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Theory predicts that dietary specialization is limited in part because of phys- iological trade-offs associated with specialization. Specifically, dietary specialists are pre- dicted to evolve mechanisms that reduce costs and enhance excretion of plant secondary metabolites (PSMs) that they frequently consume, but are less energetically and/or mech- anistically efficient at eliminating novel PSMs. Although examples of trade-offs associated with dietary specialization have been documented among insects, trade-offs have not been investigated in mammalian herbivores. To initially investigate the physiological trade-offs associated with specialization in mammalian herbivores, we determined the impact of in- gesting novel PSMs on the energy budgets of a juniper specialist (Neotoma stephensi) and generalist (N. albigula) woodrat. These results were then compared to the impact of juniper intake, the plant species preferred by the specialist, on energy budgets in these same species. Energy budgets were evaluated by measuring apparent metabolizable energy intake (AMEI, energy consumed 2 energy excreted) and energy expended on basal metabolic rate (BMR) and voluntary wheel running in specialist and generalist woodrats fed a formulated diet with and without PSMs from the novel plant creosote (Larrea tridentata). Both specialists and generalists were energetically impacted by the intake of a diet containing novel PSMs, primarily through decreased intake of creosote diet and increased excretion of energy in the urine. However, specialists were impacted to a greater extent than generalists. Specialists decreased body mass, whereas generalists maintained body mass. Specialists also reduced intake and AMEI when consuming creosote diet compared to control to a greater extent than generalists. In turn, specialists had one-sixth the energy available for energy dependent activities than generalists on a diet containing novel PSMs. These results are in stark contrast to the comparisons between specialists and generalists consuming PSMs from their natural diet. Specialists consumed more juniper, had higher AMEI and available energy than gen- eralists when fed diets containing juniper foliage. These data provide the first support that physiological trade-offs associated with dietary specialization may exist in mammalian

Ingestion of plant secondary metabolites (PSMs) presents a physiological and behavioral challenge for mammalian herbivores. Herbivores must not only detoxify PSMs, but they may also deal with energetic constraints such as reduced food intake, mass loss, increased excretion of energy, and increased metabolic demands. We hypothesized that the energetic consequences of consuming PSMs will significantly compromise apparent metabolizable energy intake (AMEI) and energy expenditure in mammalian herbivores. Furthermore, we hypothesized that foraging strategy would influence the degree to which plant consumption impacts energy budgets, such that dietary specialists would be less impacted than generalists when both are consuming the plant species preferred by the specialist. Hypotheses were tested by comparing AMEI and energy expended on basal metabolic rate (BMR) and locomotion in a juniper specialist ( Neotoma stephensi) and generalist (N. albigula) woodrat fed control diet and diet containing juniper foliage ( Jun- iperus monosperma). In general, the intake of PSMs in juniper increased the energy excreted in urine and feces in both specialist and generalist woodrats. Specialist woodrats minimized the costs associated with the intake of juniper by ingesting more juniper diet, thereby increasing energy intake, and reducing energy expended on BMR and locomotor activity. Generalist woodrats also decreased locomotor activity on a juniper diet but did not increase intake and maintained BMR. In turn, specialist woodrats had twice as much energy available for activities such as reproduction when consuming a juniper diet than generalists. These results suggest that the intake of PSMs impinges on AMEI and compromises energy ex- penditure, but that the impact of PSMs on energy budgets is relative to ecological experience with PSMs. Moreover, compensatory feeding, metabolic depression, and low activity may

Plant secondary metabolites (PSMs) can affect survival, reproduction, and distribution of herbivores. Individuals with a high capacity to tolerate PSMs will experience fewer and smaller adverse effects than less tolerant individuals. Theoretically, the capacity to tolerate PSMs can be acquired during development, modulated during adulthood, or genetically fixed. We studied tolerance to phenolic resin from creosote bush (Larrea tridentata) in two populations of desert woodrats as a first step in understanding phenotypic habituation and genetic adaptation of this species to creosote resin. One population was from Mojave desert habitat where woodrats eat creosote bush, and the other from the Great Basin desert, where creosote bush is not present and woodrats consume mainly juniper (Juniperus osteosperma). For >1 mo in the laboratory, woodrats from both populations were fed rabbit chow with increasing amounts of phenolic resin extracted from creosote bush until they lost body mass or showed any sign of sickness. Woodrats from the Mojave population maintained body mass at higher concentrations of resin and remained in the experiment longer. There were no differences between populations in food intake across all resin levels; however, maximum resin intake was 25% higher for the Mojave population. Food intake decreased with increasing resin intake. Glucuronic acid excretion in urine, one indicator of detoxification capacity, did not differ between populations. Water consumption increased with increased levels of creosote resin in the diet in woodrats from both populations. The results are consistent with the idea of differential tolerance to creosote bush phenolic resin in desert woodrat populations. Woodrats appear to be a promising natural system to study the developmental or genetic factors underlying vertebrate adaptation to plant secondary metabolites.

