THE BALANCING ACT OF FORAGING: MAMMALIAN HERBIVORES TRADE-OFF MULTIPLE RISKS WHEN SELECTING FOOD PATCHES

Thesis (Ph.D.), Environmental and Natural Resource Sciences, Washington State UniversityAnimals face multiple risks while foraging such as the risk of acquiring inadequate energy from food, the risk of predation, and the risk of thermal stress. My first objective was to predict the overall perceived risk in foraging patches with multiple types of risks using a novel modeling approach that quantifies tradeoffs among competing risks by foraging animals. I evaluated how two sympatric rabbits (pygmy rabbits, Brachylagus idahoensis, and mountain cottontail rabbits, Sylvilagus nuttallii) that differ in size, use of burrows, and habitat specialization in the sagebrush-steppe of western North America respond to different levels of perceived risks, including fiber and toxins (1, 8 cineole) in food, exposure to predation (inverse of concealment cover), and distance from a burrow refuge. Exposure to predation risk and distance from a burrow refuge were riskier for pygmy rabbits than cottontails, but the dietary toxin was riskier for cottontails than for pygmy rabbits. Pygmy rabbits consumed lower quality food, containing higher fiber or toxins, to avoid feeding in exposed patches or traveling far from their burrow to forage. In contrast, cottontails fed in exposed patches and traveled farther from the burrow to obtain higher quality food with lower fiber and toxins. My second objective was to evaluate how the interactions between ambient temperature and food quality influence selection of food patches and diets by pygmy rabbits and cottontails. I examined preferences for temperature in food patches, and the effect of temperature on diet selection, intake, digestion, passage rate, and metabolism in both species of rabbits. Both species generally chose to feed in patches that were relatively colder, and this effect was greater for the larger cottontails. Both species also chose to eat more total food and a greater proportion of high fiber food when the ambient temperature was colder, passing food more quickly through their digestive system. Temperature did not affect how much 1,8 cineole they consumed nor how thoroughly they digested food. Food quality affected dry matter digestibility, but not resting metabolic rate of the rabbits.Washington State University, Environmental and Natural Resource Science