Generalist dietary responses to individual versus combined plant toxin odors: an African elephant study

Generalist herbivores are thought to prefer consuming combinations of plant toxins over singular toxins. This might not always be the case. In this study, we show that, when making foraging decisions, African elephants did not consistently prefer the odors of combinations of plant toxins compared to single toxins (i.e., monoterpenes). This suggests that the composition and diversity of volatile toxins in plants likely determine the effectiveness of odor-based chemical defenses against generalist herbivores. The detoxification limitation hypothesis posits that, unless plant defense compounds interact synergistically or additively to increase their harmful effects, generalist herbivores will prefer consuming combinations of these compounds more than singular compounds. Monoterpenes are odoriferous defense compounds that may be toxic to mammalian herbivores when ingested in sufficient quantities. Previous research has shown that the addition of individual monoterpenes to food sources reduces consumption by generalist mammalian herbivores. By using African elephants as a case study, we aimed to determine whether odors from monoterpene combinations (i.e., two or more monoterpenes) also deter generalist mammalian herbivory, and whether generalist herbivores prefer the odors of monoterpene combinations more than individual monoterpenes. First, we tested whether the odor of monoterpene combinations that resemble the monoterpene profiles of a high-acceptability, intermediate-acceptability, and low-acceptability plant deter herbivory. We found that elephants preferred plants without the added odors of the monoterpene combinations. Second, we explored how elephants responded to individual monoterpenes found within the combinations compared with the combinations at the same set concentration, and found that the elephants did not always prefer the combinations more than the individual monoterpenes. Moreover, the more diverse the combination, the less frequently it was preferred when compared with the individual monoterpene odors. Our results imply that generalist herbivores do not necessarily prefer combinations of plant chemical defenses at comparatively lower concentrations and that, consequently, the composition and diversity of monoterpene profiles in plants likely determine the efficacy of these compounds as an olfactory defense against mammalian herbivory.