Stomaching Toxic Guts: An Avian Herbivore Strategy for Limiting Exposure to Plant Secondary Metabolites
Avian herbivores are known to use behavioral strategies to combat chemically defended plants while foraging, but this alone is insufficient to ensure survival with such dietary specialization. Many flora consumed by herbivores contain plant secondary metabolites (PSMs), so even under a highly selective foraging regime that reduces exposure, herbivores must ingest some of these potentially toxic compounds. Although physiological defenses to PSMs are well known in mammalian herbivores, the same cannot be said for avian herbivores. We used the greater sage-grouse (Centrocercus urophasianus) to test the hypothesis that avian herbivores regulate the absorption of PSMs by limiting the absorption of ingested sagebrush chemicals throughout the entire digestive tract. We used gas chromatography to detect monoterpenes (a toxic constituent of plant chemical defenses) in the sagebrush browsed by sage-grouse, in the contents within the different segments of the sage-grouse digestive tract, and from sage-grouse fecal droppings. The major monoterpenes detected in browsed sagebrush were found throughout the entire digestive tract, except the ceca, and in the excreta of the sage-grouse. The monoterpenes detected in the contents of the digestive tract and in the fecal droppings were approximately the same proportion as what was consumed. Results indicate that sage-grouse limit the absorption of ingested PSMs in the small intestine and thereby excrete these compounds unmetabolized in the feces. Regulated absorption of PSMs may be one strategy to increase their tolerance to chemically defended plants. However, further investigation is required to determine the extent to which PSMs are absorbed and elucidate other physiological strategies that may also function to limit systemic exposure to PSMs in sage-grouse.