Baby humpback whales "whisper" to their mothers to avoid attracting unwanted attention from predators and marauding males, scientists have discovered.
The quiet communication reduces the chances of being overheard by killer whales and sexually rampant male humpbacks looking for an opportunity to mate.
Humpback whales are known for their loud, haunting songs. But a study using microphone tags showed that, while swimming with their mothers, newborn whales uttered intimate grunts and squeaks that could only be heard at close range.
Dr Simone Videsen, from the University of Aarhus in Denmark, who led the research in Exmouth Gulf, Western Australia, said: "We .. heard a lot of rubbing sounds, like two balloons being rubbed together, which we think was the calf nudging its mother when it wants to nurse.
"Killer whales hunt young humpback calves outside Exmouth Gulf, so by calling softly to its mother, the calf is less likely to be heard by killer whales, and avoid attracting male humpbacks who want to mate with the nursing females."
The findings highlight the importance of keeping humpback "nursery" waters as quiet as possible, say the scientists, whose findings appear in the journal Functional Ecology.
"Because mother and calf communicate in whispers, shipping noise could easily mask these quiet calls," said Dr Videsen.
The team tagged eight humpback calves and two mothers with suction cup devices that recorded both their sounds and movements for up to 48 hours before detaching and floating to the surface.
Humpback whales breed in the tropics during winter and then migrate thousands of miles to their summer feeding grounds in the Arctic and Antarctic.
Dr Videsen said: "We know next to nothing about the early life stages of whales in the wild, but they are crucial for the calves' survival during the long migration to their feeding grounds.
"This migration is very demanding for young calves. They travel 5,000 miles across open water in rough seas and with strong winds. Knowing more about their suckling will help us understand what could disrupt this critical behaviour, so we can target conservation efforts more effectively."