Posted on Categories Discover Magazine
Though known for the sound of their hissing and slithering, snakes themselves were long believed to be deaf. Now we know that couldn’t be farther from the truth, according to a growing body of research by scientists who are working to show that snakes use sound to interact with their environment. How exactly these slithering reptiles understand noise still has the scientific jury puzzled, however. One school of thought argues that snakes sense vibrations in the ground — but new research doubles down on the challenge that there might be more. Snakes Hear Vibrations Through the Ground If snakes were to pick their favorite senses, they’d probably choose vision and taste. Yet snakes still do interpret the sound around them. Over the past two decades, a large swath of research has grown to suggest that — because snakes don’t have external ears and eardrums — they register noises through sound-induced vibrations that travel through the ground, perceived through their bodies. Read More: Roosters Have Special Ears So They Don’t Crow Themselves To Deaf Vibrations are channeled through a group of bones in their jawbone, which usually rests on the ground, and all the way through to the cochlea and the thinking brain. These vibrations can travel through everything from sand and soil to the branch of a tree they’re perched on, explains J. Leo van Hemmen, a professor of theoretical biophysics at the Technical University of Munich in Germany. Take, for example, the desert horned viper (Cerastes cerastes), which determines when a mouse walks past solely by sensing the waves its prey’s footsteps make on the sand, says van Hemmen. In a 2008 paper, he described the exact biology of how this mechanism works inside the snake’s body. Snakes Can Also Hear Through the Air Now, new research published in the journal PLOS One suggests ground vibrations aren’t the only method snakes use to sense noise — they can hear airborne sound too. To test this, researchers selected 19 slithering specimens from five different categories of captive snakes: death adders (Acanthophis), woma pythons (Aspidites), pale-headed snakes (Hoplocephalus), taipans (Oxyuranus) and brown snakes (Pseudonaja). The latter are two of the most venomous snake species in the world. Read More: These 3 Prehistoric Snakes Are the Stuff of Nightmares Then, one at a time, they plopped the snakes into a large, soundproof room and played them three different pink noise frequencies between 0 and 450 hertz. One of these also produced vibrations into the ground, while the other two solely transmitted sound through the air. “It sounds like you’re in … an airplane going slow, or an airplane going medium or fast,” explains Christina Zdenek, a researcher at the Venom Evolution Lab at the University of Queensland and one of the lead authors of the latest study. The sounds were played at 85 decibels — roughly the equivalent of a human scream. The team repeated this exercise over 300 times, noting how the snakes reacted to the sounds and paying attention to any body movements: freezing, head-flicks or tongue-flicks, hissing, dropping their jaws, and more. According to their observations, the snakes significantly reacted to both the airborne sounds and those that produced vibrations in the ground. “[If] I’m walking through the bush, I expect a snake to feel my feet. But if I’m speaking and standing still, is a snake going to hear me?” says Zdenek. “I think our study proves that they can, if you talk loud enough.” Different Snakes React to Sound Differently Of course, even within this single experiment, each species seemed to react a little individually to sound. “That makes sense from an evolutionary perspective because they have the same predators and the same foraging mode,” suggests Zdenek. “So, they would need to interpret their environment similarly and [therefore] have similar reactions.” Taipans were most likely to pounce away from the sound and get defensive. They frequently flicked their heads, hissed, dropped their jaws ready to strike, and did something called fixation — which is when snakes keep their heads still on a target but coil the rest of their body in a menacing way. “Taipans are highly sensitive, acutely aware species,” says Zdenek. In the real world, they must stay alert to avoid the carnivorous birds that fancy them as snacks. Read More: Why Do Snakes Eat Themselves? Brown snakes also moved away from the sounds on most occasions, though they went all-or-nothing. Either the snakes stayed completely still or they moved away a great deal. (Researchers have noted they also do this in paddocks in the real world when they’re approached by humans.) Woma pythons, on the other hand, didn’t shy away at all. They moved toward the noise, curious. “We interpret it as being because they’re large nocturnal predators, top predators in their environment,” says Zdenek. “So, less wary of something having a go at them.” How Snakes Actually Hear These new observations help enrich our knowledge on whether snakes hear sound through the air or not, especially looking into species of snakes we didn’t know much about previously, says Bruce Young, a professor and researcher at A.T. Still University in Missouri who was not involved in the study. Yet this new data doesn’t help answer the question of how snakes are pulling off this feat. “Studies like this show, quite clearly, that snakes do in fact respond to what we would refer to as sound,” Young says. “But there is a difference between what we see in the behavioral response, and what we see in a physiological response. There are a lot of different ways snakes may be using to respond to external stimuli.” Back in 2002, Young’s own research had suggested that diamondback rattlesnakes can respond to airborne sound. But in those results, like in these new ones, the sound could still be making the surface of the snake vibrate — and can we call that hearing? There may not be much difference between detecting the vibration from, say, the belly or the lower jaw and from the surface of the head, according to Young, who references a piece of research from 2012 that argues that snakes hear everything by detecting vibrations with their skull (especially airborne sound). “The real holy grail right now in [snake hearing] work is to figure out the pathway in the brain,” Young says. And we just don’t know that yet. “Was the sound triggering a response to sound? Or was it just triggering a response to stimuli?” Are Snakes Good at Hearing? Sure, snakes hear better than we give them credit for. But at the end of the day, they still kind of suck at it. According to van Hemmen,the sound strength of 85 dB, equivalent to human screaming, at a distance of more than 1.5 meters is still much louder than what would usually happen between predator and prey in nature. Snakes also hear lower frequencies better than higher ones, and much lower ones than humans can, says Zdenek. “Snakes don’t have nearly the range of frequency and the acuteness that humans do,” she says. “I imagine it’s a bit of a muffled sound that snakes might hear.” Snake hearing has also only been tested under 1000 Hz. But Young’s diamondback rattlesnake paper suggested snakes best respond to airborne sounds between 200 and 400 Hz. Similarly, sea snakes can hear sounds from underwater speakers between 40 and 600 Hz, peaking in performance at a mere 60 Hz. For reference, humans hear best at frequencies more than ten times higher — around 5,000 to 8,000 Hz. Unfortunately, there’s simply not enough research on snakes to be able to draw strict conclusions, Zdenek says, and we should be careful about our assumptions.