Study suggests that monkey brains are hard-wired to fear snakes

Was the evolution of high-quality vision in human ancestors driven by the threat of snakes? Work by neuroscientists in Japan and Brazil is supporting the theory.

In a paper published Oct. 28 in the journal Proceedings of the National Academy of Sciences show that there are specific nerve cells in the brains of rhesus macaque monkeys that respond to images of snakes.

The snake-sensitive neurons were more numerous and responded more strongly and rapidly, than other nerve cells that fired in response to images of macaque faces or hands, or to geometric shapes. Lynne Isbell, professor of anthropology at Davis, said she was surprised that more neurons responded to snakes than to faces, given that primates are highly social animals.

“We’re finding results consistent with the idea that snakes have exerted strong selective pressure on primates,” Ms. Isbell said. She originally put forth the theory.

Ms. Isbell originally published her hypothesis in 2006, following up with a book, “The Fruit, the Tree and the Serpent” in which she argued that human primate ancestors evolved good, close-range vision primarily to spot and avoid dangerous snakes.

Modern mammals and snakes big enough to eat them evolved at about the same time, 100 million years ago. Venomous snakes are thought to have appeared about 60 million years ago These are ambush predators that have shared the trees and grasslands with primates.

Hisao Nishijo’s laboratory at Toyama University, Japan, studies the neural mechanisms responsible for emotion and fear in rhesus macaque monkeys, especially instinctive responses that occur without learning or memory. Previous researchers have used snakes to provoke fear in monkeys, he noted. When Nishijo heard of Ms. Isbell’s theory, he thought it might explain why monkeys are so afraid of snakes.

“The results show that the brain has special neural circuits to detect snakes, and this suggests that the neural circuits to detect snakes have been genetically encoded,” Nishijo said.

The monkeys tested in the experiment were reared in a walled colony and neither had previously encountered a real snake.

“I don’t see another way to explain the sensitivity of these neurons to snakes except through an evolutionary path,” Ms. Isbell said.

Rafael Maior and Carlos Tomaz at the University of Brasilia, Brazil, also participated in the studies.

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