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. Behaviour 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. 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. It has been found that there is little hoarding done in the winter but that food is most commonly stored in the autumn and spring. Heteromyids use sight and olfactory clues to locate possible food and then use their fore-feet to manipulate objects. Kangaroo mice make greater use of clumped food where wind or water has concentrated seeds in shallow depressions or around rocks while pocket mice hunt around under vegetation or debris for individual food items. 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. These animals excavate long burrow systems with passages, chambers and multiple entrances. They are largely nocturnal, feeding and moving about at night and resting deep in their burrows during the day. Here it is cooler and more humid which conserves moisture and the animals may temporarily block the entrances to augment this. They