GRANIVORY AND PLANT-SELECTION BY DESERT GERBILS OF DIFFERENT BODY-SIZE

Plant selection, energy intake, and digestibility were studied in nonr eproducing Gerbillus henleyi (10 g; n = 9), Gerbillus dasyurus (24 g; n = 6), and Meriones crassus (90 g; n = 9). These gerbils coexist in t he Negev Desert Highlands, have similar dietary habits (being primaril y granivores), and do not require drinking water, but they differ grea tly in body mass. Each species was offered different levels (0% and si milar to 40% and 80% of maintenance energy requirements) of millet see ds and ad libitum fresh vegetation consisting of Atriplex halimus, Mor icandia nitens, Zygophyllum dumosum, Lycium shawii, Anabasis articulat a, and Thymelaea hirsuta, the dominant plants in the area inhabited by the rodents. Drinking water was not available. We hypothesized that: (1) smaller animals would be more dependent than larger animals on hig hly digestible food items such as seeds, and (2) smaller animals would be more selective than larger animals for highly digestible and energ y-rich plant parts. The minimum levels of millet seeds needed to maint ain body mass were 97%, 94%, and 36% of maintenance energy requirement s for G. henleyi, G. dasyurus, and M. crassus, respectively. Therefore , G. henleyi was the most dependent on seeds, and M., crassus the leas t, supporting hypothesis 1. At similar levels of seeds consumed, appar ent digestibilities of dry matter (DMD) and energy of the total diet w ere highest for G. henleyi. In the absence of seeds, vegetation DMD av eraged 88% in G. henleyi and 70% in M. crassus, yet G. henleyi lost bo dy mass, as a percentage, twice as fast as M. crassus. The three roden t species were selective in consuming vegetation and made similar choi ces. A. halimus and M, nitens were preferred, comprising similar to 48 %, 64%, and 67% of total plant dry matter consumption in G. henleyi, G . dasyurus, and M. crassus, respectively. Plant species not selected w ere relatively low in water and protein contents and were high;n pheno lic compounds and condensed tannins compared to preferred species. Gro ss energy, ash, and fiber levels did not appear to affect selectivity. Selection of plant parts within each plant species, as indicated by t he leaf:stem (L:S) ratio, differed among rodent species. The L:S ratio for A. halimus was 4.8, but the L:S ratios ingested were 1.7, 4.0, an d 7.4 in G. henleyi, G. dasyurus, and M. crassus, respectively, indica ting selection of stems by G. henleyi, selection of leaves by M. crass us, and basically no selection by G. dasyurus. The ratio of seed : gre en vegetation (S:G) intake was highest in G. henleyi and lowest in M. crassus. A similar inverse relationship between body mass and S:G rati o within seasons was found in other Old World and New World free-livin g granivorous desert rodents.