( ) Journal of Arid Environments 1998 40: 97107 Article No. ae980418 Osmoregulatory capacity of an arboreal rodent Thallomys nigricauda Jennifer L. Frean, Colleen T. Downs  & Barry G. Lovegrove Department of Zoology and Entomology, University of Natal, Private Bag X 01, S cottsville, Pietermaritz burg, 3209, Republic of South Africa ( ) Received 11 December 1997, accepted 17 April 1998 The Black-tailed tree rat, T hallomys nigricauda, is a southern African xeric- adapted nocturnal rodent which diurally remains in its arboreal nest. Its urine concentrating ability was investigated in a series of dehydration experiments, and kidney morphology was examined. Unlike several other southern African xeric rodents, T . nigricauda did not excrete allantoin precipitate in their urine. ( ) H owever they had a similar efficient urine concentrating ability UCA reflected by their urine osmolality and urea concentration when deprived of free water. Renal structure indicated efficient UCA and was also similar to that of other xeric rodents. Because they are unable to maintain body mass on a dry food diet, T . nigricauda require a diet high in preformed water to maintain a constant body mass, despite their efficient UCA.  1998 Academic Press Keywords: urine concentrating ability; T hallomys nigricauda; kidney; renal morphology; urea; osmolality Introduction Despite great ecological stresses in xeric zones, including large daily and seasonal temperture fluctuations, unpredictable food resources and water scarcity, many rodent ( species successfully inhabit these zones Louw et al., 1972; Christian, 1978; de Graaf, ) 1981; Skinner & Smithers, 1990; Degan, 1997 . Preformed and metabolic water from ingested food are the only reliable, but limited sources of water for xeric-adapted rodents, therefore water loss must be minimized to maintain positive water balance ( ) Urison & Buffenstein, 1994 . Body water loss is a summation of pulmocutaneous ( evaporation, and faecal and urinary water loss Schmidt-Nielson, 1964; M acM illen , ) 1972 . Daytime summer ambient temperatures reach 45°C while soil surface ( temperatures reach 70°C in xeric regions Schmidt-Nielson, 1964; Downs & Perrin, ) 1989; Lovegrove et al., 1991 . An animal’s behaviour, microhabitat and diet play ( important roles in reducing pulmocutaneous evaporative water loss Louw et al., 1972;  To whom correspondence should be addressed. 0140196398010097  11 $30.000  Academic Press