Bill McQuay

Scientists eavesdropping in trees have decoded a high stakes game of hide and seek. Katydids rely on ultrasound to find mates and listen for bats, which use ultrasound to find the bugs, and eat them.

The National Park Service is racing to record soundscapes of each park that capture nature for the ear. "If we start to lose sounds of wilderness, we start to lose a piece of us," one scientist says.

Editor's note: One of the most intriguing stories we ran in 2015 looked at — and listened to — how the invention of the stethoscope changed medicine. We're presenting it again, in case you missed it in July.

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Over the years, scientists have mostly interpreted the world through ...

Sound gets into our brains and processed so quickly that it shapes all other perceptions, says neuroscientist Seth Horowitz. "You hear anywhere from 20 to 100 times faster than you see."

It can take more than just a keen ear to figure out what animals are saying. Sometimes, scientists are learning, you have to talk back to map the rich networks of conversation in a forest.

For some insects, sound waves or vibrations are the real social media — high-speed rumbles sent through the air and along leaf stems to help the bugs claim territory, send warnings and find mates.

The trumpeting roar of an elephant is loud. But scientists living with herds in the forests of central Africa say the deep rumbles that humans can't hear, but can feel, carry crucial messages, too.

Christopher Clark, an engineer turned whale biologist, wired the world's oceans with hydrophones. Whales sing as they migrate, he learned. And the ship sounds clouding the ocean can deeply interfere.

Male humpback whales create "songs" together, scientists say. Katy Payne was the first to hear the shifts in pitch and pattern in the collective calls as complex music — haunting, evolving tunes.

The stethoscope seems so simple — a 19th century tool for listening more closely to the human heart or lungs. It also sparked a culture of listening that is transforming the way scientists learn.

Nir Kalron was once an Israeli commando, then private security consultant to African leaders, and a dealer of legal arms. Today he's working with African locals to hunt ivory poachers via satellite.

Andrea Turkalo spent 22 years in central Africa, studying rare forest elephants. Then civil war forced her to flee — and poachers killed many of the elephants she'd shared a life with.