Olfactory and visual plant cues as drivers of selective herbivory

Food quality is an important consideration in the foraging strategy of all animals, including herbivores. Those that can detect and assess the nutritional value of plants from afar, using senses such as smell and sight, can forage more efficiently than those that must assess food quality by taste alone. Selective foraging not only affects herbivore fitness but can influence the structure and composition of plant communities, yet little is known about how olfactory and visual cues help herbivores to find preferred plants. We tested the ability of a free-ranging, generalist mammalian browser, the swamp wallaby Wallabia bicolor, to use olfactory and visual plant cues to find and/or browse differentially on Eucalyptus pilularis seedlings grown under different nutrient conditions. Low-nutrient seedlings differed from high-nutrient seedlings, having lighter coloured leaves, red stems and lower biomass and nitrogen content. In the absence of visual cues, wallabies used odour to differentiate vials containing cut seedlings. They visited and investigated patches with high-nutrient seedling odour most, followed by patches with low-nutrient seedling odour, and patches with no added odour least. However, when visual and olfactory cues of seedlings were present, wallabies reversed their foraging response and were more likely to browse low- than high-nutrient seedlings. This browsing difference, in turn, disappeared when long-range visual cues were reduced by pinning seedlings horizontal to the ground. We suggest that visual cues overrode the effects of olfactory cues on browsing patterns of intact seedlings. Our study shows that herbivores can respond to odours of higher nutrient plants but in ecologically realistic scenarios they use a variety of visual and olfactory cues, with a context-dependent outcome that is not always selection of high nutrient food. Our results demonstrate the importance of testing the sensory abilities of herbivores in realistic multi-sensory settings to understand their function in selective foraging.