Kidney form and function vary along an aridity gradient in the African striped mouse, genus Rhabdomys

Abstract Rodents display various adaptations to maintain water balance, involving morphological, physiological and behavioural mechanisms adapted for their habitats. We investigated variation in kidney form and function in four sister species of African striped mice (genus Rhabdomys ), occurring along an aridity gradient in South Africa. We collected urine from mice housed in metabolic chambers and measured urine osmolarity using a vapor pressure osmometer. Thereafter, mice were euthanised, and their kidneys were removed and stored in buffered formalin. We dissected both kidneys sagittally and calculated size-corrected average renal medullary thickness (RMT). The arid-occurring R. pumilio had larger RMT and more concentrated urine than individuals of species from mesic environments, although no association occurred between RMT and urine concentration within all species except for R. bechuanae, which had larger RMT but not higher urine osmolarity than mice from the other mesic locations. R. d. chakae from Karkloof Nature Reserve (most mesic environment) had the smallest RMT. While kidney RMT and function are under strong selection pressures in arid African striped mice, mesic populations vary in form but not function. Phylogeny appears to modulate relative RMT in the genus Rhabdomys , but RMT and urine osmolarity are likely driven by local environmental conditions along the west–east rainfall gradient in South Africa.