Specialist and generalist herbivores differentially regulate food and water intake while on diets containing plant secondary compounds

Background/Question/Methods

The “generalist” feeding strategy of mammalian herbivores is hypothesized to be a mechanism that permits herbivores to regulate intake of plant secondary compounds (PSCs) thereby decreasing the possibility of over-ingestion of large quantities of similar toxins present in a single species diet. This hypothesis however, presupposes that generalist herbivores are capable of detecting PSC concentrations of foods and halting a meal before suffering ill effects from any particular plant, and if necessary selecting a different food item to meet energetic demands. Alternatively, specialists are predicted to have evolved biotransformation pathways that can process large doses of the host plant’s compounds, therefore they are not predicted to require behavioral regulation. We tested these hypotheses by comparing the spontaneous feeding behavior two sympatric herbivorous rodents: a juniper specialist, Neotoma stephensi, and a generalist species, N. albigula. Juniper (Juniperus monosperma) contains marked concentrations of terpenes, particularly alpha pinene. Animals were fed diets with increasing concentrations (0-90%) of juniper for three days per concentration. Animals that lost more than 10% of starting body mass were removed from the trial. We measured the microstructure of food intake as well as the 24 hour water intake of all animals.

Results/Conclusions

All N. stephensi completed the trial (N=8) whereas five of twelve N. albigula were removed for mass lost during the 90% juniper treatment. The specialist did not regulate terpene intake at any juniper concentration and ingested significantly higher quantities of terpenes per meal than the generalist. Furthermore the specialist did not adjust water intake with increasing juniper concentrations. The generalist, in contrast regulated terpene ingestion. The amount of terpenes consumed in a single meal did not differ between 50% and 90% juniper because the generalist decreased meal size in a dose dependent manner. Also, the generalists doubled water intake between the control diet and the 90% juniper diets. The percent increase in water intake between the control and 90% diets was a statistically significant predictor of the animals’ ability to remain in the trial. It is unclear whether or not increased water consumption is a direct effect of juniper consumption or if this is a behavioral modification the gereralists use to increase their capacity to process a toxic diet. These data provide evidence for the hypothesis that the generalist foraging strategy is a mechanism to regulate intake of PSCs.