Influence of Plant Secondary Metabolites on intake, Detoxification Costs, and Microbial Communities in Deer.

Plants available to wild herbivores, especially browsers, often contain plant secondary metabolites (PSMs). Herbivores have evolved behavioral, physiological, and microbial mechanisms for avoiding and detoxifying PSMs. The detoxification limitation hypothesis suggests that herbivores can reduce toxicity by consuming a mixture of PSMs to avoid overloading a particular detoxification pathway. Although this hypothesis has been examined for smaller mammalian hindgut-fermenters, less is known about responses to PSM mixtures in wild ruminants. To assess the role of host and microbial responses to PSM composition, we used controlled feeding trials to measure voluntary dry matter and PSM intake, urinary excretion of glucuronic acid (GA, a byproduct of PSM detoxification through conjugation), and the diversity and relative abundance of gastrointestinal bacterial families in the feces of two species of captive-raised deer (Odocoileus hemionus, O. virginianus). Deer were fed five mixtures of four purified PSMs that included two same-chemical class mixtures, two different-class mixtures, and one 4-way mixture of all chemicals. Overall, we found that PSM composition had minimal effect on intake, that GA was a consistent physiological biomarker of PSM intake regardless of PSM composition, and that dietary phenolics may influence microbial communities more than monoterpenes. Our results partially conformed to the detoxification limitation hypothesis, where deer consumed less of one same-class mixture (monoterpenes) than different-class mixtures. Our results point to the complexity of the interplay between different behavioral, physiological, and microbial mechanisms that can mediate the consequences of PSMs.