The unique abilities of kangaroo rats to survive such low levels of water availably have been a direct result of the geological and environmental changes that occurred in its ancestor’s history. By examining the phylogeny and evolutionary history of kangaroo rats, the selective forces that have pushed the genus to its impressive tolerance of arid conditions can be seen. The Family Heteromyidae first appeared in the Tertiary period of the Cenozoic era. These rodent populations existed in what was then the neotropical region of central Mexico and the southwestern United States. These regions were somewhat arid and were inhabited by sclerophyllous and microphyllous plant species, similar to modern day desert shrubs. (Macmillen and Hinds 1983). While these heteromyids were somewhat adapted to survive dry conditions, the majority of the adaptations came along with the geological change occurring during the Tertiary period.
During the middle of the Tertiary period, the climate of the area inhabited by heteromyids became increasingly arid, as rainfall became less frequent and seasonal. This transformation into more desert-like habitats was accompanied by coevolution of flora and heteromyid species. The decrease in primary production resulting from the more arid environments is thought to have selected for granivory, a diet of primarily dry grains. (Macmillen and Hinds, 1983). During this time period, the family took several different evolutionary paths, including those dwelling in more humid environments (genera Heteromys and Liomys) and those able to live in the more arid environments (Microdipodops, Perognathus, Dipodomys). The capacity for bipedality (from where kangaroo rats got their name) was developed in Microdipodops, and this ability was further enhanced in Dipodomys (Macmillen and Hinds, 1983). The development of the family from the more ancestral Liomys genus to the highly specialized Perognathus and Dipodomys shows a gradient of the development of several characteristics. As xeric environmental pressures increased, water and energy needs decreased, locomotion became more efficient, and behavior changed. (Macmillen and Hinds, 1983). |
