The foraging tightrope between predation risk and plant toxins: a matter of concentration

1. Plants defend and predators attack, provoking the foraging dilemma faced by herbivores and frugivores of how to eat enough without being eaten. High toxin concentration in leaves and fruits inhibits consumption, while predation risk reduces feeding opportunities, as prey forage to avoid encountering predators. Thus, both factors vary and define the quality of the landscape. How foraging animals directly quantify, compare and respond to these two costs has rarely been tested. 2. We show that free-ranging bushbabies – small, frugivorous primates – change their behaviour and use of artificial food patches based on the interplay between toxin concentration in food and patch safety. Using a titration experiment, we demonstrate that bushbabies quantify the relative costs of toxin and fear. We pinpoint where these costs are equivalent and show that animals seek food patches with the lower net cost. 3. We conclude that the ecological effectiveness of plant toxins as defence against consumers needs to be considered in the context of a landscape of fear – and the relative impact of antipredator tactics and plant defence is strongly shaped by the concentration of these defences. 4. A corollary is that plants may benefit from fear as a substitute for their own chemical defence, adding a new dimension to the concept of indirect plant defence. Whether, from the plant’s perspective, the benefits derived from fear can be considered evolutionarily adaptive rather than simply ecologically serendipitous remains to be tested.