When a wolf howls and a cricket chirps, it is easy to treat both as the same kind of event: an animal making a noise. Underneath, they have almost nothing in common. Animals have evolved at least half a dozen separate ways of making sound, and the method often explains what a species can and cannot say.
Mammals: Air Over a Larynx
Most land mammals, from dogs to deer to people, make sound by pushing air from the lungs past vocal folds in the larynx. The tension of the folds and the shape of the throat and mouth change the pitch and tone. Size matters: larger animals generally have longer vocal folds and a longer vocal tract, which lowers the pitch. Some species stretch the rules. Male red deer lower their larynx during the autumn rut, which makes their roars sound like they come from a bigger animal. Howler monkeys have an enlarged, hollow hyoid bone that acts as a resonating chamber, so a primate of a few kilograms can be heard across a forest.
Birds: A Voice Box with Two Sides
Birds do not use a larynx for song. They have a syrinx, an organ at the point where the windpipe splits into the two bronchi. Because the syrinx has two sides, each can be controlled independently. A songbird can produce two different notes at the same time, or switch rapidly between sides, which is part of how nightingales and thrushes manage such complex songs. Emperor penguins use the same two-voice ability to give each individual a call that is hard to confuse with any other.
Frogs: A Vocal Sac as an Amplifier
A frog calls by pushing air back and forth between its lungs and its mouth over its vocal cords. Males then inflate a stretchy vocal sac under the throat, which works like a resonator and makes the call far louder. A spring peeper weighs a few grams, yet a chorus of them can be heard from hundreds of metres away. The shape and size of the sac, and how fast the frog pulses its call, help females identify males of the right species.
Insects: Rubbing, Buckling and Beating
Insects do not have lungs, so they cannot use air in the same way. Each group has found a different trick:
- Stridulation: crickets and katydids rub a scraper on one wing against a file of tiny ridges on the other. Each stroke makes a pulse of sound, and the pulses merge into a chirp. Warmer temperatures speed up the muscles, so chirps get faster as the air warms.
- Tymbals: male cicadas have ribbed membranes on each side of the abdomen. Muscles buckle the ribs inward, making a click, and a large hollow abdomen amplifies the clicks into a loud buzz.
- Wingbeats: the hum of a bee or mosquito is simply the wings beating hundreds of times a second. It was not designed as a signal, but some species have learned to use it as one; female mosquitoes and males adjust their wingbeat tones to each other during courtship.
Whales and Dolphins: Sound Made Without Breathing Out
Toothed whales such as dolphins and sperm whales produce clicks and whistles using structures called phonic lips in the nasal passages. They push air past these lips and recycle it within the head instead of releasing it, which lets them make sound underwater for long periods. A fatty organ in the forehead, the melon, helps focus the clicks into a beam for echolocation. Baleen whales, including humpbacks and blue whales, lack this equipment and produce their calls using a laryngeal structure that recirculates air in a similar way. Low-frequency sounds lose little energy in water, which is why the calls of large whales can travel so far.
Reptiles: Hisses, Rattles and Bellows
Reptiles generally have simpler voices. Snakes hiss by forcing air through the glottis. A rattlesnake’s rattle is not a vocal sound at all but a set of hollow keratin segments clicking against each other when the tail shakes. Alligators produce deep bellows, and the lowest part of the sound is so low that it makes water on their backs ripple.
Why the Method Matters
How an animal makes sound limits what it can say. Insects that rely on a fixed file-and-scraper make highly stereotyped calls, while birds with a flexible syrinx can learn and rearrange phrases. Animals that live in dense forest or deep water have evolved low or sustained sounds that travel well, while those in open habitats can afford short, high calls. When scientists study the sounds of wildlife, they often learn as much about the body and the habitat as about the message itself.
To see these ideas in practice, read the profiles of the humpback whale, periodical cicada and American bullfrog, or continue with our article on how animal communication works.