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The rainbow of the marmoset and tamarins

The behavior of Brazilian monkeys indicates the existence of four-color vision standards

A study carried out over the last three years at the Primatology Center of the University of Brasilia (UnB) with fifteen (15) Brazilian saguin and capuchin monkeys suggests that these primates show at least four distinct forms of perceiving color. Two females managed to discriminate all colors, more or less as would a normal human being. Another two animals of the female sex had the limited capacity to distinguish tones of green, as if they had had a type of daltonism. A third female exhibited problems in recognizing red matrices as if she suffered from another form of color blindness. A fourth group of ten monkeys, which included eight males and two females, erred in sorting out the colors of the visible spectrum of colors between red and green, which include orange and yellow. “We imagine that in their natural environment the animals of this final group had difficulty in recognizing a red or orange colored fruit among the green foliage”, says Valdir Pessoa, from the UnB, the experiment’s coordinator, which involved six Golden-headed Tamarin (Leontopithecus chrysomelas), five Black Tufted-ear Marmosets (Callithrix penicillata) and four Black-handed Tamarin (Saguinus midas niger).

The major part of work on vision of the so called New World monkeys, the group of upper primates with snubbed noses that came about in South and Central America approximately 30 million years ago, comes from the study of the biology of the marmosets and tamarins. In these studies, in general there is a measuring of four types of color receptor cells (the cones) that exist on the retina of the animals. If, as in a normal human being, the capuchin or saguin monkey has three photo pigments available to it (one for blue, another for green and a third for red), this animal is called trichromat. When it presents receptor cells of only two of the fundamental colors, it is denominated dichromat. Sometimes the researchers also included an analysis of the monkey’s DNA, where mutations can well be found that lead to distinct forms of trichromatism or dichromatism. In order to study the vision of the small Brazilian primates, Pessoa and his colleagues chose another pathway: they anchored their work on the analysis of the animals’ behavior. “This is our differential”, says the UnB researcher.

How did the Brazilian scientists deduce that an animal was capable of recognizing one color from another? Were they literally to observe the tamarin and marmoset in monkeys in the forests to see if they ate fruit of the most varied matrices? Not at all. Firstly the scientists trained the primates, which were houses in nurseries integrated into the natural landscape of the Clear Water Farm where the Primatology Center is located, to associate a color to the presence of a type of food. Every time that they would remove an orange colored disc, which covered a hole, they would find a piece of fruit hidden inside. Once they had learned this standard of behavior the animals were submitted to a choice: they had to choose between two discs, one always being of an orange hue and another of a distinctly different color. It was expected that if the marmoset or tamarin could differentiate between the colors it would opt, in the majority of times, to remove the orange colored disc and not the other, in the hope of obtaining the prize. In total, the primates were shown 96 distinct pairs of discs, each one of them with a color, shine and specific level of saturation.

Adaptive advantage
To discriminate blue from orange was a piece of cake. All of the primates, independent of the sex and species to which they belonged, managed to differentiate the two colors in at least 65% of the cases, the lowest level of assertion considered sufficient to validate the recognition of tonality. “A lower level than this could  have been derived from a chance color choice on behalf of the animals”, says the biologist Daniel Pessoa, another of the authors of the study with the three species of Brazilian monkeys that is shortly going to be published in the scientific magazine American Journal of Primatology. As was expected, none of the animals had success in distinguishing pairs of discs that contrasted two distinct tones of orange. At the end of the experiment, the researchers concluded that the behavior of the tamarins and marmosets had provided evidence that they had four distinct standards for color perception. “But as yet we don’t know what adaptive advantages these standards of vision may well give to the animals in their natural environment”, ponders Daniel.

Molecular biology studies carried out abroad with other species of tamarins and marmosets reached the finding of six standards of vision derived through genetic alterations. The Brazilian researchers did not analyze the DNA of the Primatology Center animals. For this reason they are not able to say if these mutations are related to some of the four forms of color recognition that they identified in the primates’ behavior. For some 20 years now evidence has been collecting that the New World monkeys, a group of simians that, in evolutionary terms, is the most distant from man and the large anthropoid primates, show some color vision standards that are very particular to them. The majority of the primates, including man, is always trichromat – or that is to say, can view all of the tonality mixtures surrounding the three primary colors (green, blue and red). However, the marmoset and capuchin monkeys of South and Central America can be tri-chromate or dichromate. In general, the males perceive only the matrices surrounding two colors. The females can be trichromat or dichromate, with the apparent predominance of the former situation.