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In a new paper published in the journal 

, a former professor of evolutionary primatology at the University of Cambridge, reports a high rate of venomous snake encounters by his team of primatologists seeking to observe unhabituated wild chimpanzees in Mount Assirik, Senegal, West Africa.
McGrew's snake-encounter analysis in the paper 

Snakes as hazards: modelling risk by chasing chimpanzees

 is one test of what's known as the snake-detection theory of primate origins, a set of hypotheses that suggest we (along with other primates) owe certain features of our evolution to the risks posed by death and injury from snakes.


During the 609 days that make up the core period of the analysis, McGrew and his team encountered a snake in Assirik, on average, once every 4.3 days — totaling 132 snake encounters. During the entire study period of four years, 142 snakes of 14 different species were identified. Of these, 64 percent were venomous: 33 cobras, 27 vipers and 24 rear-fanged snakes.

In the mixed habitat of Assirik, snakes were encountered in the forest twice as often by the researchers as expected by chance, at chance rates in the woodland, and half as often as expected in the grassland. The dry season was considerably more risky a time for snake encounters than the wet.

McGrew acknowledges that the sample of identified snakes is "pitiful by herpetological standards," notes that the methodology was non-random because the researchers walked more in habitats preferred by chimpanzees, and specifies that no chimpanzee-snake encounters were witnessed.

So, do these published data matter? I think they do.

"Our encounter rate," McGrew writes in the paper, "seems notably high, as does the proportion of venomous snakes. If anything, our encounter rate may have been lower than that of the apes, if party size is positively correlated with snake detection efficiency, proxied by number of vigilant eyes. Median party size for the chimpanzees was four; for observers it was two."


It's this "notably high" encounter rate that supports the snake-detection theory, developed by anthropologist 

The Fruit, The Tree, and the Serpent: Why We See So Well

, explained some of her core ideas regarding snake detection to NPR's 

 story notes, "Primates in parts of the world with lots of poisonous snakes evolved better vision than primates elsewhere, Isbell found out. It's no accident that lemurs in Madagascar have the worst vision in the primate world, she says. There are no venomous snakes [in Madagascar]."


Last week by email, Isbell elaborated for me on the two "thrusts" of her snake-detection theory (SDT):

Centrally, then, the SDT posits that the possibility of death or injury from contact with snakes was an evolutionary selection pressure that put our lineage on its path toward visual acuity and highly developed brains.

I asked Isbell why other animal lineages weren't similarly affected, since surely other animals too would have encountered snakes. Ancestral placental mammals, she explained, were probably nocturnal, not active during sunlight hours. By contrast, the lineage leading to primates "would have spent the day in shrubs and trees (because all primates have grasping hands and feet for arboreal living), where sunlight would have penetrated." As a result, that lineage "could take advantage of sunlight to detect their predators visually, which resulted in increased visual improvement over time."

In the past few years, neuroscientists have been at work testing some of Isbell's specific hypotheses. Isbell told me:

More field studies in primate habitats like McGrew's will, Isbell hopes, join with the neuroscience work in further testing the SDT.


Taken together, the findings from primatology and neuroscience regarding the SDT may help explain why many of us react strongly to the sight of a snake. Our response may be deeply ingrained, even when the chances of encountering a poisonous snake are quite low, and when comparatively speaking, the risks to most of us from snakes today 

If you startle or otherwise react strongly and rapidly at a snake sighting, then, just consider yourself a well-evolved primate.

Barbara J. King, an anthropology professor at the College of William and Mary, often writes about human evolution, primate behavior and the cognition and emotion of animals. Barbara's 

was released in paperback in April. You can keep up with what she is thinking on Twitter: 

Science

Environmental News

Barbara J. King

Author

Why can we see in color? Why is our depth perception and visual acuity so good? New research supports the theory that we needed these abilities to spot and avoid venomous snakes.

We may have snakes to thank for our acute vision

Go Deeper.

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