At nearly 50 feet long, North Atlantic right whales are so large that most living things in the sea shouldn’t worry them—but that’s not the case for their babies. Newborn calves are vulnerable to attacks by sharks and orcas. To keep them safe, a new study has found, right whale mamas take things down a notch, “whispering” to their young so they don’t attract any hungry predators lurking nearby.

Right whales typically communicate with one another using a vocalization called an up-call, a rising “whoop” sound that can last two seconds and travels very far. With their babies, however, they use a quieter, shorter grunting sound that can only be heard in the immediate vicinity. The new study appears in the journal Biology Letters.

“They allow the mother and calf to stay in touch with each other without advertising their presence to potential predators in the area,” lead author Susan Parks, a marine biologist at Syracuse University, says in a press release.

Parks and her team found the grunting sound after attaching small, non-intrusive recording tags to juvenile, pregnant and mother-calf pairs of North Atlantic right whales while the animals were in their calving grounds in Florida and Georgia. Compared to the juvenile and pregnant whales, the mothers caring for calves significantly reduced the loud noises they made, instead producing more of the quieter grunting sounds.

Understanding the North Atlantic right whales is critical for the survival of the endangered species, which only has 420 individuals left.

“Right whales face a number of challenges, including a very low number of calves born in recent years, combined with a number of deaths of reproductive females by collisions with large ships or entanglement in fishing gear,” Parks says. “There are still many things we don’t know about their behaviors, and it is my hope that studies like this will help to improve efforts for their conservation.”

The North Atlantic right whale isn’t the only whispering whale. In July, researchers in Europe reported that they had identified similar whispering in Southern right whales, a different species that lives in the oceans of the Southern Hemisphere. The softer, more intimate calls, the researchers found, could only be heard about 1,000 feet away. In 2017, researchers also found that humpback whales have a special, quieter type of communication between mothers and calves.

The existence of these quieter languages raises the possibility that human-generated noise in the oceans, like the noise of shipping vessels, is doing more damage to marine mammals than previously believed. A study published last year found that ship noise disrupted the mating calls of humpback whales, causing some whales to go completely silent. Another study has found that the whales change their diving and foraging behavior in the presences of ship noise.

All of these whale whispers also highlight the possibility that biologists are overlooking a whole subset of animal communication reports Carolyn Wilke at Science News. Typically, field researchers focus on the loudest noises animals make, but that may only be part of the story, as Peter Tyack from the University of St. Andrews in Scotland, not involved in the studies, tells Wilke.

“There may be a repertoire among the calls of lots of animals that are specifically designed only to be audible to a partner who’s close by,” says Tyack.