By Lewis Smith
Baby humpback whales have learnt to “whisper” to avoid being heard by predatory killer whales.
The calves may be five metres long when born but are still no match for orcas, which grow up to eight metres and are known to target young humpbacks.
Adult male humpbacks are known for their “haunting” and penetrating singing but the calves, to avoid signalling their whereabouts to hungry killer whales, communicate with their mothers with quiet grunts and squeaks - the cetacean equivalent of whispering.
Often the calves use even quieter means, researchers found after tagging whales with recording devices, of telling their mothers they are hungry - simply by deliberately bumping into the adult.
Humpbacks give birth in sheltered tropical waters such as Exmouth Gulf off the western coast of Australia, but have to migrate 5,000 miles to higher latitudes to feed. Some head to the Arctic, others to the Antarctic.
“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,” said Simone Videsen of the University of Aarhus in Denmark.
As soon as they are born the calves are in a race to get as big as possible as quickly as possible to enable them to reach the feeding grounds and to become too large to need worry about killer whales.
They can grow up to one metre in length each month but to manage such a feat they need a steady supply of their mother’s milk.
As they travel to their feeding grounds, humpback calves and their mothers break off from swimming and remain stationary while suckling.
They can suckle at the surface or when submerged deep under the surface but need to remain quiet to reduce the chances of orcas finding them.
Recordings from tags fitted with suction caps, which fall off the animals within two days and float to the surface, revealed that mother and calf will remain stationary, either at the surface or deep down, for up to 20 per cent of the time it takes to complete their migration.
Dr Videsen added: “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.”
One of the problems identified by the study team, who reported their findings in the journal Functional Ecology, is noise from shipping.
“From our research, we have learned that mother-calf pairs are likely to be sensitive to increases in ship noise," said Dr Videsen. "Because mother and calf communicate in whispers, shipping noise could easily mask these quiet calls.”
Photo: a humpback mother and calf in Exmouth Gulf. Credit: Fredrik Christiansen