of seeds eaten.[4][6] They carry most of the seeds they find back to their burrows in exterior cheek folds lined with fur rather than in cheek pouches inside the mouth as do hamsters. This is because carrying the food in the mouth would involve wetting it and moisture needs to be conserved as much as possible in dry environments. Most species store the seeds they gather in special chambers in the burrow where they absorb moisture from the humid air. Others, such as Merriam's kangaroo rat, bury them in shallow caches in the open air.[6]