Polymorphic colour vision and foraging in white-faced capuchins: Insights from field research and simulations of monkey vision

I investigated the consequences of colour vision variation on foraging by white-faced capuchins (Cebus capucinus) inhabiting a seasonally dry forest. Capuchins, like other Neotropical monkeys, possess a polymorphic M/L opsin gene, leading to the presence of multiple dichromatic and trichromatic colour vision phenotypes. My goal was to determine if and how colour vision affected foraging choices, behaviours and efficiency. Capuchins are highly omnivorous and I investigated their foraging patterns for both fruits and insects. My methods included behavioural observations of free-ranging monkeys, as well as computer-based foraging trials in which capuchin colour vision phenotypes were simulated for human participants. I conducted 13 months of field research on four groups of capuchins in north-western Costa Rica between January 2007 and September 2008, and spent 10 months acquiring digital photographs of monkey food items and running computer trials in the Vision and Aging Laboratory at the University of Calgary between January 2009 and August 2010. Genotyping of the monkeys was completed via DNA amplification and sequencing at the University of Tokyo. I found that trichromatic individuals, especially those with the most spectrally-separated photopigments, were more accurate than dichromats during close-distance foraging for conspicuous (reddish) fruits. These fruits were an important component of the capuchin diet and provided a critical source of moisture during the hottest and driest months. These findings indicate that frugivory may exert selection pressures favouring trichromacy. Dichromatic monkeys were able to compensate behaviourally for their poorer chromatic vision by increasing their attempt rates and use of non-visual senses during fruit foraging. Furthermore, trichromat advantage was not universal. Dichromatic monkeys had higher capture efficiency of surface-dwelling invertebrates, while trichromatic monkeys increased their search times to achieve the same net prey intake. Advantages associated with insectivory may therefore favour the persistence of dichromatic phenotypes in this polymorphic system. In sum, advantages can be identified for both dichromacy and trichromacy. Future studies should examine the associated energetic costs and benefits, as well as the long-term reproductive success of dichromatic versus trichromatic primates to better understand the evolutionary mechanisms maintaining polymorphic colour vision.