Then Ferrara — who works for the Wildlife Conservation Society, a U.S.-based conservation group — heard it: a quick, barely audible pop within the shells.

The hatchings seemed to saying to one another, she said, “‘Come on, come on, it’s time to wake up. Come on, come on.’ And then all the hatchlings can leave the nest together.”

Researchers for decades thought of aquatic turtles as hard of hearing and mostly mute. One popular 1950s textbook claimed turtles “make no appreciable use of sound in their daily routine.” In the world’s rumbling rivers and cacophonous oceans, the lumbering reptiles appeared to tread along without much to say.

But recent recordings of these turtles’ first “words” — before they even hatch — challenge notions not just of the turtles’ capacity to communicate, but also of their instinct to care for young. Now the discovery has spurred an urgent count of this talkative turtle’s numbers, and may shape protections for shelled creatures in the Amazon and beyond.

When Ferrara began studying turtle communication, “so many people looked at me and said, ‘Oh, how? I don’t think that turtles use sound to communicate,’” she said.

“I said, ‘Let’s see.’”

‘Something really new’

Known locally as the arrau or tartaruga da amazonia, the giant South American river turtle lives throughout the Amazon and its tributaries. During the dry season, thousands of females at once crawl onto beaches along the river to lay their eggs.

For other kinds of turtles, the mothering usually ends at the beach. Many turtle hatchlings are left by their parents to fend for themselves.

But that’s not the case with the arrau. After nesting, females often hover by the shore for up to two months waiting for their eggs to hatch.

So Ferrara and her colleagues wondered: are mother turtle and child turtle communicating with one another? To test the idea, her team sent months taping the turtles — on land and underwater, in the wild and in a swimming pool.

The team recorded a wide repertoire of whisper-quiet calls from arrau of all ages.

Embryos appear to chirp together to coordinate hatching and digging up to the surface. With so many jaguars and other predators lurking, it is safer for baby turtles to move en masse toward the river.

The mothers, meanwhile, approach and respond to the calls of their young. Once the hatchlings reach the water, the baby turtles migrate down the river with the adult females, Ferrara’s radio-tracking research shows.

When her team published an early study on turtle vocalizations a decade ago, Ferrara said, academic journals resisted putting the phrase “parental care” in the title of a study about turtles.

“At that time it was very hard to publish,” she said. “It was something really new.”

But Ferrara and her colleagues have gone on to record vocalizations from more turtle species, including the pig-nosed turtle in Australia, Blanding’s turtle in Minnesota and Kemp’s ridley sea turtle in Mexico, one of the world’s most endangered. “Probably most of these species use sound to communicate,” she said.

Other researchers may have missed turtle noises since they tend to be quiet, infrequent and low-pitched — just at the edge of human hearing. Leatherback sea turtles, for instance, appear to have ears tuned to the frequency of waves rolling ashore. Some species can take hours to reply to each other.

For injured turtles, a return to the sea

“Had we had a bit more expansive imaginations, we might have caught this earlier,” said Karen Bakker, a fellow at Harvard’s Radcliffe Institute for Advanced Study who wrote about turtle vocalization in her book, “The Sounds of Life.”

“We’re looking for sounds in the frequencies we can hear,” she added. “We’re looking for sounds at a temporal rate that is as quick as we speak. And so we have blinders on our ears.”

Conducting a turtle census

As a group, turtles are more ancient than dinosaurs, and are central to many cultures’ creation stories. Yet today they rank among the species at most risk of extinction. Nearly three in five species may vanish, according to a recent assessment, with climate change, habitat loss and hunting posing risks.

The Amazon once teemed with so many turtles, it was difficult to navigate. While Indigenous people have long relied on turtles for meat, the arrival of Europeans accelerated their decline.

Colonists rounded up the turtles as a ready source of fresh meat. Missionaries declared turtles counted as fish, so Catholics could eat them during Lent. Fat from their eggs was rendered for street lighting in Brazil and cooking fat in Europe.

A sea turtle neared extinction. A trove of eggs shows it can be saved.

The species still faces serious threats. A boom in dam construction threatens to cull their numbers. And a continued appetite for turtle meat sustains a lucrative illegal trade, where middlemen can buy an arrau for $50 and sell it downriver for $450.

Residents of Brazil’s Amazonas state alone, according to one estimate, consume about 1.7 million turtles and tortoises every year. Conservationists are pushing to have the International Union for Conservation of Nature declare the species endangered.

Researchers are now racing to count how many arrau turtles remain in the wild. In September and October, Ferrara and other Wildlife Conservation Society scientists conducted a turtle census along the Guaporé River, which forms the border between Brazil and Bolivia.

With flying drones fixed with infrared cameras, the researchers counted the nesting site, which they say is probably the largest concentration of any freshwater turtle species in the world.

The team is still analyzing the images, but it estimates that a staggering 80,000 giant turtles nested along the river. Over the past couple of weeks, millions of hatchlings have crawled out of their shells and scurried into the river.

“We need to know its biology, its population,” said Omar Torrico, a biologist and drone pilot with the group. “Maybe climate change is going to be one of the problems for the future. And so we think assessing the population is one of the most important things to know.”

The warming climate is making baby sea turtles almost all girls

Ferrara now wants to figure out if noise pollution drowns out turtle chatter. “We can observe with those impacts with the other types of animals, like for whales or dolphins. We know that the ship noise can impact their communication.”

But for her, the real fight is not in the field, but in the cities, convincing regular Brazilians to refrain from eating turtle meat. For her, changing the minds of just a few folks would be a victory.

“What I want is to see is two or three people stop.”

This article is part of Animalia, a column exploring the strange and fascinating world of animals and the ways in which we appreciate, imperil and depend on them.

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