National Geographic News
Photo of parrot snake in defense pose.

Humans may be hardwired to fear snakes.

Photograph by Robert Pickett, Papilio/Corbis

Liz Langley

For National Geographic

Published October 29, 2013

Snakes on a Plane, snakes in a can, a rubber snake under someone's chair—just thinking we see a snake is enough to make some of us leap away like a suddenly expert dancer. And it might be those slithery serpents that helped us evolve to see as well as we do.

What is it that makes us react to snakes so quickly? The combined efforts of neuroscientists and one anthropologist may provide some answers about the place of the snake in our brain and our hissssstory.

The Beginning

Lynne Isbell, an anthropologist at the University of California, Davis, had theorized back in 2006 that snakes were the main reason that primates developed the best vision of all mammals. She published her findings in the book The Fruit, the Tree, and the Serpent: Why We See So Well.

Isbell says there were two "pulses of selective pressure" that brought about the unusually sharp vision we have as human beings.

The first pulse was about 100 million years ago on the supercontinent Gondwana, when constricting snakes were evolving at the same time as our ancestors, the rodent-like mammals who would become primates. Those mammals rested in the trees in the light of day, where they fell prey to the snakes. The combination of access to bright light and the presence of those constricting snakes created the selective pressure for those early mammals to develop better vision.

The second pulse, Isbell says, was when venomous snakes began to evolve 40 million years later, putting pressure on Old World primates to refine and improve their vision so they could avoid the even-more-dangerous reptiles.

Primate vision varies, and it's not nearly as good in Madagascar, where there are no venomous snakes, Isbell says. The primates there "don't have a fovea, a pit in the retina that allows our central vision to be very sharp so that we can see fine detail. So their visual acuity isn't the best. It's still better than other mammals'; it's just not as good as ours," she says. This is likely because primates in Madagascar did have the constrictors but not the venomous snakes to contend with, which would have improved their vision even further.

What Does the Study Show?

Isbell worked with colleagues in the neurosciences, specifically Hisao Nishijo, whose lab had previously studied fear, emotion, and instinctive responses in macaques and who thought Isbell's theory might explain the fear of snakes in monkeys.

A series of emails, which also involved scientists at the University of Brasilia, eventually led to the study that appeared this week in the Proceedings of the National Academy of Sciences. The study shows that neurons in a very specific area of the brain—pulvinar neurons—in macaque monkeys responded selectively to images of snakes, and responded more quickly and strongly to snake images than to images of monkey faces and hands or geometric shapes.

The Japanese macaques involved in the study lived in a high-walled enclosure and hadn't had exposure to snakes, Isbell says, suggesting that the quick neural response is a hardwired one.

These findings "provide neuroscientific evidence in support of the Snake Detection Theory, which posits that the threat of snakes strongly influenced the evolution of the primate brain," according to the study.

What Doesn't the Study Show?

Yes, primates have a quick response to snakes—we can see them and get the heck out of their way, Isbell says. But that doesn't explain the paralyzing phobia some people have of the slithering reptiles.

"People want to connect what we've done with fear and phobias, but in fact we haven't addressed emotion in this study," Isbell said. "We've only looked at vision. So we've looked at the very first step. How does the image of the snake get into our brains to begin with?"

Fear, she says, is something that comes later for some people. "There's another step that leads to fear, but our study doesn't address the emotional aspect of it."

What Does It Mean?

If you live in an area where there are venomous snakes, you might have had instant empathy with the speedy reaction of those neurons to the images of a snake. People still die of snakebite, especially in the tropics, says Isbell, who encounters snakes in her fieldwork.

"It's important to be able to see them before they see us so we can stop in time to do that acrobatic jumping away," she said.

Follow Liz Langley on Facebook & Twitter.

26 comments
David Todtenhagen
David Todtenhagen

If I put a rubber snake near the glass walls of my turtle aquarium, these baby turtles immediately attack the imposter.  These turtles have never been 'wild' and their response isn't the same as a food prey response, it is attack exhibition.

Another thought:  I have friends that are terrified by snakes and others that are not.  Is this a genetic defense mechanism based on ancestry.  I know in many cases it is a learned response and in others "a deep rooted response".

