Seed Properties and Water Balance in Desert Granivores

Abstract

-Chemical composition, caloric yield, and water content of 11 seed species were measured to identify 1) seed properties that influence hygroscopic capacity and 2) relationships between preformed water content, metabolic water production, and energetic yield. Preformed water content increased with increasing ambient humidity, and varied significantly among seed species. Preformed water content and total water yield (preformed plus metabolic water) varied significantly among seed species and were positively correlated with carbohydrate content and negatively correlated with lipid and protein content and with caloric content. These results suggest that consumption of different seeds may have marked effects on water balance of desert granivores, and that seed selection on the basis of energetic yield may be expensive in terms of water gain. Granivory is common and widespread in desert animals, especially ants, birds, and rodents (see review by Brown et al., 1979). Many desert-dwelling granivores are quite selective in their dietary choices; seed selection by these animals has received considerable attention, generally from points of view of optimal foraging theory and of understanding patterns of competition and coexistence among sympatric species (Brown, 1975; Brown et al., 1979; Davidson, 1977a, 1977b; Price, 1978; Reichman, 1975, 1977; Reichman and Oberstein, 1977). Most of this research has implicitly or explicitly emphasized energetic perspectives and has, for the most part, ignored other factors that may influence the seed preferences of desert granivores. One such factor that may warrant attention is water balance. For most desert animals, granivores and others, water resources are closely coupled to food resources (MacMillen and Christopher, 1975). That is, the major (or sole) source of water for most of these animals is the diet, including preformed water contained in the food and water formed by the oxidation of food (metabolic water; Schmidt-Nielsen, 1964, 1975a). This restricted access to water is particularly significant for granivores because the seeds they ingest contain little water relative to other possible food sources, such as succulent plant tissues or insects. It is therefore important to gain a better understanding of seed qualities that influence water relations of these animals, and to explore the possible role of water balance considerations in dietary seed selection. Several aspects of seed and animal water-balance relationships are poorly understood. These include 1) factors affecting the preformed water content of seeds; 2) relationships between preformed water yield and other routes of water exchange that may be influenced by seed chemical composition; and 3) the degree of variation in water yield among seed species. It has been well demonstrated that the water content of seeds and other plant material increases with humidity (Harrington, 1960; Morton and MacMillen, 1982; Roberts and Roberts, 1972; Schmidt-Nielsen, 1964; Taylor, 1968), but THE SOUTHWESTERN NATURALIST 29(2):181-188 MAY 30, 1984 This content downloaded from 207.46.13.13 on Thu, 25 Aug 2016 06:16:41 UTC All use subject to http://about.jstor.org/terms TABLE 1.-Weights, composition, caloric yield, and water yield/g of 11 species of seeds identified by scientific and common name and family. See text for sample sizes and methods of analysis. Values under Caloric Yield are (1) computed from composition data and (2) determined by bomb calorimetry.