To what extent do elephants use pre-ingestive cues and/or postingestive feedback when making diet choices? (#295)
Plant quality varies dramatically both among and within species, creating a patchy foraging environment. Elephants, like all other herbivores, can use various sources of information to assess plant quality, make dietary choices, and move among patches. Both pre-ingestive cues (appearance, smell, or taste of a plant) and postingestive feedback measures (crude protein, digestibility, tannin content) can potentially contribute to their process of dietary selection. The role of these mechanisms in influencing foraging of elephants is poorly understood. Previous studies have focused on postingestive measures and have yielded mixed results regarding their importance. Because all plants emit odours composed of various volatile organic compounds (VOCs), elephants could obtain information about a plant prior to ingestion. However, the degree to which they use pre-ingestive odour cues when making foraging decisions is unknown. We assessed the importance of postingestive measures and/or pre-ingestive cues (specifically plant odours). Dietary choices of foraging African elephants were compared to attributes of the available plant species that provided pre-ingestive cues and postingestive measures. We found that avoided plant taxa contained both high levels and a wide diversity of plant secondary metabolite odour cues, particularly monoterpenes, known antiherbivore compounds. Our behavioural studies and laboratory analyses suggested that these volatile monoterpenes played a stronger role in elephant diet choice than phenolics (such as tannins). This is likely because monoterpenes are a pre-ingestive cue that incurs postingestive costs. Furthermore, analysis of elephant saliva indicated that elephants likely can cope with a certain amount of tannins via salivary tannin-binding proteins.