| Granivorous rodents living within rich communities of scatter-hoarding species partially rely on olfaction as a foraging strategy to locate buried seeds. While abiotic factors are well known to influence the olfactory capabilities of foraging rodents, we know less about how seed olfactory cues may change over time, how attributes of seed resources themselves affect olfactory cues, or exactly what type of odors are being detected. Such questions are worth investigating, as olfactory cues may influence resource partitioning in arid ecosystems through their importance to a rodent's foraging strategy. I investigate three hypotheses in the first chapter, 1) the olfactory signal of seeds buried in dry soil diminishes over time, 2) the presence of a seed's hardened seed coat acts to diminish olfactory cues, and 3) the olfactory signal of buried seeds increases following rain events. Data collected using artificial foraging grids in Little Valley, NV, and laboratory foraging trials, suggest that a seed's soil residence time does not alone significantly affect rodents' ability to detect cached seeds, and in fact, localized disturbances associated with seed burial may slightly increase cache detection. Alternatively, a seed's hardened, durable shell does appear to play an important role in reducing detection by naive foragers. Further, my experimental evidence showed that rodents were more likely to retrieve their own caches, than pilfer, in moist soil conditions. In chapter two I investigated the hypotheses that 1) Jeffrey pine seeds contain volatile terpenoids (VTs) that contribute to seed odor, 2) VTs are used as cues when rodents forage for buried seeds, and 3) rodents can detect individual macronutrients. I used similar investigative techniques as those in chapter 1, with the addition of solid phase micro-extraction (SPME) paired with gas chromatography/mass spectrometry (GC/MS) headspace analysis. Jeffrey pine seeds were found to contain 16 compounds, most of which were released from whole, wetted seeds. The majority of compounds appeared to emanate from the seed coat rather than the nutrient-containing seed embryo. One of these, (-)ss-pinene, was found to illicit digging behavior in the lab, while another, (-)R-Limonene, had no such effect. When isolated macronutrients were presented to chipmunks, lipids and proteins were found most successfully, while carbohydrates were not. Cumulatively these data suggest that the chemosensory information available within seed resource may be as important in making foraging decisions as they are in social and predator awareness contexts. As effective density dependent predators, rodents pressure plant resources to balance selective costs of dispersal with loss to seed predators, pathogens, and fungi. The high preference of rodents for Jeffrey pine seeds and the presence of a variety of terpenoid compounds within them suggest that these balancing mechanisms have already been at work shaping plant-animal interactions in an animal-dispersed pine. |
