Scientists from Denmark and Australia detected very quiet calls and grunts between mothers and calves, which increased during dives, in Exmouth Gulf, Western Australia.

The sounds were 40 decibels lower than a singing male humpback whale, and 20 to 70 decibels lower than the normal social sounds of an adult humpback whale. In low-visibility water, these quiet vocalisations seemed to keep the pair together, yet allowed them to avoid detection by predators.

There were fewer vocalisations by the calf before suckling. Rather, it seemed calves rubbed against their mothers when hungry—which makes a sound like two balloons rubbing together.

These findings may mean that noise disturbances by other marine activities—such as whale watching and shipping—could potentially drown out these tiny vocalisations and raise the chances of a calf being separated from its mum.