Although known for their hissing and slithering sounds, snakes themselves were long thought to be deaf.

Now we know that couldn’t be further from the truth, according to a growing body of research from scientists working to show that snakes use sound to interact with their environment.

how exactly however, these slithering reptiles understand that noise still baffles the scientific jury. One school of thought holds that snakes sense vibrations in the ground — but new research doubles down on the challenge that there may be more.

Snakes hear vibrations through the ground

If snakes had to choose their favorite senses, they would probably choose sight and taste. Yet snakes still interpret the sound around them.

Over the past two decades, a large body of research has grown to suggest that—because snakes lack external ears and eardrums—they register noises through sound vibrations that travel through the ground and are perceived through their bodies.

Read more: Roosters have special ears so they don’t crow to the point of deafness

The vibrations are channeled through a group of bones in their jawbone, which usually rests on the ground, and all the way to the cochlea and the thinking brain. These vibrations can travel through anything from sand and soil to the tree branch they’re perched on, he explains J. Leo van Hemenprofessor of theoretical biophysics at the Technical University of Munich, Germany.

Take for example the desert horned viper (Cerastes cerastes), which determines when a mouse is passing by solely by sensing the ripples its prey’s footsteps make on the sand, van Hemen says. IN 2008 paperhe describes the exact biology of how this mechanism works in the snake’s body.

Snakes can also hear in the air

Now, new published research in the diary PLoS One suggests that ground vibrations aren’t the only method snakes use to sense noise—they can also hear airborne sound.

To test this, the researchers selected 19 slithering specimens from five different categories of snakes in captivity: deadly snakes (Acanthophis)python woman (aspiditis)pale-headed snakes (hoplocephalus)taipans (oxyuran) and brown snakes (Pseudonym). The latter are two of the most venomous snake species in the world.

Read more: These 3 prehistoric snakes are nightmares

Then, one by one, they dropped the snakes into a large, soundproof room and released three different ones pink noise frequencies between 0 and 450 hertz. One of them also produced vibrations in the ground, while the other two transmitted sound only in the air.

“It sounds like you’re in… a slow moving plane or a medium to fast moving plane,” he explains Christina Zdenekresearcher at the Venom Evolution Lab at the University of Queensland and one of the lead authors of the latest study.

Sounds are played at 85 decibels – roughly the equivalent of a human scream.

The team repeated this exercise more than 300 times, noting how the snakes responded to the sounds and paying attention to every body movement: freezing, head or tongue strikes, hissing, jaw relaxation, and more.

According to their observations, snakes react significantly both to sounds in the air and to those that cause vibrations in the ground.

“[If] I walk through the bushes, waiting for a snake to feel my feet. But if I talk and stand still, will a snake hear me?” Zdenek says. “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 seems to respond somewhat individually to sound.

“It makes sense from an evolutionary point of view because they have the same predators and the same way of foraging,” Zdenek suggests. “So they will have to interpret their environment in a similar way and [therefore] have similar reactions.”

Taipans are most likely to retreat from the sound and defend themselves. They would often shake their heads, hiss, drop their jaws ready to strike, and do something called fixation — which is when snakes hold their heads still on the target but coil the rest of their bodies in a threatening manner.

“Taipans are highly sensitive, highly aware species,” says Zdenek. In the real world, they have to be alert to avoid the carnivorous birds that like them as a snack.

Read more: Why do snakes eat themselves?

Brown snakes also moved away from the sounds in most cases, although they went all or nothing. Either the snakes remained completely still or moved very far away. (Researchers have noted they also do this in real-world paddocks when approached.)

The Woma pythons, on the other hand, were not shy at all. They moved toward the noise, curious.

“We interpret it that way because they are large nocturnal predators, the top predators in their environment,” says Zdenek. “So, less wary of anything affecting them.”

How do snakes actually hear

These new observations help us to expand our knowledge about whether or not snakes can hear sound in the air, especially looking at snake species about which we previously did not know much, says Bruce Youngprofessor and researcher at AT Still University in Missouri, who was not involved in the study.

But this new data doesn’t help answer the question how snakes accomplish this feat.

“Studies like this show quite clearly that snakes do respond to what we would call sound,” says Young. “But there’s a difference between what we see in the behavioral response and what we see in the physiological response. There are many different ways that snakes can respond to external stimuli.

In 2002, Young’s own research had suggested that diamondback rattlesnakes can respond to noise in the air. But in those results, as well as in these new ones, sound can still cause the snake’s surface to vibrate—and can we call that hearing?

There may not be much difference between detecting the vibrations from, say, the abdomen or lower jaw and from the surface of the head, according to Young, who cites 2012 study who claims that snakes hear everything by detecting vibrations with their skulls (especially airborne sound).

“The real holy grail is right now [snake hearing] the job is to understand the pathway in the brain,” says Young. And we just don’t know that yet. “Did the sound evoke a response to sound? Or just trigger a stimulus response?”

Are snakes good?

Of course, snakes hear better than we give them credit for. But at the end of the day, they’re still gross.

According to van Hemen, a sound level of 85 dB, equivalent to a human scream, at a distance of more than 1.5 meters is still a lot stronger than what usually occurs between predator and prey in nature.

Snakes also hear lower frequencies better than higher frequencies and much lower frequencies than humans, Zdenek says. “Snakes don’t have nearly the frequency range and acuity that humans do,” she says. “I guess it’s a bit of a muffled sound that the snakes might hear.”

Snake hearing is also only tested at 1000 Hz. But Young’s diamond paper for rattlesnake suggest that snakes respond best to airborne sounds between 200 and 400 Hz.

Likewise sea serpents can hear sounds from underwater speakers between 40 and 600 Hz, reaching peak performance at just 60 Hz. FYI folks hear best on frequencies more than ten times higher – about 5000 to 8000 Hz.

Unfortunately, there just isn’t enough research on snakes to draw firm conclusions, Zdenek says, and we have to be careful with our assumptions.

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