BODY-SIZE VARIABILITY AND WATER-BALANCE - A COMPARISON BETWEEN MAINLAND AND ISLAND POPULATIONS OF MASTOMYS HUBERTI (RODENTIA, MURIDAE) IN SENEGAL

Generally rodents are found to be larger on islands than on the mainla nd. However, there an some exceptions to this rule, and the aim of thi s paper is to examine one of them. On the mainland of Senegal, Mastony s huberti occupies humid habitats. However, it occurs also on dry and sandy islands (Saloum delta), where its representatives are dwarf. Sin ce water availability appeared to be the limiting factor in these isla nds when compared to the mainland, we studied water turnover character istics in relation to body size, in mainland and island populations at the end of the dry season, under both field and laboratory conditions . All populations were found to be water balanced in their natural hab itats. They presented similar rates of water turnover, even though isl and animals were subjected to stronger constraints than mainland ones. Laboratory experiments suggested that the physiological plasticity of one of the island populations may be reduced. Island populations have a higher kidney size to body weight ratio than those from the mainlan d. We propose that smaller size in the islands allows the maintenance of water balance with a smaller amount of water, and that a higher rat io of kidney filtration surface to body size may help Mastomys huberti to survive in dry islands. We discuss the factors responsible for bod y size variability and variation in water exchange characteristics and conclude that different factors could explain body size variation amo ng island populations, depending on the species considered and the eco logical constraints met within the islands.