Are primates hard-wired to be scared of snakes?

Posting will be light today as there’s a Horseman afoot (see next post), but I wanted to call attention to a paper that’s of some interest. It can even be construed as a decent bit of research on (horrors!) evolutionary psychology.

The paper by, Quan van Le et al. in Proc. Nat. Acad. Sci. USA (reference and download below), suggest that selection has molded primate brains to render them particularly sensitive to snakes.  I won’t be able to do it justice in the time I have to write, but let’s have a look.

The authors implanted electrodes in individual neurons of brains of three macaque monkeys (Macaca fuscata) and then did recordings from the neurons as the monkeys were exposed to four sets of pictures: snakes, angry monkey faces, monkey hands, and other geometrical shapes, like circles. The monkeys had been trained to look at the screen, and were rewarded when they did.  The authors’ “Snake Detection Theory” (“SDT”; what a name!) led them to hypothesize that these neurons—in a region of the brain called the “medial and dorsolateral pulvinar”, a region unique in primates—would respond especially quickly to images of snakes.  The pulvinar has been shown to be involved in visual processing of information from the eyes, and, as the authors note, “fast processing of threatening images.”  This hypothesis is of course based on the supposition that primates have an innate fear of snakes (these monkeys were reared in captivity and never exposed to snakes), a fear bred into them by natural selection. Those monkeys who weren’t especially attentive to snakes, the SDT suggests, were those monkeys who didn’t leave descendants! That would lead to natural selection for more attentiveness, perhaps detectable by neuronal response.

What the authors found supports the SDT, but weakly.  Of the 91 neurons tested with all stimuli, 37 were more sensitive to snake images, 26 to angry-face images, 17 to hand images, and 11 to shape images. That is weakly significant (hash mark), but only if you use the p < 0.1 criterion that is used in psychology but not biology (we use p < 0.05, and physicists are far more stringent).  Here’s the figure from their paper showing that snake neurons are more numerous (A), have a higher magnitude of response than the other categories of neurons (B), and have a lower latency of response than other categories (C; i.e., they respond faster). The brackets show the groups compared, and an asterisk or hash sign over a bracket means the comparison is significant.

As expected, then, “snake neurons” were more numerous and more sensitive to the relevant stimuli than neurons in the other three groups, and in second place was the number and response of “angry monkey face” neurons, which I suspected from the outset since monkeys are surely selected to be attentive to each other’s expressions.

There were other experiments as well, involving scrambling the images, and these supported the main result.  I won’t go into more detail except to say that these result are suggestive, although the higher probabilities involved in some comparisons, particularly those of “hand” versus “face” neurons, are a bit worrisome.  The authors conclude:

. . . since the origin of primates, snakes have been a universal threat; both primates and snakes that can kill them (i.e., constrictors and venomous snakes) have their greatest diversity in tropical ecosystems (1, 2, 40, 41). Our data provide unique neuronal evidence supporting the hypothesis that snakes provided a novel selective pressure that contributed to the evolution of the primate order by way of visual modification (2, 5). We urge neurophysiologists to engage in similar studies across a wide range of primate species and closely related mammals to further examine the phylogenetic fingerprint of fast snake detection.

Now there’s an easier way to see if primates are innately scared of snakes: just expose a bunch of naive primates to snakes versus other animals, or snake-toys versus other kinds of toys, and see if they show a fright reaction. The authors mention that this has been done: snakes are detected “visually more quickly than innocuous stimuli” in both humans and other primates. What the authors show here is, perhaps, the neurological basis for that difference.

As they say, this study needs fleshing out with more species, especially those species that have never encountered snakes during their evolution. Maybe mammals are attentive to long skinny things for other reasons. But it’s a start, and it’s the kind of evolutionary psychology that I consider pretty good. My last statement will, of course, immediately trigger a group of quibblers to find faults with this work.  But you have problems with the paper, take them up not with me but with David Hillis, who was the editor on this one—and a reader here!

One more thing: they need to do studies like this with spiders, too—at least judging by the reaction of our commenters this week!

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Van Le, Q., L. A. Isbell, J. Matsumoto, M. Nguyen, E. Hori, R. S. Maior, C. Tomaz, A. H. Tran, T. Ono, and H. Nishijo. 2013 Pulvinar neurons reveal neurobiological evidence of past selection for rapid detection of snakes.
Proceedings of the National Academy of Science, early edition: 10.1073/pnas.1312648110