The influences of plant secondary metabolites on the foraging behaviour and carrying capacities of the African elephant, Loxodonta africana.
Schmitt, Melissa Holbrook.
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Herbivore diet selection is influenced by plant quality and availability. For browsing and mixed-feeding herbivores, such as elephants, which consume trees and shrubs, plant quality is influenced by the concentrations of crude protein, fibre, and plant secondary metabolites (PSMs). To date, the driving factors behind elephant diet selection have not been well understood. The broad aim of this study was to better understand how pre-ingestive cues such as volatile organic compounds (VOCs) and postingestive cues such as the effects of plant secondary metabolites (PSMs) influenced the foraging behaviour of African elephants. I also examined the salivary tannin-binding proteins of elephants and how these influenced carrying capacities. I found that elephants were able to differentiate between plant odours in the form of VOCs across multiple spatial scales. This could help elephants to reduce their search time while foraging across a landscape. I also found that these pre-ingestive cues were a better indicator of diet preference than the tested postingestive cues such as digestibility, crude protein, tannins (a type of total polyphenol) and the ratio of palatable: unpalatable indices. The VOCs in preferred plant species had significantly lower concentrations and diversity of PSMs than avoided species, particularly with respect to monoterpenes, a known anti-herbivory VOC. In contrast, avoided plant species were more digestible and had lower levels of polyphenols and tannins. Ultimately, I found that terpenes play a stronger role in elephant diet choice than phenolics (such as tannins) and other postingestive feedback measures. This is likely because terpenes are a pre-ingestive cue that also incur postingestive costs, which the elephants can use prior to making foraging decisions, unlike other postingestive feedback measures. While it seems counterintuitive that preferred plants have high concentrations of tannins, which reduces the amount of crude protein available, I found that elephants have salivary tannin-binding proteins, which neutralize the influences of tannins by an average of 75%. This neutralization influences the amount of available crude protein, which would, in turn, influence the carrying-capacity for elephants. I constructed an optimal-foraging model for carrying capacity that incorporated the negative effects of tannins and the neutralization of these chemicals by the tannin-binding affinity of elephant saliva. I also included diet breadth and dietary contribution of browsed species, browse quality, and the available standing crop of browsed species. As a result, my model produced more plausible estimates of elephant carrying capacity than conventional standing-crop models.