Review: Using physiologically based models to predict population responses to phytochemicals by wild vertebrate herbivores.

To understand how foraging decisions impact individual fitness of herbivores, nutritional ecologists must consider the complex in vivo dynamics of nutrient-nutrient interactions and nutrient-toxin interactions associated with foraging. Mathematical modeling has long been used to make foraging predictions (e.g. optimal foraging theory) but has largely been restricted to a single currency (e.g. energy) or using simple indices of nutrition (e.g. fecal nitrogen) without full consideration of physiologically based interactions among numerous co-ingested phytochemicals. Here, we describe a physiologically based model (PBM) that provides a mechanistic link between foraging decisions and demographic consequences. Including physiological mechanisms of absorption, digestion and metabolism of phytochemicals in PBMs allows us to estimate concentrations of ingested and interacting phytochemicals in the body. Estimated phytochemical concentrations more accurately link intake of phytochemicals to changes in individual fitness than measures of intake alone. Further, we illustrate how estimated physiological parameters can be integrated with the geometric framework of nutrition and into integral projection models and agent-based models to predict fitness and population responses of vertebrate herbivores to ingested phytochemicals. The PBMs will improve our ability to understand the foraging decisions of vertebrate herbivores and consequences of those decisions and may help identify key physiological mechanisms that underlie diet-based ecological adaptations.

The balancing act of foraging: mammalian herbivores trade-off multiple risks when selecting food patches.

Animals face multiple risks while foraging such as the risk of acquiring inadequate energy from food and the risk of predation. We 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 types and levels of perceived risks (i.e., fitness cost × probability of occurrence), including fiber and toxins in food, exposure to predation, and distance from a refuge. We measured food intake by the rabbits at paired food patches that varied in these risks and used the method of paired comparisons to create a relative ranking of habitat cues, which revealed an animal's perceived risk on a single scale representing an integrated response to a variety of risks. Pygmy rabbits perceived exposure to predation risk and distance from a burrow as riskier than did cottontails, whereas cottontails perceived dietary toxin as riskier. Pygmy rabbits consumed lower quality food, containing higher fiber or toxins, thereby avoided 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. We have shown how risks can be integrated into a single model that allows animals to reveal their perceptions of risks on a single scale that can be used to create a spatially explicit landscape of risk.

Ambient temperature influences diet selection and physiology of an herbivorous mammal, Neotoma albigula.

The whitethroat woodrat (Neotoma albigula) eats juniper (Juniperus monosperma), but the amount of juniper in its diet varies seasonally. We tested whether changes in juniper consumption are due to changes in ambient temperature and what the physiological consequences of consuming plant secondary compounds (PSCs) at different ambient temperatures might be. Woodrats were acclimated to either 20 degrees C or 28 degrees C. Later, they were given two diets to choose from (50% juniper and a nontoxic control) for 7 d. Food intake, resting metabolic rate (RMR), and body temperature (T(b)) were measured over the last 2 d. Woodrats at 28 degrees C ate significantly less juniper, both proportionally and absolutely, than woodrats at 20 degrees C. RMRs were higher for woodrats consuming juniper regardless of ambient temperature, and T(b) was higher for woodrats consuming juniper at 28 degrees C than for woodrats eating control diet at 28 degrees C. Thus, juniper consumption by N. albigula is influenced by ambient temperature. We conclude that juniper may influence thermoregulation in N. albigula in ways that are helpful at low temperatures but harmful at warmer temperatures in that juniper PSCs may be more toxic at warmer temperatures. The results suggest that increases in ambient temperature associated with climate change could significantly influence foraging behavior of mammalian herbivores.