Colour Preferences in Relation to Diet in Chimpanzees(Pan troglodytes), Gorillas (Gorilla gorilla) and Mandrills (Mandrillus sphinx).

Although trichromatic colour vision has been extensively studied as it grants significant advantages for Old World primates, it is unknown which selective pressures were behind the trait's evolution. The leading hypothesis would be that colour vision arose as a foraging adaptation because it allowed individuals to spot food more efficiently. To test this, we exposed 3 chimpanzees (Pan troglodytes), 5 gorillas (Gorilla gorilla) and 3 mandrills (Mandrillus sphinx) to colour cardboard plates to assess whether colours related to diet were the most preferred. The experimental setting was divided into two phases. During the first, animals were provided with colour cardboard plates of only 1 colour per data collection session. The order of colour presentation was randomly determined: white, black, yellow, green and red. In phase 2, primates were simultaneously provided with cardboard plates of all colours. Behavioural interactions with plates were measured using a one-zero group focal sampling (10-s sampling intervals and 20-min observation periods). Results showed that when animals were exposed to only 1 colour at a time, they exhibited different colour preferences depending on the species considered. Chimpanzees preferred red and yellow, the colours linked to fruits, while gorillas selected red and white. Mandrills exhibited fewer differences between colour preferences, with red being the most selected. Furthermore, when all colours were simultaneously provided, individuals chose colours related to diet over black and white. Although there were clear individual differences, our results support that trichromatic colour vision is an advantage in detecting and selecting red items. In the wild, it could be important in the detection of reddish fruits and leaves.