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. 'Our encounter rate,' McGrew writes in the paper, 'seems notably high, as does the proportion of venomous snakes. So, do these published data matter? I think they do. 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.

The dry season was considerably more risky a time for snake encounters than the wet. 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. Of these, 64 percent were venomous: 33 cobras, 27 vipers and 24 rear-fanged snakes. During the entire study period of four years, 142 snakes of 14 different species were identified. 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.ĭuring 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. McGrew, 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. In a new paper published in the journal Primates, author William C. Next time you run into an African bush viper like this one, be thankful for your forward-facing eyes.

Chimpanzee eyes