Generalist mammalian herbivores typically avoid plant species containing high levels of plant secondary compounds because generalists are thought to lack the ability to process large quantities of these chemicals. Here I propose and test two hypotheses: (1) that food-hoarding herbivores behaviorally overcome consumption limits imposed by secondary compounds by storing food until the toxins degrade; and (2) that the presence of secondary compounds in cached plant material facilitates the preservation of these items, as well as other cached items that lack such compounds. To evaluate these hypotheses, I conducted a number of field and laboratory experiments using the North American pika, Ochotona princeps, a generalist herbivore that consumes low-phenolic vegetation in the summer while it simultaneously collects and stores high-phenolic vegetation for subsequent consumption during winter. In experiments investigating decomposition of summer and winter diets of pikas, after 10 mo of storage, the winter diet retained 20.5% more biomass, and was higher in energy, lower in fiber, and equal in nitrogen compared to the summer diet. Moreover, a common food item in the winter diet, Acomastylis rossii, which contains high levels of phenolics, was the only plant extract to deter bacterial growth in a bioassay. Acomastylis rossii leaves with experimentally reduced phenolic levels retained significantly less biomass than leaves with natural phenolic concentrations. However, the presence of A. rossii in artificial caches containing a low-phenolic species, Trifolium parryi, did not facilitate the preservation of T. parryi. Approximately halfway through the typical storage period, phenolic concentrations of pika winter diet samples in artificial caches decreased to levels readily consumed by pikas in their summer diet. Examination of natural haypiles of pikas before and after storage revealed that pikas do increase their intake of A. rossii from the haypile sometime during the winter. In experiments with captive pikas, pikas preferred A. rossii with experimentally reduced phenolic concentrations over those with natural concentrations. Observations of pikas foraging from natural and artificial haypiles suggested that pikas do not increase their intake of A. rossii from the haypile until phenolics levels decrease. Taken together, the results support both of the hypotheses. Pikas manipulate plant chemistry by storing plants rich in allelochemicals and by delaying consumption of these plants until the toxins decay. Moreover, plants with high levels of secondary compounds exhibit superior preservation qualities so that more biomass and nutrients are retained during storage. As food caching is a common strategy among several animal classes and many foods contain potentially deleterious compounds, the manipulation of food toxins by storage may be a prevalent phenomenon.

Climate change, human disturbance, and disease can have large impacts on the dynamics of a species by affecting the likelihood of survival and reproduction of individuals. We investigated the roles of precipitation, off-road vehicle (ORV) alteration of habitat, and infection with Sin Nombre virus on the survival and reproductive probabilities of deer mice (Peromyscus maniculatus). We used generalized linear mixed models to estimate the effects of these factors and their interactions by fitting capture-recapture data collected seasonally from 2002 to 2007 at 17 sites in the Great Basin Desert of central Utah, USA. During periods with high precipitation, we found no difference in survival and reproductive probabilities between seasons, but during drier periods, we found a reduction of overwinter survival and fall reproductive activity. Precipitation also interacted with disturbance to affect survival probabilities and female reproduction; in periods with low precipitation, deer mice on highly disturbed sites had extremely low survival probabilities and low reproductive probabilities of females compared to those of individuals from low-disturbance sites. However, high precipitation ameliorated the effect of disturbance on both parameters. Deer mice from sites with high impact of ORV disturbance also had low survival over summer. Additionally, male reproductive probabilities were diminished on highly disturbed sites in both seasons; in contrast, they were reduced only in the fall on low-disturbance sites. Density had an overall negative effect on survival and reproductive probabilities of deer mice. For females, the negative effect on reproductive activity was amplified in highly disturbed sites. We found no effect of hantavirus infection on survival probabilities of deer mice. Overall, this study revealed complexity in the determinants of deer mouse survival and reproduction given by the effects of a number of significant interactions among explanatory variables. Thus, factors that may not appear to have a strong effect when investigated alone can still be influential by modulating the effect of a different factor.

The diet of the predominantly herbivorous Bonaire Island whiptail lizard (Teiidae: Cnemidophorus murinus) was examined to assess three models of diet selection by generalist herbivores. These models were: single-nutrient maximizing, toxin avoiding, and nutritional wisdom. At three sites we gathered data on the diet of the lizard over a full year (both wet and dry seasons), on the relative abundance of all plant food types available to the lizards during that period, and on the nutritional composition of each plant type. Thirteen nutritional variables were measured, including content of energy, protein, minerals, water, and potentially toxic plant secondary compounds, and digestibility of protein. The lizards were generalist feeders, consuming a wide variety of flowers, fruits, leaves, nectar, and some animal material. Most stomachs contained more than one food type, suggesting C. murinus typically samples several kinds of plant materials each day. The lizards were selective with regard to foods eaten; most foods were not taken in proportion to their availability in the environment. Potential plant foods varied in nutritional quality, but no one nutrient was correlated with dietary preferences. Multivariate analysis revealed that preference or avoidance of a potential food type could be predicted by a combination of nutritional properties, but these differed among sites. Despite differing plant assemblages at each site, annual intake of nutrients by the lizards was similar among sites. The results best support the nutritional wisdom hypothesis; the Bonaire whiptail lizard may assemble its diet to obtain the proper balance of required nutrients, while avoiding dangerous levels of plant secondary compounds.