Heteromyid rodents are distributed from the New World tropics to the deserts of North America, but their habitation of deserts is relatively recent. Their evolutionary history, though, is associated with progressive aridity, with larger quadrupedal taxa being more mesic (and primitive), and smaller quadrupeds and all bipeds more xeric. The correlation between water regulatory efficiency and body mass is strongly negative in heteromyids; bipedal Dipodomys spp. have water regulatory efficiency fixed at an intermediate level independent of mass. Heteroymids generally have basal metabolic rates reduced below the eutherian level, with the greatest reductions occurring in desert species. The use of torpor as an energy-conserving device is cosmopolitan in small (<40 g) heteromyids, but is inconsequential or lacking in larger ones. Bipedalism, characteristic of Dipodomys, confers no direct energetic advantages as revealed in treadmill studies.
Large quadrupedal heteromyids have adaptive physiologies suitable for more mesic habitats, but smaller quadrupeds are suited for xeric existences; these characteristics likely reflect an evolution from a tropical ancestry to rather recent habitation of deserts. Bipedalism occurs only in xeric-adapted forms and has no directly discernible energetic benefit; yet it appears to relieve in some unknown way the energetic constraints of foraging.
MacMillen, Richard E.
"Adaptive physiology of heteromyid rodents,"
Great Basin Naturalist Memoirs: Vol. 7
, Article 5.
Available at: http://scholarsarchive.byu.edu/gbnm/vol7/iss1/5