Just like primates, snakes are one of the most unique vertebrates on the planet. They have evolved to adapt for various environmental conditions, occupying various niches across ecosystems, across the entire world. Ophidiophobia, the psychological fear of snakes, is one of the most common fears that humans seem to innately develop, often without ever encountering a snake. It’s believed that common phobias, especially those dealing with real objects, may stem from a deeply-ingrained evolutionary instinct.
Written by — Gabriel Stroup
Snake Detection Theory (SDT), an overarching hypothesis within evolutionary theory, consisting of numerous sub-hypotheses, posits that many primate characteristics came about as results of interactions with predatory snakes, among other evolutionary pressures. Anthropologist Lynne Isbell (2006) has put forward a detailed, hypothetical timeline of evolutionary changes that may have occurred to the anthropoid clade of mammals since their inception in the Cretaceous, highlighting the role of snakes as predators or otherwise sources of danger, which may have subsequently influenced the evolution of primates (Figure 1).
Adapted from Isbelle, 2006.
In primatology, SDT is supported by many studies that investigate non-human primate reactions to different snake species; moor macaques, for example, have been shown to discriminate local and dangerous snakes from non-local and non-dangerous snakes, as well as to discriminate constrictors from venomous snakes (Clara et al, 2021).
Moreover, psychological studies have shown that humans have a particular talent for picking out snake silhouettes much more quickly than any other potentially-dangerous animal (Kawai, 2016). In this experiment, researchers tested a handful of undergraduate students by presenting images of various animals that are initially obscured by random white noise, but then gradually revealed through 20 steps (see Figure 2). The results show that most participants were able to correctly identify the snake between steps 6 and 9, while most other animals were not identified until steps 10 and later.
Other papers since Isbell’s publication have largely shown support for SDT. In cultural anthropology, some of the oldest human cultures still continuing on the planet, like the various Agta people from the Philippines, developed ways of detecting and preventing deaths from snakes, particularly from reticulated pythons, the longest species of snake in the world (Headland and Greene, 2011). These large constrictors most resemble the earliest snakes that the earliest primates would have made contact within prehistory. On a broader scale, snakes have long been feared or revered as mythological symbols for various ideas throughout many independent human cultures through time.
One neurological study has shown that human infants have an innate brain response to snake-specific stimuli, even when compared to snake-like caterpillars, which would be consistent with an evolutionarily-ingrained instinct to quickly identify a potential predator of primates (Bartels et al, 2020). Another study performed three different eye-tracking experiments, all of which cumulatively suggest that, compared to spiders, snakes are quite easily identified in challenging visual conditions (Saores et al, 2014).
Snake detection also manifests in being able to identify snake scales, which are remarkably unique in the animal kingdom. One study has shown that one part of the human brain, which is associated with emotional responses to stimuli, becomes vastly active when confronted with patterns resembling snake scales, compared to patterns that resemble lizard scales or bird plumage (Van Strien & Isbell, 2017. See Figure 3).
Perhaps due to the influence that snakes have potentially exerted onto the physiology of primates, there is evidence to suggest that snakes are evolving to survive against preemptive attacks by primates. Harris et al (2021) has provided reason to suspect that the advent of cobra venom was caused by the deadly interactions between cobras and Afro-asian primates; primarily from the fact that these primates bear resistance to cobra venom, where prosimians have not. This is in spite of the fact that cobra venom had previously evolved through at least three separate lineages of cobra.
The delivery of cobra venom, evolving as a way for the snake to defend itself from a distance, is itself possibly a result of primates that initially interacted with the snake from a distance. If accurate, these points would be more solid evidence to suggest that snakes and primates have gone through (and continue to go through) a unique coevolutionary process.
Despite the mounting support for SDT, there are still some valid questions that have yet to be thoroughly explored. Coelho et al (2019) have put forward some fair criticisms and questions towards the theory. Some of these criticisms/questions have been addressed by later research, but some still remain. Some examples are:
- Numerous animal groups evolved different ways to create and deliver venom, which are not unique to snakes, so it would make sense for primates to evolve a generalized pathway to detect such species, because evolving pathways for each individual group would be biologically expensive and impractical.
- The selective habituation hypothesis posits that prey animals begin with a general image of a predator, seemingly sensitive to many potential predator-stimuli, but then acclimate/habituate to their specific surroundings, learning to distinguish harmless environmental clues from useful or harmful clues. This would render it unnecessary for primates to evolve ways to detect snakes in particular, and instead have primates learn to recognize all threats local to their particular area.
It is also worth mentioning that despite the prevalence of ophidiophobia, there is also a large fraction of primates (humans specifically) that find joy from finding and interacting with snakes, and such joy occurs across human cultures just as much as fear occurs. Landová et al (2018) surveyed college students in Azerbaijan and the Czech Republic, about their attitudes and perceptions on various snake species presented to them. They found that both groups agreed that the most fear-inducing species (vipers in particular) were also the most beautiful. These results occurred even when the Azerbaijani group harbored a more negative attitude on snakes compared to the Czech group, and despite both groups having similar educational backgrounds (biological sciences). The researchers conclude that there must be both a generalized fear and generalized joy of snakes across socio-political boundaries (see Figure 4).
Snake Detection Theory is a fascinating idea which suggests that the coevolutionary relationship between primates and snakes stretches far back into early mammal times. SDT helps to explain a very common phobia, the innately-primate ability to distinguish snakes from other species in the animal kingdom, and other quirks of the primate experience. Future studies will be necessary before SDT becomes fully accepted, but as of now, it has much reason to be taken seriously. As a snake-keeper myself, even I get short flashes of fear when I see venomous snakes online, or even when I watch my own snakes explore their environment. While I encourage everyone to be educated on snake biology and behavior, I also understand that their fear of snakes may be a product of millions of years of primate evolution, which may have contributed to our very success as a species.
Bibliography
Bertels, J., Bourguignon, M., de Heering, A. et al. Snakes elicit specific neural responses in the human infant brain. Sci Rep 10, 7443 (2020). https://doi.org/10.1038/s41598-020-63619-y
Harris, R.J., Nekaris, K.AI. & Fry, B.G. Monkeying around with venom: an increased resistance to α-neurotoxins supports an evolutionary arms race between Afro-Asian primates and sympatric cobras. BMC Biol 19, 253 (2021). https://doi.org/10.1186/s12915-021-01195-x
Headland, Thomas N., and Harry W. Greene. “Hunter–gatherers and other primates as prey, predators, and competitors of snakes.” Proceedings of the National Academy of Sciences 108.52 (2011): E1470-E1474.
Hernández Tienda, Clara, et al. “Reaction to snakes in wild moor macaques (Macaca maura).” International Journal of Primatology 42 (2021): 528-532.
Isbell, Lynne A. “Snakes as agents of evolutionary change in primate brains.” Journal of human evolution 51.1 (2006): 1-35.
Kawai, Nobuyuki, and Hongshen He. “Breaking snake camouflage: Humans detect snakes more accurately than other animals under less discernible visual conditions.” PLoS One 11.10 (2016): e0164342.
Landová, Eva, et al. “Association between fear and beauty evaluation of snakes: cross-cultural findings.” Frontiers in psychology 9 (2018): 333.
Soares, Sandra C., et al. “The hidden snake in the grass: superior detection of snakes in challenging attentional conditions.” PLoS one 9.12 (2014): e114724.
Van Strien, Jan W., and Lynne A. Isbell. “Snake scales, partial exposure, and the Snake Detection Theory: A human event-related potentials study.” Scientific Reports 7.1 (2017): 46331.
Cover image: (C) Gabriel Stroup, GigabyteSpyder Photography
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