Mihajlo Filipovic
Mihajlo Filipovic

I believe if you're afraid of rabbits, you could develop an interesting theory based upon remnants of a primal fear from Megalobunnis hoppitus or something even more horrifying, and there would be no-one to dispute it.

To my mind, there are people (and other animals) who are afraid of snakes, and there are those who aren't. And that's about all there is to it.

http://yourshot.nationalgeographic.com/photos/1155424/ for instance... ;)

As to the theory that some animal or other does not see well, it does look like some sorts would have eyes just for ornamentation - and this simply is not so. The last theory along that line silently sank when it was (re) discovered that sharks have perfect eyesight, although it was believed that they sight is bad, so they had to accompaned by the pilot fish which "led it toward its prey".

In short, the eye is not what sees: it is the way the image is interpreted, which simply can't be reproduced by studying the optical element! We would need to understand the signal that arrives to the brain, the software that analyses that signal, and simply said, the "personality" of the species which uses the derived information. And that's a no-go.

Remember the old horror movie about the giant ant, and the way the ant's vision was interpreted (a raster of images)? Well, it was funny even then. Multi-faceted eyes do not necessarily render multi-faceted interpretation, same as today's multi-shot cameras deliver the single final processed image. Let alone what we (don't) know of the light frequencies some living beings can perceive, and we can't (such as auras etc.).

Some scientists shoud re-think the way they present their findings or "findings" to the general public. There is that definition of assume... ;)

Justin Waters
Justin Waters

I've been keeping snakes as pets since I was a kid.  I think most people fear snakes because they cannot differentiate the poisonous snakes from the non-poisonous ones.  As an adult, I got into the fad of jogging through the forest trails barefoot, until I almost stepped on a rattlesnake in the strike pose.  Fortunately, due to my evolved eyesight and hearing (it was rattling) I did not step on it.  - from Florida

TERESA GRACE MULLEN
TERESA GRACE MULLEN

I talk to animals. If I notice a serpent observing me I speak in soft tones of our friendship. I got the idea from The Jungle Book. I say that "we are of the same blood you and I." If I see one dashing away in the woods, I just say, "hello neighbor." I am amazed at a story about the cobra who shielded the Buddha. I am also amazed with neuroscience.

James Sibert
James Sibert

Wow.  I love Nat Geo and really respect them for a lot of what they do, but when they try to make connections like this it really makes me laugh.  It's remarkable how far people will strain to find things that support their worldview.

Dr Bob Rhoda
Dr Bob Rhoda

I grew up on a farm in upstate N.Y. and quickly learned to identify venomous snakes like the Eastern Diamondback and the Copperhead.  Avoidance was always a good idea.  I never played with snakes.  Fear is a learned response.  I was taught to respect all wildlife including snakes. Snakes are very beneficial.  In barns and fields they keep a rein on the mice & rats protecting feed stores and crops.  Overall, they were "Good Guys & Gals".   However, I have to admit, coming on a snake unexpectedly is still a gut wrench.  I still remember running through the tall grass on our farm one day and stepping on a VERY LARGE BLACK SNAKE, (completely harmless).  It responded by whipping itself around my foot.  I almost wet my pants but we both departed unharmed.  Anyway, I work with BSA a lot and spend time teaching the boys about reptiles.  Since we are in the field a lot, they need to know about venomous reptiles of all kinds.  Where I live you NEED to know what a Coral snake, (Red on Yellow, like a traffic light, KILL A FELLA), looks like and be able to identify it from its harmless look alike. GOOD PIECE. THANKS. DrBob

Kenny Flanagan
Kenny Flanagan

"Now the serpent was more crafty than any of the wild animals the Lord God had made... the serpent said to the woman. “..when you eat from it your eyes will be opened...", she took some and ate it. She also gave some to her husband, who was with her, and he ate it. Then the eyes of both of them were opened.." Genesis 3:1-7

I love it when those that try to hard so deny and overthrow Creation and God quote it in their own works.
This "discovery" has been known for about 3500 years.

Leslie Sokolow
Leslie Sokolow

I think people are afraid of snakes because their body form accentuates a mouth full of teeth.

