Strategies in herbivory by mammals revisited: The role of liver metabolism in a juniper specialist ( Neotoma stephensi ) and a generalist ( Neotoma albigula )

Although herbivory is widespread among mammals, few species have adopted a strategy of dietary specialization. Feeding on a single plant species often exposes herbivores to high doses of similar plant secondary metabolites (PSMs), which may exceed the animal's detoxification capacities. Therefore, theory predicts that specialists will have unique detoxification mechanisms to process high levels of dietary toxins. To evaluate this hypothesis, we compared liver metabolism of a juniper specialist, Neotoma stephensi (diet >85% juniper), to that of a sympatric generalist, N. albigula (diet ≤30% juniper). Specifically, we quantified the content of a key detoxification enzyme, cytochrome P450 2B (CYP2B) in liver microsomes, as well as the microsomal turnover of α-pinene, the most abundant terpene in the juniper species consumed by the specialist woodrat. In both Neotoma species, a 30% juniper diet increased CYP2B content (2-3x) and α-pinene turnover rates (4-fold). In N. stephensi, higher levels of dietary juniper (60% and 100%) further induced CYP2B content and increased turnover rates of α-pinene. Although no species-specific differences in α-pinene turnover rates were observed at 30% dietary juniper, CYP2B content was 1.7x higher in N. stephensi than in N. albigula (p<0.01). In N. stephensi, the rates of α-pinene turnover increased with dietary juniper and were positively correlated with CYP2B content. The ability of N. stephensi to elevate CYP2B content and rate of α-pinene turnover with increasing levels of juniper in the diet (from 30-100%) may facilitate juniper specialization in this species.