Which animal has the best hearing?

As a bat swoops through the night sky, it chirps out high-frequency calls and listens for the echoing sound waves to navigate the dark forest. Those chirps may be above the hearing range of most animals, but not the greater wax moth, which can hear frequencies of up to 300 kilohertz — about 15 times greater than the upper range of human ears. Sensing the bat's call, the moth knows its predator is approaching, and it ducks out of the way just in time.

Both bats and greater wax moths are frequently cited as having some of the best ears in the world. But which animals really have the best hearing?

"What is 'best' is always relative," said Christine Köppl, a professor of cochlear and auditory brainstem physiology at the University of Oldenburg in Germany. Sensitivity, the ability to distinguish between similar sounds, and the ability to localize sounds are all different factors that create an animal's sense of hearing, Köppl told Live Science in an email. Factors like these make it difficult to rank animals' hearing. However, there are some major standouts.

Hearing to pinpoint prey: Owls

When it comes to animal auditory systems, Köppl is a big fan of barn owls (Tyto alba).

"I have worked on barn owls, so they are on top of my list," she said. "Its entire auditory system has been shaped by its nocturnal hunting habits and the ability to precisely locate prey by listening."

Owls hunt at night, so they use their ultrasensitive hearing to supplement their vision in low-light conditions. They can detect the rustling of a mouse scuttling underneath thick layers of snow or leaves and quickly pinpoint their prey.

Owls accomplish this through a couple of key adaptations. First, the feathers around their faces create a dish shape, which helps funnel sound waves into their ears. Their right and left ears are also at slightly different heights, meaning sound waves reach the two ears at slightly different times. The owls can use this tiny difference in the sound detected between their left and right ears to calculate a sound's location.

Related: Which animal has the largest ears?

Hearing to map the world: Bats and dolphins

While they live in completely different environments, bats and dolphins share one impressive trait: the ability to echolocate.

"I love dolphin and bat ears, because they're not simply picking up sound and processing it, but they're using it for active imaging of their environments," said Darlene Ketten, a research scholar emeritus at Woods Hole Oceanographic Institution.

Echolocation involves producing a sound that bounces off the objects in the environment and then listening for the returning sound waves. Through this method, dolphins and certain bat species can create a mental map of their surroundings using sound. Sonar is based on the same technique, but dolphin and bat echolocation is more advanced than the human-made technology, Ketten said.

"We're pretty much constantly being surprised by the complexity, the sophistication of what they can do with sound," Ketten said of her research on dolphins.

Bats have a couple of features that help support echolocation. First, their large outer ears help collect sound waves. Second, the structures in their brains that process auditory information are huge — similar to how a big chunk of the human brain is devoted to visual processing.

Dolphins need a lot of brainpower to echolocate — their auditory nerve is about two or three times as thick as many land mammals' — but their ears work a little differently than those of land animals like bats. That's because dolphins' external ear openings are just small holes, and they don't seem to play a large role in conducting sound. Instead, dolphins likely use fat deposits around their jaw to detect sound waves traveling through water.

Both bats and dolphins also have special mechanisms in their ear that help cancel out the noise of their echolocation calls so they aren't deafened by their own vocalizations.

Hearing above and below water: Pinnipeds

Brandon Southall, president and senior scientist for Southall Environmental Associates, thinks the crown for the best hearing should go to an often-overlooked group of animals: pinnipeds, which include seals, walruses and sea lions.

"They do the nearly impossible thing of having to hear both above and below the water," Southall said.

Every animal has an auditory system that's tuned to the environment where it spends most of its time. Think about how when you dunk your head underwater, sound becomes muffled, distorted and hard to pinpoint. That's because our ears are designed to detect sound waves traveling through air, so sound waves traveling through water sound distorted and strange to us. The same goes for marine animals — their ears are designed to detect sound waves traveling through water.

"If you take a dolphin and you put it in air, it's virtually deaf and has no directionality," Southall said.

But because pinnipeds hunt in the ocean and mate and raise young on land, they need to hear well both on land and in water. In fact, Southall said, some pinnipeds can hear almost as well as owls on land, and some can hear almost as well as dolphins underwater. While working in the field, Southall has even seen seals react to the sound of crunching snow as far as 1 mile (1.6 kilometers) away.

To hear so well both on land and in water, pinnipeds use a unique mechanism: They fill the empty space in their middle ears with blood while swimming. This allows underwater sound waves to continue travelling through a liquid in the middle ear, minimizing any sound distortion. When the seals return to land, their ears fill with air again, allowing them to hear sound waves traveling through air.

It's "mind-blowing," Southall said.