Can snakes recognise themselves? A controversial study says yes

An experiment that uses snakes' chemical senses "could change people’s opinions of reptiles away from that of slow, dumb, dull, instinct machines," scientist says.

Published 25 Mar 2022, 17:51 GMT
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An eastern garter snake (pictured in Maine's Acadia National Park) flicks its tongue, a sensory behavior that picks up chemicals in its environment.
Photograph by George Sanker, Nature Picture Library

Scientists who study snake behaviour are often faced with a challenging conundrum: How do you get into the mind of a reptile?

Primates can gesture with their hands, elephants can signal with their trunks, and ravens can peck with their beaks to indicate a particular response to an experiment. But serpents?

That’s why researchers have designed a test just for snakes by playing to their strengths: their chemical senses.

Snakes rely on chemical cues to recognise prey, escape from predators, find mates, and navigate their habitats. In addition to their sense of smell, snakes also flick their tongues to carry chemical cues—say sex pheromones from a potential mate—to a specialised organ in the roof of the mouth, an action roughly equivalent to sniffing.

A recent study on common garter snakes, a species widespread in North America, found that the reptiles can discriminate their own chemical signature from that of littermates fed the same diet. Study leader Gordon Burghardt, an ethologist and comparative psychologist at the University of Tennessee, Knoxville, says it’s a demonstration of self-recognition—a snake version of recognising your mirror reflection.

Common garter snakes, native to North America, are non-venomous reptiles found throughout the continent.
Photograph by Tim Fitzharris, Minden Pictures

Recognising one’s reflection is considered an advanced ability, and it’s only been confirmed in a handful of species, such as chimpanzees, orangutans, and dolphins. 

“Snakes demonstrate many of the same cognitive and perceptual mechanisms as other animals if you study them in the right way, ask the right questions, and respect their biology and way of dealing with the world,” Burghardt says. (Read about 14 animals that have shown us how smart they are.)

Know thyself

In the study, published recently in the journal Behaviour, Burghardt and colleagues studied 24 garter snakes born in a single litter in their Tennessee lab.

The snakes had been housed individually since birth and fed either an all-fish or all-worm diet, making it possible to differentiate the snakes’ faeces chemically. 

When the snakes were four months old, the team exposed them individually to four different stimuli: their own dirty cage liner, the dirty cage liner from a same-sex sibling fed the same diet, the dirty cage liner from a same-sex sibling fed a different diet, and a clean cage liner.

During each experiment, the scientists measured the rate of each snake’s tongue-flicking and its overall movement around the cage. (In pictures: amazing amphibians.)

Snakes flicked their tongues less when exposed to the dirty cage liner from a littermate fed the same diet than they did to their own dirty cage liner. 

Burghardt says this behaviour shows that the garter snakes can recognise their own chemical cues as distinct from those of other snakes, even closely related snakes eating the same diet.

Differing interpretations

About 50 years ago, American psychologist Gordon Gallup and colleagues developed a mirror self-recognition test that is still the standard for many experimental studies.

Researchers put a mark somewhere on an animal’s body that it can only see in its mirror reflection. If the animal looks in the mirror and then touches or examines the mark on its body, it passes the test. People tend to pass the test as toddlers, and some great apes—chimpanzees, orangutans, and bonobos—recognise themselves. A select few non-primates, such as elephants and dolphins, may qualify as well.

But Gallup and his colleagues are skeptical of the evidence for mirror self-recognition in any species other than humans and great apes. They have also critiqued non-visual self-recognition studies, such as those using odours or other chemicals, that purport to be equivalent to mirror tests.

That’s why one of Gallup’s frequent collaborators, psychologist James Anderson of Kyoto University, says that even though the snakes in Burghardt’s study may demonstrate chemical self-recognition, it’s not the same as an ape or human recognising their appearance in a mirror.

“Many researchers overlook the spontaneity of great apes’ [and our own] use of mirrors simply to check on our appearance, perhaps to rearrange it or observe it from different viewpoints,” he said in an email. “No convincing demonstration exists of any so-called ‘chemical mirror’ being used in this way.”

Burghardt says, “I don’t make the claim that these snakes are self-conscious. But they seem to have an awareness of themselves as a different entity than another organism.”

Scientists are also divided about what an animal recognising itself in the mirror really means in terms of cognition. Gallup and Anderson contend that passing the test is equated with self-awareness, self-consciousness, and possibly even the awareness of others’ unique selves. 

Some researchers think self-awareness is more likely to exist on a continuum, with different species demonstrating varying levels. It’s possible self-recognition may be one of the more basic abilities on this continuum, one that is common among many animals, Burghardt says.

“No matter how careful these authors [of the snake study] were, the biggest hurdle will be convincing some scientists that the findings mean anything beyond a rudimentary response to stimuli,” says Gordon Schuett, an ecologist at Georgia State University, in Atlanta.

But it’s good to push these boundaries, he adds: The new study is useful in that it may challenge some preconceived notions about snakes’ cognitive capacities.

More than dumb machines 

A swelling stream of research supports the idea that snakes can learn, adapt on the spot, and even make friends.

For instance, scientists have documented northern Pacific rattlesnakes moving branches out of the way before hunting, seemingly altering their habitat to help them ambush prey. Common garter snakes are shown to have personalities and prefer to socialise with specific individuals. And recent studies of rattlesnakes have revealed that male diamondbacks find and stay in the same den year after year, while juvenile and pregnant female timber rattlesnakes prefer to “cuddle up” with close relatives.

Burghardt says that snakes have a lot of the same basic tools to navigate the world as mammals, but their anatomy and lifestyle may keep these capacities hidden.

“My career,” he says, “has been devoted to trying to change people’s opinions of reptiles away from that of slow, dumb, dull, instinct machines."

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