No abstract

Ancestrally, oryzomyine rodents have a sylvan habit, but some species have colonized the semi-arid Brazilian Caatinga. Therefore, the clade provides a suitable model for studying adaptation to such environments within a phylogenetic framework. We investigated physiological responses to short-term water deprivation in two oryzomyine rodents kept in captivity. Oligoryzomys nigripes (Olfers, 1818), a small-bodied rodent, occupies the Caatinga, whereas Nectomys squamipes (Brants, 1827), which is larger and semi-aquatic, lives in moist habitats outside this region. Measurements were also carried out in three additional related species of intermediate body size. Neither O. nigripes nor N. squamipes displays exceptional ability to conserve water, and this condition appears to be plesiomorphic for the clade. The results for O. nigripes, such as the great absolute water intake and the high ratio of water ingested to urine voided, probably derive from allometric specializations correlated with its small body size and greater evaporative water loss. In N. squamipes, low urine osmolality and a reduction of food intake during water deprivation may be related to its semi-aquatic lifestyle. Therefore, the Oryzomyini exemplify Neotropical rodents that can occupy semi-arid environments with no remarkable physiological adaptation for water conservation.