Roles of Odor, Taste, and Toxicity in the Food Preferences of Lambs: Implications for Mimicry in Plants
Provenza, F. D., Kimball, B. A. and Villalba, J. J. 2000. Role of odor, taste, and toxicity in the food preferences of lambs: implications for mimicry in plants. – Oikos 88: 424–432.
In the traditional sense, food ingestion consists of prehending, masticating, swallowing, and digesting plant matter. It is also possible to ingest plants without eating them. Volatile compounds are inhaled directly into the lungs and transported from the lungs into the bloodstream. Volatiles in high concentrations could presumably produce toxicosis, without an herbivore ever ingesting a plant in the customary sense. Volatile compounds may be aposematic, serving to warn potential predators of toxins in plants. We conducted three experiments to explore the roles of odor, taste, and toxicity in the food preferences of lambs. The first experiment determined if brief exposure to a novel odor followed by lithium chloride (LiCl)-induced toxicosis caused lambs to avoid a familiar food that contained the odor. Lambs that sniffed coconut-flavored barley and then received LiCl subsequently ate less coconut-flavored barley than lambs that did not receive LiCl. The second experiment determined if lambs were deterred from eating a familiar food by the odor of Astragalus bisulcatus. A. bisulcatus is a malodorous (to humans) sulfur-containing herb considered unpalatable and toxic. Neither odor nor intraruminal infusions of A. bisulcatus deterred lambs from feeding. The third experiment also determined how the degree of familiarity with the odor of A. bisulcatus, along with toxicosis, influenced preference of lambs for food with or without the odor of A. bisulcatus. Lambs with 8 d exposure to the odor but not given LiCl ate similar amounts of food, with and without the odor of A. bisulcatus, whereas lambs given LiCl showed a mild aversion to food with the odor during testing. Lambs with 1 d exposure to the odor but no LiCl ate similar amounts of food, with and without the odor, whereas lambs given LiCl showed a strong but transient aversion to food with the odor. Collectively, these findings show that lambs responded strongly to novel odors, but their response was transient and depended on the postingestive consequences of toxins and nutrients associated with odor inhalation. Thus, we submit that odor alone, in the absence of toxicosis or nociception, is not a deterrent to herbivores that continually sample foods and adjust intake based on the postingestive effects of toxins and nutrients. It also is unlikely that non-toxic plants can mimic the odors of toxic plants to avoid herbivory (Batesian mimicry), unless the odors are indistinguishable by herbivores, again because herbivores constantly sample foods.