Abstract
Among New World primates, only one genus out of 18 is known to have routine trichromatic color vision like humans. All other species investigated to date have diverse color vision genotypes that suggest potential diversity in their visual ecology. Some evidence indicates that specific color vision phenotypes, particularly dichromats, are more successful when foraging in dim light or foraging for cryptic prey. Using light spectra measured during the transition from darkness to daylight, and behavioral data collected during ongoing studies, we analyzed the timing of the earliest activities requiring vision. For four sympatric primates at the Tiputini Biodiversity Station in Amazonian Ecuador we found no evidence of activity occurring before the onset of nautical twilight (~48 min before sunrise). Observers on the ground frequently thought that monkeys began their morning activity in darkness, but measures of the quantum flux between 400 and 700 nm showed that only 1-2% of the light at canopy level reaches the ground, so the light available to the animals is ~2 log units higher. The larger monkeys (Ateles and Lagothrix), whose diets include a high proportion of ripe fruit, left their sleeping trees and began to move through the canopy earlier than the smaller monkeys whose diets are more diverse (Callicebus) or include more cryptic, unripe fruits (Pithecia). Although human observers at canopy level were able to distinguish colors and fine detail at 21-23 min before sunrise, most of the monkeys waited until after sunrise to begin feeding. For the larger monkeys, 80-90% of the feeding bouts occurred after sunrise, and for the smaller monkeys, 100% of the feeding bouts occurred after sunrise. Thus, it appears that the monkeys wait for good viewing conditions before feeding, and they are not exploiting the dim light of twilight where dichromacy might confer some advantages.
Meeting abstract presented at VSS 2017