The color red plays an important role in our society: it’s how we know when to stop in traffic, how we see if fruit is ripe, and how we symbolize love. We are so used to seeing the color red that we do not often think about what our lives would be like without it, but the truth is that most other mammals live in a world without the rosy hue (1). Dogs, elephants, tigers, bears, and squirrels are all animals that see in blues and yellows. Only certain primates, including us, can see the red rawness of fresh meat or the delectable hue of a ripe raspberry.
The way that animals perceive colors depends on the types of color receptors in their eyes. Each type of receptor absorbs a different wavelength of light, enabling animals to see different colors. Fish and birds live in a world rich with colors. They have four color receptors, sensitive for ultraviolet, blue, green, and red, and are therefore called tetrachromats. In contrast, the receptors for seeing ultraviolet and red were lost in most mammals over the course of evolutionary time. These dichromats were left with blue and green sensitive color receptors, likely because they helped the first mammals, which were nocturnal, to better see in the dark (2). However, some primates managed to gain back the red color receptor, so are known as trichromats.
Why did some primates, including our ancestors, re-evolve the red color receptor? One theory has to do with the fruit that the primates ate (2). If we rewind 30 million years, primates were already at home in forest ecosystems and spent their days foraging for fruits and leaves. If there was less fruit one year, perhaps due to a drought, then all of the fruit-eating animals would compete for these few fruits. A primate with a mutation that allowed it to see the color red would be better able to pick out fruit with bright colors like yellow, orange, and red from the lush backdrop of green foliage (2). In contrast, a primate without such a mutation might climb right past the brightly colored fruit because it would be less likely to see it. Primates with a mutation for trichromatic vision therefore could be more likely to find enough food to eat and survive, and thus pass their genes on to the next generation. Because primates would eat and disperse the seeds of plants with conspicuously colored fruits, these would spread and become more common as well. If this theory is right, we would expect areas of the world where fruit is scarce to have increased numbers of primate species that can see red, as well as more species of brightly colored fruit.
A study by Dr. Renske Onstein and colleagues (3) examined this prediction. To do so, the scientists compared the distribution and color vision of more than 400 primate species with the fruit colors of more than 1700 species of palm trees worldwide. The researchers focused on palm trees because palm fruits are keystone resources for fruit-eating animals and almost all of their seeds are dispersed by animals. Onstein and colleagues hypothesized that there would be the most species of red-seeing primates and palm trees with bright fruit in the subtropics, where fruit is a limiting factor and there is more heated competition among animals for the fruit that is there.
The researchers found that in the Americas and Asia, only 34% of palms had bright fruits, and correspondingly, there were very few trichromatic primates. The primates that can see red in these regions do not appear to use this ability to seek out colorful fruit; they instead forage for leaves or rely on many different colors of fruits. This means that the theory that red color vision evolved in primates to help them spot brightly colored fruit is not supported in these regions of the world.
However, Onstein and colleagues did find a relationship between the number of trichromatic primate species and the number of palm species with brightly colored fruit in Africa. Across the continent, most primates can see the color red and almost 70% of the African palms had bright fruit, and there was a clear pattern to how the species were distributed. In current-day subtropical zones in Africa, where the forest is relatively dry, there were more trichromatic primates and palm trees with bright fruit than in other parts of the continent. The researchers hypothesized that this was due to a scarcity of other plant-based foods in these regions, increasing competition for palm fruits. As a result, primates that could see red would have been especially favored. A similar explanation may underlie why the scientists found support for their hypothesis in Africa, which had arid and cool conditions several million years ago, when trichromatic primates and bright fruit were just starting to evolve. The Americas and Asia were spared these tough climatic conditions, which could be why they do not demonstrate a similar relationship between bright fruit and trichromatic vision.
All in all, this study demonstrates support for the theory of how primates re-evolved color vision, at least for African species. Stressful circumstances that resulted in low availability of food and high competition seemed to have caused the mutation for red-color vision to spread, which also helped brightly colored fruit to spread. The evidence for this is that trichromatic primates are present in the areas where there is bright fruit, and where the climate conditions are or were stressful. However, this study does not show that trichromatic primates caused the evolution of more bright fruit palm species or that bright fruits caused the evolution of more trichromatic primate species. It is also unclear why primates in other parts of the world evolved red color vision.
Africa, in addition to being a hotbed of red color vision in primates, is also where ancient hominins first split away from our red-seeing primate ancestors. Therefore, our perception of the world was shaped by our predecessors and can be traced back to the primal need for nutrition. Although our own red color vision seems to be a relic from our ancestors’ frugivore days, we still use it to tell if strawberries, raspberries, and apples are ripe to eat, even if our lives may no longer depend on it.
(1) Bowmaker, J. Evolution of colour vision in vertebrates. Eye 12, 541–547 (1998). https://doi.org/10.1038/eye.1998.143
(2) Osorio, D. & Vorobyev, M. (1997) Colour vision as an adaptation to frugivory in primates. Proc. R. Soc. B. 263593–599. DOI: 10.1098/rspb.1996.0089
(3) Onstein RE, Vink DN, Veen J, Barratt CD, Flantua SGA, Wich SA, Kissling WD (2020). Palm fruit colours are linked to the broad-scale distribution and diversification of primate colour vision systems. Proc. R. Soc. B 20192731. DOI: 10.1098/rspb.2019.2731
(4) Schaefer, H.M., Valido, A., Jordano, P. (2014) Birds see the true colours of fruits to live off the fat of the land. Proc. R. Soc. B. 28120132516. DOI: 10.1098/rspb.2013.2516