Color vision in the black howler monkey (Alouatta caraya)

Electrophysiological and molecular genetic studies have shown that howler monkeys (Alouatta) are unique among all studied platyrrhines: they have the potential to display trichromatic color vision among males and females. This study examined the color discrimination abilities of four howler monkeys (Alouatta caraya) through a series of tasks involving a behavioral paradigm of discrimination learning. The animals were maintained and housed as a group in the Zoological Gardens of Brasilia and were tested in their own home cages. Stimuli consisting of pairs of Munsell color chips were presented in random brightness values to assure that discriminations were based on color rather than brightness cues. All the animals (three males, one female) successfully discriminated all the stimulus pairs, including those that would be expected to be difficult for a dichromatic monkey. These results are consistent with the earlier predictions suggesting that howler monkeys are routinely trichromatic.