13 December 2011

Fear of snakes? This could be why

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Our early relatives would have made an easy meal for large snakes, according to new research that provides solid evidence for the threat snakes posed to primitive humans and other primates.
fear of snakes

Reticulated python, total length 6.9 m. Shot by Kekek Aduanan, the adult male Agta on right, on 9 June 1970, at the headwaters of the Koso River in the Philippines.

Credit: J. Headland.

fear of snakes

Skin of same python as in Figure 1, after the two hunters and Headland butchered it, thereby providing ~25 kg of meat to the men’s families and fellow group members.
Credit: J. Headland.

LONDON: Our early relatives would have made an easy meal for large snakes, according to new research that provides solid evidence for the threat snakes posed to primitive humans and other primates.

The finding, published in Proceedings of the National Academy of Sciences today, helps to settle the long disputed question of why humans have a fear of snakes, even though they pose little threat in modern society.

“I’m not sure if anyone till now has demonstrated with data just how dangerous large snakes must have been to early hominin species in the ancient past,” said anthropologist Thomas Headland from the not-for-profit organisation SIL International, who conducted the research with Harry Greene from Cornell University.

Fear of snakes in the grass

Snakes tend to eat their pray whole, making it hard to find fossil evidence of snakes preying on humans and other primates. In the rare cases where the fossilised contents of snake’s stomachs have been found, they did not contain traces of primates.

However, studies on humans looking at the response of both adults and children to images of snakes shows that children, like adults, are much quicker at spotting snakes in a scene than other objects. That suggests that our fear of snakes is in-built, rather than something we learn as we grow up.

The new study shows just why such an in-built response would have been so important, proving that humans and other primates have a long-standing, and often fatal relationship with snakes.

Hunter-gatherer lifestyle

Greene and Headland made the discovery by studying 20th century hunter-gatherers in the Philippines. Headland had already been working with an indigenous people called the Agta Negritos and spoke their language. “It was a lucky break that I happened to be living with [Agta Negritos] for most of the past half-century, where I stumbled on this startling phenomenon of intense symbiosis interactions between the Agta and giant pythons,” he said.

Until recently, the Agta retained a hunter-gatherer lifestyle, foraging in the forests for food and eating wild meat. Starting in 1972, Headland interviewed a total of 120 Agta – 58 men and 62 women – about their experiences with snakes. The team also looked at natural history records in order to identify species of non-human primates that attack or are eaten by snakes.

Fatal snake attacks

The results showed that 15 of the Agta men (26%) and one of the women had been attacked by pythons, and two had been attacked twice. Interviewees also reported six cases of fatal snake attacks between 1934 and 1973.

The authors believe that before the Agta had iron tools such as knives they must have suffered not just high attacks from giant snakes, but most of them were probably then fatal. The study also shows that Agta people ate pythons, and would also have competed with them for other food, revealing just how complex their relationship with snakes would have been.

To further show our evolutionary link with snakes, the team looked for evidence in natural history records that our non-human primate ancestors also had close interactions with snakes. They found evidence of 26 primate species that were preyed on by snakes, and also found examples of primates who prey on snakes.

A human universal

Taken together, these findings demonstrate the complex historical relationship that our human and non-human ancestors had with snakes, which might well explain our innate fear response to snakes today. The same might be true for other common phobias, such as a fear of spiders.

“I can easily entertain as logical the hypothesis that human fear of spiders and scorpions is genetic, as is our fear of snakes. These fears are a human universal and, I think, different from the fear we may have when a robber holds a gun on us,” Headland said. “Men have it just as much as women, and certainly the Agta people have it, at least for snakes. We learned to be afraid of cars from our parents. But our fear of snakes and spiders seems to be not learned, but genetic.”

Anthropologist Lynne Isbell from the University of California in Davis, U.S was impressed by the results. “This paper uncovers a unique and complex natural relationship that the authors effectively argue has been in existence for millions of years. I especially like it because it may help to explain the paradoxical response of humans to snakes: we are both strongly repelled by and attracted to snakes,” she commented.

Isbell also pointed out that as modernisation spreads, this kind of evidence will be harder to find. “The intensity of snake attacks and mortality rates from snakes historically in the Agta Negrito population of the Philippines is incredible,” she said. “I doubt such data would be possible to collect now that there are few human populations remaining that live so intimately with the land in the tropics where giant snakes also live.”

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