Plant secondary metabolites compromise the energy budgets of specialist and generalist mammalian herbivores

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 (Juniperus 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 expenditure, 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 be strategies employed by specialist woodrats that can mitigate the cost of processing PSMs, but these strategies may be constrained in generalist woodrats.