Araujo, A., Didonet, J., Araujo, C., Saletti, P., Borges, T., & Pessoa, V. (2008). Color vision in the black howler monkey (Alouatta caraya).Visual Neuroscience, 25, 243-248.

Previous genetic and microbiological studies have shown that both male and female Howler monkeys have the potential to routinely express trichromatic color vision. Although this feature is common in Old World monkeys, Howler monkeys are the only New World monkeys studied to date where males routinely express trichromatic vision. The purpose of this study was to determine whether Howler monkeys could distinguish colors using a series of behavioral discrimination learning tasks. Easy discrimination tasks involved distinguishing between yellow-reds and purple-blues while more difficult tasks were between yellow-reds and green-yellows. The color chips were created based on Munsell Book of Color, and were specified in three dimensions: hue, brightness, and saturation. Brightness was varied to ensure the animals could not have used brightness cues instead of color cues to discriminate between the chips. The experiments were conducted in diffused daylight. All of the monkeys tested were successful at discriminating between the colors, even those colors that are assumed to only be distinguishable for trichromatic animals. Four monkeys were tested, one female and three males. The researchers concluded that both male and female Howler monkeys routinely express trichromatic color vision.

This study shows that Howler monkeys are an exception to a general rule we learned about primates. In the light reception lecture, we learned that the molecular distinction between dichromatic and trichromatic color vision is in the number of different cone types that an organism expresses. Trichromatic vision requires at least three different cone proteins while dichromatic vision only requires two. We further learned that in primates the genes for cone type are X-linked. In most New World monkeys, the cone genes have become separated. As a result, trichromatic vision is usually restricted to heterozygous females. Howler monkeys, however, appear to be an exception to what we learned: both males and females express trichromatic vision. The paper did not discuss why Howler monkeys might display this unique trait.

In its methodology, this paper also addressed some other issues we discussed in class. For example, the researchers used a three-dimensional model to define their colors similar to the color models we saw in class. Also, the researchers were careful to control the type of light that was present during the experiment. As we learned in class, this is important to control because a light source that is filtered or saturated in one wavelength could change the perceived color of the chips.