At sunset on 13 March 1973, a reticulated python (Python reticulatus) slithered into a thatched hut in the Philippines and killed two siblings: a 4-year-old girl and a 3-year-old boy. The third child in the family was saved when his father returned to the hut and killed the reptile with a bolo knife.
That's one of many observations described in a new analysis of snake attacks on the Agta Negritos, a rural hunter-gatherer culture that thrived in the isolated mountain regions of Luzon island until the 1990s. The new data, based on interviews with Agta adults, reveal that snakes were more than just a rare nuisance to the people. They were prey, predator, and competitor all at once. The complex relationship helps reveal the evolutionary pressures that humans and snakes once put on each other.
"Until these data, most people thought that there was a one-way relationship: snakes occasionally harming people," says herpetologist Harry Greene of Cornell University, who helped put together the new analysis. "But this is the strongest evidence yet that it's a much more complicated relationship."
In 1976, anthropologist Thomas Headland of the Southern Methodist University in Dallas interviewed 120 adult Agta about python attacks. At the time, Headland, who had learned the language of the Agta, was studying their lifestyle of hunting, sleeping in small temporary structures, and living in small kin-related groups. Recently, he teamed up with Greene to analyze the interviews. Of the men interviewed, nearly 26% had been attacked by a snake, and almost all of them had substantial scars from python bites. A traumatic python incident—leading either to a fatality or an injury—occurred every 2 to 3 years from the 1940s to the 1970s. If these numbers on attacks were consistent throughout multiple generations, the researchers concluded, the mortality within the population could have reached more than 8% in earlier days, when iron weapons such as knifes were rare in the Agta population. But Agta Negritos, the interviews revealed, also hunted pythons. Adult snakes are often more than 5 meters long and provide 20 kilograms or more of meat. All Agta men interviewed had killed at least small pythons.
Moreover, pythons and the Agta competed for the same prey, the team reports online today in the Proceedings of the National Academy of Sciences. "From time to time they would find in the stomachs of the snakes they butchered the same other prey species that the Agta really liked: wild deer, wild pigs, and monkeys," says Greene. "So the Agta knew, from eating the pythons, that the snakes were their competitors for other food."
Greene also combed the literature for observations of encounters between other primates and snakes. He found evidence that many species of monkeys and lemurs have had both relationships with snakes: hunter and hunted. Likewise, he analyzed data from recorded incidents of snake attacks in rural Indonesia and Malaysia and found similar frequencies of attacks and fatalities. That strengthens the idea that these complex encounters have likely been prevalent throughout the evolutionary history of snakes and primates  including humans.
"These are data that you can't get any longer," says anthropologist Lynne Isbell of the University of California, Davis, who was not involved in the study. By the 1990s, the Agta Negritos' habitat had been widely developed and they had been exposed to modern cultures—the Agta are now threatened with extinction. "People don't live under the hunter-gatherer conditions that the Agta Negritos population lived in," she says, "so we don't have the opportunities to look at these snake encounters anymore."
Isbell calls the relationship between snakes and primates throughout history "an evolutionary arms race." She says there is evidence that pressure to detect well-camouflaged snakes likely helped improve early humans' eyesight. Likewise, the increasing intellect of early humans may have pressured snakes to develop new methods of camouflage and defense. Greene plans to investigate how this could be reflected in the biology of snakes today. The new numbers on frequency of primate interactions with snakes, Isbell says, strengthens those ideas. "This brings quantitative data to the story for the first time."
Today in the United States, dangerous snake encounters are rare, but the new analysis of anecdotes brings to life the relationship that snakes and humans once had, she says.