FORAGING SUCCESS OF GRANIVOROUS RODENTS - EFFECTS OF VARIATION IN SEED AND SOIL-WATER ON OLFACTION

The ability of yellow pine chipmunks (Tamias amoenus), deer mice (Pero myscus maniculatus), and other rodents to detect buried Jeffrey pine ( Pinus jeffreyi) and antelope bitterbrush (Purshia tridentata) seeds us ing olfaction was investigated during dry and moist conditions at the Whittell Forest and Wildlife Area in Little Valley, Washoe County, Nev ada, USA. Removal of cached seeds was monitored on two Jeffrey pine fo raging grids (each with 100 caches of five seeds buried 5 mm deep) and on two bitterbrush foraging grids (each with 100 caches of 15 seeds b uried at the same depth). Under dry conditions, soil typically contain ed <0.5% water, whereas Jeffrey pine and bitterbrush seeds contained 4 .37 +/- 1.46% water (mean +/- 1 SD) and 7.45 +/- 1.83% water, respecti vely. Under these conditions, rodents found a mean of 0.33% of availab le Jeffrey pine seed caches each day and 0.14% of available bitterbrus h seed caches each day. After rains, soil moisture increased to as muc h as 8.5%, and Jeffrey pine and bitterbrush seeds contained 17.50 +/- 12.04% water and 28.04 +/- 16.35% water, respectively. When seeds and soil were moistened, the rate at which rodents removed caches increase d dramatically; 8.8% of available Jeffrey pine seed caches and 2.1% of available bitterbrush seed caches were removed each day. Similar resu lts were obtained when other foraging grids were artificially watered during a summer drought. Seeds are very hygroscopic, rapidly absorbing water whenever the environment around them becomes moistened. This mo isture appears to trigger the release of odorant molecules from the se eds, which rodents can readily detect. Under dry conditions, release o f odorant molecules is apparently reduced to the point that some roden ts have difficulty detecting buried seeds. These results may have impo rtant implications for rodent-seed interactions, but more generally, t hey suggest that, for terrestrial animals inhabiting arid environments , variation in environmental water level may modulate olfactory sensit ivity in many other contexts where olfaction is important, such as ass essing food patch quality, social behavior, predator-prey relations, a nd interspecific competition.