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.

Other adaptations include partially fused vertebrae in the neck, short fore limbs and much enlarged bullae (bubble-shaped bones in the skull).

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.

[4] The upper incisors are grooved and the enamel on the molars is quickly worn away by chewing leaving the dentine exposed.

The spiny pocket mice have coarser fur and spiky bristles on the back near the tail.

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.

Most species store the seeds they gather in special chambers in the burrow where they absorb moisture from the humid air.

It has been found that pocket mice forage more efficiently than kangaroo rats and this is thought to be due to the fact that they handle their finds more and are better able to distinguish between food and non-food items.

[4] Kangaroo rats and pocket mice form a part of the diet of many predatory creatures.

[8] Merriam's kangaroo rat primarily moves rapidly from one bush to another gathering seed but overlooking anything lying on the desert floor.