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Go to Brilliant.org/SciShow to learn more. [♪ INTRO]. In 1984, biologist Katharine Payne spent a week observing elephants at a zoo in Oregon, and she noticed something strange.

Every now and then, she could feel what she described as a faint throbbing in the air, like silent thunder. But it wasn't thunder she was feeling- it was the elephants' secret language. You might've heard elephants trumpeting and grunting before, but those noises are just a fraction of the sounds they actually produce.

The rest are so low-pitched that our ears can't pick them up. Elephants do most of their chatting at these low registers, and it actually has some pretty big advantages for them. Months after Payne first heard the rumbles, she went back to the zoo and recorded the elephants.

When she and colleagues played the tapes back at high speed and higher pitch, they could hear that the elephants were as noisy as a bunch of cows on a farm! We humans can only hear frequencies between about 20 hertz and 20,000 hertz. Sounds below that range are called infrasonic.

And that infrasonic range is where most elephant communication happens, between around 14 and 35 hertz. This secret banter isn't just good for talking about other animals behind their backs, though. Infrasound allows elephants to communicate over impressively long distances.

Lower frequency sounds have longer wavelengths- imagine widely-spaced ripples instead of closely-spaced ripples. And since they vibrate more slowly, they lose energy more slowly, too, meaning they can travel farther before fading out. On top of that, high-frequency sounds tend to be blocked or absorbed by obstacles like trees, but low-frequency sound waves are larger than most barriers in their path, so they can pass around them and keep going.

So those high-pitched trumpeting noises elephants make? Those tend to peter out after traveling around 100 meters. But infrasonic calls can travel much, much farther.

So Payne and other researchers started looking for evidence that elephants communicate over long distances … and they found it! By tracking the movements of elephants with radio collars, the researchers observed that family groups were able to spend days coordinating their movements with each other, even when they were separated by as much as five kilometers. And when fertile female elephants belted out their infrasonic mating calls, eager male elephants as far as a kilometer out would start walking their way!

They can achieve this incredible long-distance communication in part because infrasound just naturally travels far, and also because elephants are ridiculously loud. The loudest recorded elephant calls are around 117 decibels— that's about as loud as construction equipment or a rock concert. Imagine being at a rock concert and not being able to hear anything except maybe a faint rumble in the air!

Elephants also take advantage of environmental factors, which can have a big impact on how far their voices carry. In forests, their voices have to push through the trees and compete with lots of other noises, so they usually fade out in less than a kilometer. They tend to do a lot more talking at quieter times of day, like early evening, when a forest elephant's voice might reach up to three kilometers.

But out on the savanna, with fewer obstacles and less competing noise, the major limiting factor is the atmosphere! During the day, the air over the ground is warmer than the air higher up. Since sound travels faster through warmer air, the bottom of the sound wave moves faster than the top of the sound wave, which causes it to bend upward.

So daytime sound waves only travel so far before turning away from the ground. But at night, the air near the ground cools down much faster than the air above it. Now the temperature gradient flips, creating what's called a temperature inversion.

And instead of bending upward, sound waves bend back toward the ground. The layer of cool air basically becomes a channel that sound can travel through for much longer distances. Around sunset and sunrise, when the temperature inversion is the strongest and the winds are calmest, it's been estimated that elephant calls could travel as far as 10 kilometers!

And while observing elephants in the wild, researchers have seen that the animals will occasionally raise their ears and stand still to listen to the distant calls of other elephants. Studies have shown that the inner ear structures of elephants are extremely large, making them especially good at gathering low-energy infrasonic sound waves. But they might not just be listening to each other … they might also be listening to the sounds of Mother Nature.

Widespread legends in African and Indian cultures say that if you see an elephant after several dry months, it means the rain is finally coming. And like a lot of folklore, it seems to have some truth to it. See, rainstorms are very noisy, especially at low frequencies, and there's evidence that elephants might be able to use their super-hearing to forecast the weather.

Research in the early 2000s tracked elephant movements across the seasons and found that elephants tend to exhibit different movement patterns in wet conditions versus dry ones. When the dry season turns to the rainy season (or even when there's a particularly rainy day during the dry season) elephants tend to switch from short-distance foraging to long-distance migration or vice versa. Exactly why they do this isn't totally clear, but what is really interesting is that the elephants usually changed their behavior days or weeks before the rains even reached them.

It's like the elephants could hear the storm coming. In many cases, the researchers saw that elephants in different locations would simultaneously react to the same distant storm. In at least one instance, the animals responded to a storm nearly 300 kilometers away!

So elephants are clearly tapped into a world of sound that we've only recently started to appreciate. And in a world where scientists are desperately trying to protect these majestic animals from extinction, their super-hearing may be key to understanding how they fit into their ecosystems. It takes a lot of different kinds of science working together to understand how elephants communicate: biology, atmospheric science, the physics of sound.

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