Color sensitivity helps primates detect predators in nature

Ability to see colors helps identify predator or catch insects. Study helps to understand 'color blindness' found in American monkeys.

Images of predators hidden in the wild were shown to human volunteers with normal vision and color blindness; In the photo, a taxidermized leopard was hidden in the middle of the foliage. Color-blind people took longer to notice the animals.

The way primates see colors - in more or less detail - may be related to their need to detect predators in the wild.

The conclusion of a study conducted at the Sensory Ecology Laboratory of the Federal University of Rio Grande do Norte (UFRN) and presented on August 28, 2014 at a symposium at the XXIX Annual Meeting of the Federation of Experimental Biological Societies (FeSBE), in Caxambu, Minas Gerais, it helps to understand why primates in the Americas and the Old World see colors differently.

It is well known that while primates in Africa and Asia are trichromat, that is, they have a human-like color vision, primates in the Americas have more variability in color vision: there are trichromat individuals, but most are dichromatas, those that have a limited color distinction, similar to that of color-blind people.

Many of the explanations proposed so far for this difference suggest that color vision has evolved in each region to better detect food or sexual partners. The UFRN study, conducted by researcher Daniel Marques de Almeida Pessoa, was the first to successfully test the possibility that color vision in primates has evolved into better predator detection.

To reach this conclusion, the researchers did two experiments. The first of these quantified the color contrast between the coat of seven carnivorous predators - such as jaguar, ocelot and bush cat - and natural vegetation. A statistical model was created to predict how dichromate and trichrome primates would see these contrasts.

The conclusion was that, among trichromatas, the contrast is much higher: that is, they could more easily distinguish predators in the wild.

The second experiment tested this hypothesis in humans with dichromate and trichrome, that is, people with normal vision and people with color blindness. Tables containing four different landscapes were presented to the volunteers. One of them contained a hidden animal.

The researchers calculated how long it took each person to identify which landscape contained the animal. The conclusion was that the "color-blind" actually took longer to distinguish the predator.

Native primates in the Americas are much smaller than those in Africa and Asia. The smaller the primates, the more dependent they are on eating insects. Previous studies have shown that dichromate primates are more successful at identifying insects for food. On the other hand, the tiny size also makes these animals more vulnerable to potential predators, so they would also benefit from trichromatism to more easily identify these threats.

These two needs may explain why primates with these two types of color vision can be found in the Americas. The primates in Asia and Africa, because they are larger, do not feed on insects and must protect themselves from even larger predators. This would explain why there they have, in their entirety, a detailed view of the colors.

“It was the first research that tested the possibility of predators as an important ecological influence,” says Pessoa. “To date no other factor studied has been able to explain this distribution so well.”

He notes that the model even explains why the howler monkey is an exception, being the only totally trichromate in the Americas. “It's a big beast, it doesn't eat much insects and at the same time is subject to a great predation pressure.”