Statistically there are far fewer deaths by snake than by animal species we routinely cuddle; of the 7,000-8,000 people bitten by venomous snakes in the U.S. every year, only 5 die. Dogs, on the other hand, cause an average of 30 deaths/year and yet we don't freak out at the sight of a docile dog.

I have 5 pet snakes and can attest that there are many species that are naturally docile.

Elvis Nmehielle
Elvis Nmehielle

Snakes are dangerous. Lets be careful in dealing with them because they can kill.

nidhi ranji
nidhi ranji

i think snake is the only thing i"m really scared of .it just freaks me out!

Trisha Zimmerman
Trisha Zimmerman

I like snakes and have a pet snake, but I'd still jump if I saw a snake near me unexpectedly. 

John Gunnett
John Gunnett

I never thought I was scared of snakes until I was working in a new house tiling a bathtub surround.

I was in the tub setting tiles when I caught something in my peripheral vision, a six foot black snake staring at me face to face. My hardwired instincts shot adrenaline through my body and I leaped out of the tub like I was shot out of a cannon.

After I realized it was a non venomous snake I laughed at my reaction and decided to remove it.

I took my 4 foot level and went to pin it's head to the floor and it struck the end of my level so quick I called my boss and said I'm taking off work till that snake is gone!

Taylor Hanson
Taylor Hanson

@TERESA GRACE MULLEN 

Just to you know, snakes don't have ears, they can't hear you. But a calm demeanor always helps with wildlife, and it doesn't hurt to talk to them. 

TERESA GRACE MULLEN
TERESA GRACE MULLEN

@Dr Bob Rhoda  Are the venomous coral snakes likely to strike because you come upon them too quickly and if so, how can you announce your approach into their resting place, to prevent their alarm? 

Senthilkumar K
Senthilkumar K

@Kenny Flanagan : This is the difference between scientific folks and religious folks. Religion never accepts the ignorance. But science does. That's why science is gaining momentum over religion.

By the way, the Bible never said anything about the now known continent "America's". Does that mean America doesn't exist?

Doc Holiday
Doc Holiday

@Leslie Sokolow actually due to media hype people do freak out at the sight of a docile american pit bull terrier.  Even a small 40 lb one with nothing but a wagging tail and a big tung ready to lick your face off.

Peter Bensen
Peter Bensen

@Senthil K @Kenny Flanagan The Bible doesn't say a lot about many things. It's not exactly the phone book or Wikipedia. Jesus' caveat said, "Scripture cannot be untrue". There is wisdom and knowledge there, a handbook for life. If you expect more than that, not only will you be disappointed, you will have missed the entire point of Holy Scripture. 

Meredith Heffernan
Meredith Heffernan

@Senthil K @Kenny Flanagan I am a Christian, but I don't think the people of Moses' day would understand that the earth is billions of years old. In the book of Job (the oldest story of the Bible), they even mention the Leviathan in a metaphor, which is basically a dragon. I think ancient people saw dinosaur bones and created monsters from them.

The Bible isn't a book of science. It isn't about how the earth was made - the point of the creation story is to say that God made it and that he is in control. The story in the Bible is about God and his people, not trying to explain science. I understand "fearing" God when I think of it like that - the kind of fear that is awe, or reverence. God made the Earth in a very complicated way, so the ancient people couldn't have understood all of it. Moreover, it didn't really matter.

Peter Bensen
Peter Bensen

@Senthil K @Meredith Heffernan First of all, you would have to define "day" if you ask if God created the World in SIX days. (He rested on the seventh). We always assume a day of 24 hours, an Earth day as measured in the year 2013. Science will tell us the length of an Earth day has varied since Earth's formation. To paraphrase Jesus, "you are looking from man's point of view, not God's".

Senthilkumar K
Senthilkumar K

@Meredith Heffernan : I like your comment!! But what I am trying to say is Religion[Any Religion] as such never accepts our human kinds Ignorance.
Its good to question our own beliefs time to time and improve it.. Even if its of God.. No body can surely tell whether God created earth in seven days or what happened before the big bang for now. 
But science is making great progress, i hope we will get the answer soon.

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