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

Abstract

The ability of yellow pine chipmunks (Tamias amoenus), deer mice (Peromyscus maniculatus), and other rodents to detect buried Jeffrey pine (Pinus jeffreyi) and antelope bitterbrush (Purshia tridentata) seeds using olfaction was investigated during dry and moist conditions at the Whittell Forest and Wildlife Area in Little Valley, Washoe County, Nevada, USA. Removal of cached seeds was monitored on two Jeffrey pine foraging 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 buried at the same depth). Under dry conditions, soil typically contained <0.5% water, whereas Jeffrey pine and bitterbrush seeds contained 4.37 ± 1.46% water (mean ± 1 sd) and 7.45 ± 1.83% water, respectively. Under these conditions, rodents found a mean of 0.33% of available Jeffrey pine seed caches each day and 0.14% of available bitterbrush seed caches each day. After rains, soil moisture increased to as much 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 increased dramatically; 8.8% of available Jeffrey pine seed caches and 2.1% of available bitterbrush seed caches were removed each day. Similar results 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 moisture appears to trigger the release of odorant molecules from the seeds, which rodents can readily detect. Under dry conditions, release of odorant molecules is apparently reduced to the point that some rodents have difficulty detecting buried seeds. These results may have important implications for rodent–seed interactions, but more generally, they suggest that, for terrestrial animals inhabiting arid environments, variation in environmental water level may modulate olfactory sensitivity in many other contexts where olfaction is important, such as assessing food patch quality, social behavior, predator–prey relations, and interspecific competition.