Influence of visual information on sniffing behavior in a routinely trichromatic primate

Most catarrhine primates are considered to be strongly visually oriented animals, obtaining information about conspecifics and their environment from a diversity of visual cues. Other sensory modalities may provide information that is redundant and/or complimentary to visual cues. When cues from multiple sensory modalities are available, these may reinforce or suppress each other, as shown in a range of taxa from insects to humans. In the present study, we tested how the presence and ambiguity of visual information affects the use of olfactory cues when exploring food and non-food items in semi-free ranging Barbary macaques at Affenberg Salem, Germany. In experiment 1, we presented monkeys with pipes containing food (peanuts, popcorn), non-food (stones, feces) or no items. The ends of the pipes were either transparent or opaque, and we assessed whether animals looked, sniffed and/or grabbed into the pipes depending on visibility of the contents. In experiment 2, we manipulated the visual appearance of familiar food items (popcorn) with food colorant and scored if monkeys sniffed at and/or ate the popcorn depending on whether it was uncolored or dyed. In both experiments, the available visual information modulated sniffing behavior. Both visual as well as olfactory information further affected, whether or not monkeys attempted to retrieve the items from the pipes in experiment 1, while data were insufficient to assess this in experiment 2. Hence, reliance on the olfactory sense was modulated by the available visual information, emphasizing the interplay between different sensory modalities for obtaining information about the environment.

Methods
We presented monkeys with pipes containing food (peanuts, popcorn), non-food (stones, feces) or no items in transparent or opaque containers and continuously monitored the pipe setups with wildlife cameras. Behaviours of approaching monkeys were scored from the recorded videos. We further manipulated the visual appearance of familiar food items (popcorn) with food colorant and presented dyed and undyed pieces to the monkeys. Behaviours were recorded with a handheld video camera and scored from the videos.