Along with dolphins, whales are widely considered the smartest mammals in the sea, having developed brains and behavior that suggest intelligence and sophistication rarely seen in nature.

But a new study theorizes that some species of whales have taken that intellect to a new level — by "whispering."

The research, published in the Journal of Experimental Biology, suggests that female southern right whales "whisper" to their calves to avoid being overheard by killer whales.

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Researchers used multisensor tags on nine lactating whales for approximately 63 hours in a Western Australian breeding ground, using SoundTrap to estimate the acoustic background noise and were astonished by what they heard — or barely heard.

"It was difficult to assign the calls to either the calf or the mom, because they are so close to each other," the study's lead author, Mia Nielsen, said in a statement. Even though southern right whales calves are between 16 and 26 feet long (5 and 8 meters), they are vulnerable to attack, placing an importance on keeping a low profile when predators are near.

Whale calls are usually audible for miles, but the moos and grunts of the female southern right whales were barely audible over the pounding waves, often at very low decibel levels and less frequent then usual.

Nielsen, who said that one of the initial challenges was understanding the whales in the area, noted that these mammals are "very physical with each other," including actions such as the calf rubbing up against the mother. This made it difficult for the tags to stay attached to the calves for longer than 40 minutes.

"We conclude that such acoustic crypsis in southern right whales and other baleen whales decreases the risk of alerting potential predators and hence jeopardizing a substantial energetic investment by the mother," the study's abstract states.

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