We examined the effects of food-safety interactions upon the foraging of free-ranging Sykes’ monkeys (Cercopithecus albogularis erythrarchus) at Lajuma Research Center, South Africa. To this end, we conducted giving up density (GUD) experiments with peanuts previously soaked in solutions of secondary compounds offered in safe and risky microhabitats. Our preliminary goal was to assess baseline preferences for quebracho tannin-treated and oxalic acid-treated foods, as well as to assess what the monkeys perceive to be safe vs. high predation risk microhabitat. Our primary goal was to examine the tradeoffs made by the monkeys when offered peanuts treated with tannin or oxalic acid in safe (2m high in trees) and risky (ground) microhabitats. We expected the monkeys to forage less from patches on the ground than from patches in trees when the food contained tannins and to leave high but equal GUDs between tree and ground when the food contained oxalic acid. Finally, we predicted that increasing concentrations of secondary compounds in safe patches would raise GUDs in those patches, driving the monkeys down to forage more and leave lower GUDs on untreated peanuts in risky patches.
As expected for the preliminary experiments, the monkeys left 14% higher GUDs on the ground than in trees, and GUDs were significantly higher in patches containing oxalic acid-treated peanuts than in patches with tannin-treated and untreated peanuts. Contrary to our predictions, when the same treatment-type of peanut was presented on the ground and in the tree above, GUDs differed between ground and tree patches by 18% for oxalic acid and by 8% and 7% for tannin and untreated peanuts, respectively. Conforming partially to expectation for the final experiment, the monkeys reversed foraging from safe to risky microhabitats where safe food patches contained peanuts soaked in a 30% solution of quebracho tannin and risky patches contained untreated peanuts. In spite of this 30% tipping point, GUDS did not continue to decrease on the ground where peanuts with 40% and 50% solution concentrations were offered in the safe microhabitat above. In safe patches with oxalic acid, ground-tree differences in GUDs did not differ with varying concentrations of the compound. We conclude that the interaction of plant secondary compounds with predation risk can significantly influence the foraging of Sykes’ monkeys. However, the exact pattern of response in socially foraging animals, like the Sykes’ monkey, may differ from what one might expect for non-social foragers.