Are We Hard-Wired to Fear Snakes?

Karlene holding an Ontario Foxsnake hatchling Photo by: Calvin Knaggs

Karlene holding an Ontario Foxsnake hatchlings
Photo by: Calvin Knaggs

Work by neuroscientists in Japan and Brazil are looking into Lynne Isbell’s, a professor of anthropology at the University of California, hypothesis that our primate ancestors evolved good, close-range vision primarily to spot and avoid dangerous snakes.

Hisao Nishijo and Quan Van Le at Toyama University studied the neural responses that occur without learning or memory, such as instinctive actions that cause emotions such as fear. Nishijo found that some nerve cells in macaque’s brains get more excited when exposed to images of snakes. Interesting enough, the macaques studied were reared and housed in a colony with no previous encounters with a real or artificial snake.

Lynne Isbell has studied the behavioural responses of macaques when shown a series of images. When confronted by an image of a snake the macaques exhibited fear or a sensitivity response. Neuroscientists found specific neurons were firing stronger, faster and in higher numbers when images of snakes where shown compared to images of macaque faces or hands. Lynne Isbell found this surprising as macaques are very social animals, and one would expect a higher neuron response to images of their primate pals.

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

Many of today’s mammals and big snakes evolved together about 100 million years ago, with venomous snakes popping up about 60 million years ago. It would have been a great advantage to be able to see and respond to these predators quickly since our primate ancestors shared the same tree and grassland habitat.

“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.

“I don’t see another way to explain the sensitivity of these neurons to snakes except through an evolutionary path,” Isbell said. “I don’t do neuroscience and [the neuroscientists] don’t do evolution, but we can put our brains together and I think it brings a wider perspective to neuroscience and new insights for evolution,” she said.

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Journal Reference:

Quan Van Le, Lynne A. Isbell, Jumpei Matsumoto, Minh Nguyen, Etsuro Hori, Rafael S. Maior, Carlos Tomaz, Anh Hai Tran, Taketoshi Ono, and Hisao Nishijo. Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes. PNAS, October 28, 2013 DOI: 10.1073/pnas.1312648110