Bushbaby foraging ecology: walking the tightrope between predation risk and plant toxins

Background/Question/Methods: Foraging animals face many constraints while obtaining food. For small fruit-eating mammals, two prominent ones include predators and plant toxins. Effective foraging involves balancing the cost of avoiding predators against the physiological costs of consuming too much toxin. The units quantifying the proximate costs of these two factors differ, but the ultimate cost of both is on fitness. Using the fruit-eating omnivorous primate, the thick-tailed bushbaby (the brown greater galago, Otolemur crassicaudatus), we titrated predation risk against increasing concentrations of a plant toxin. We sought the “tipping point” where the animals equate a particular concentration of plant toxin with a certain amount of predation risk. To do this, we measured the giving-up densities (GUD) of free-living animals from five paired feeding patches at each of four sites in forest at the Lajuma Nature Reserve, South Africa. For each pair, one patch was placed in a Tree (“safe”) with one of five concentrations (0 – 20 %) of the plant toxin cineole (or, in a second trial, gallic acid). The other patch was placed on the Ground, without cineole but with fresh leopard scat nearby (“risky”). We filmed animals as they fed from the food patches (plastic tubs stocked with dry cat food mixed into sawdust as an inedible matrix). Each trial ran five days, as a Latin-square design.  Results/Conclusions: For the cineole trial, the Ground-GUD was higher than the Tree-GUD at zero cineole. The Ground-GUD remained constant but Tree-GUDs increased with cineole concentration, intersecting at 5% cineole. Bushbabies spent most (77%) of their time feeding, otherwise they were vigilant (33%); irrespective of treatment. They were more often alone and showed a more heightened state of vigilance on the ground. The non-linear cumulative time that bushbabies spent at the patches throughout the night shifted towards later as cineole concentrations increased, thus their foraging time was progressively extended and delayed. Gallic acid produced similar patterns as cineole. Our results demonstrate that bushbabies quantify and compare the two foraging costs, and alter their behaviour – in different ways – in response to both costs. The tipping point occurred at 5% cineole, when toxin-rich food in trees was equivalent to the perceived risk of predation on the ground. The decisions these animals make are highly responsive to variation in toxin load, which has implications for fruit selection; but further, they demonstrate the sophisticated ecological balancing acts that animals use when foraging.