Animal acoustics

In the January issue of Physics Today, acoustic biologist Megan McKenna reviews different sources of sound in the environment and how animals react to them . Whether it’s picking up the faint squeak of a mouse or compensating for noisy surroundings by resorting to visual cues to attract a mate, animals have adapted to navigate their acoustic environments.

Consider barn owls, nocturnal hunters that use an impressive set of auditory capabilities to capture their prey. Their ears can quickly locate the horizontal position of a mouse’s squeak by taking advantage of the mismatch in the time that it takes for a sound to reach each ear. Humans and other animals also use that capability for sound source localization. But unlike most of those creatures, barn owls and other owl species have another acoustic specialization: asymmetrically placed ears. The difference in the placement of the ears allows barn owls to determine the sound source’s vertical location. By precisely localizing targets, the owls can often catch their prey within about 10–15 minutes over a hunting area of several square kilometers, and they do so repeatedly throughout the night.

Lizards, amphibians, and birds, including the barn owl, have inner ears that contain a membrane called the basilar papilla. Hair cells on the membrane convert the mechanical motion of a sound wave into an electrical signal that’s delivered to the brain. Biologists have found that longer basilar papillae are associated with the ability to respond to higher-frequency sounds. Among birds, the barn owl has the longest basilar papilla and an upper frequency limit of around 10 kHz; lizards’ limit is 4–8 kHz.

On top of having long papillae for lizards, geckos also enjoy specialized frequency-selective hearing. Different locations on their basilar papilla respond to sounds of different frequencies . The adaptation makes the gecko the only type of nocturnal lizard that can practice auditory communication. Male geckos produce a series of clicks to establish territory or dominance; females and juveniles may alert other individuals to a stressful situation with short squeaks.

Invertebrates also have evolved enhancements to their hearing mechanics without ears. The antennae of mosquitoes act as receivers that vibrate as a sound wave passes. Attached to the base of the antennae, a collection of cells called Johnston’s organ contains thousands of sensory neurons that detect nanometer-size displacements of the antennae and convert them to electrical responses. Researchers have found that mechanoreceptor cells in Johnston’s organ improve mosquitoes’ sense of hearing by amplifying sounds at specific frequencies , an adaptation not so different from auditory structures found in the human ear.

The ability of an animal to hear specific communications and respond to them degrades in noisier environments. But animals do have some recourse when those situations arise. Just as people have the tendency to speak louder in a crowded restaurant, other animals also employ the technique, which is known as the Lombard effect . Auditory analysis of king penguins reveals that they make louder and more frequent calls to one another in windy conditions .

Sometimes an environment’s soundscape becomes too loud for animals to exploit auditory signaling. Human noise pollution from traffic, heavy machinery, and other sources can alter animals’ listening capabilities, as can natural noise, such as the sound of a stream’s rushing water. When vocal calls get lost in the noise, some animals resort to visual displays to get their point across. Some frog species, including the dancing frogs in India, extend a back leg in the air to attract a potential mate’s attention in noisy environments, a practice known as foot flagging .

The aforementioned examples represent only a small sample of the acoustic adaptations of animals. To learn more, read McKenna’s article . And to hear (and see) more animals, especially birds, check out the Cornell Lab of Ornithology’s Macaulay Library .