Renal physiology of two southern African Mastomys species (Rodentia: Muridae): a salt-loading experiment to assess concentrating ability Phumza Ntshotsho a , Rudi J. van Aarde a, * , Sue W. Nicolson b , Tim P. Jackson a a Conservation Ecology Research Unit, Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa b Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa Received 26 April 2004; received in revised form 27 September 2004; accepted 27 September 2004 Abstract Aspects of renal physiology were examined to test the hypothesis that two cryptic species of the genus Mastomys (Mastomys natalensis and Mastomys coucha) are geographically separated by differences in aridity tolerance. Laboratory-bred females of each species were subjected to different levels of salinity in their water source (distilled water, 0.9% NaCl, and 1.5% NaCl; 10 conspecifics in each group) from weaning until sexual maturity. Individuals of the two species exhibited similar rates of water consumption and urine production. The salinity treatments caused sodium diuresis in both species, evident in increased urine volume, decreased osmolality and increased osmotic output. Urine concentration, kidney mass and kidney relative medullary area (RMA) did not differ between species. The results of our study do not support the hypothesis that differences in osmoregulatory ability separate these two cryptic species. Nor do they support the use of salt loading to elicit maximum urine concentrations in mammals. D 2004 Elsevier Inc. All rights reserved. Keywords: Mastomys coucha; Mastomys natalensis ; Osmoregulation; Kidneys; Urine concentration 1. Introduction In South Africa, the widely distributed rodent genus Mastomys has been traditionally considered to comprise a single species, Mastomys natalensis , but subsequent studies have revealed the existence of two cryptic species, M. natalensis (A. Smith, 1834) and Mastomys coucha (A. Smith, 1836) (Gordon, 1984). These species are two of the most common and widely distributed southern African rodents (Green et al., 1980; Gordon, 1984; Smit et al., 2001). As one of the first species to recolonise disturbed habitats, M. natalensis can undergo rapid population increases, making it a major agricultural pest (Leirs et al., 1996; Stenseth et al., 2001; Jackson and van Aarde, 2004). Because of this and both species’ medical importance as vectors of bubonic plague (Grats, 1997), they have been the focus of considerable research. Information available on their distribution suggests that within South Africa M. natalensis is confined to high rainfall areas (greater than 600 mm annual precipitation) along the eastern coastal region, whereas M. coucha predominantly occurs at higher altitudes in central regions with lower and more variable rainfall (Skinner and Smithers, 1990; Venturi et al., 2004). Potential factors influencing these distributions have not been examined. Water is a limiting factor for many terrestrial rodent populations (e.g., Reaka and Armitage, 1976; Moro and Bradshaw, 1999a) and physiological adaptation of small rodents to arid conditions is achieved mainly through concentrating ability of their kidneys (Beuchat, 1996). The present paper tests the hypothesis that observed differences in the distribution of M. natalensis and M. coucha can be 1095-6433/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpb.2004.09.018 * Corresponding author. Tel.: +27 12 420 2535; fax: +27 12 420 4523. E-mail address: rjvaarde@zoology.up.ac.za (R.J. van Aarde). Comparative Biochemistry and Physiology, Part A 139 (2004) 441– 447 www.elsevier.com/locate/cbpa