Newborn humpback whales "whisper" to their mothers to avoid being detected by predators such as killer whales, new research suggests.

Never captured before, the baby whale call recordings were collected using tags placed temporarily on the whales by a team of ecologists in Denmark, Australia and Scotland. Their findings were published Thursday in the journal Functional Ecology.

Lead author Simone Videsen, a marine biologist from Aarhus University in Denmark — along with colleagues from Murdoch University in Australia and the University of St. Andrews in Scotland — tagged eight humpback calves and two mothers with suction-cup-like devices that record sound and movement for 48 hours before floating to the surface.

They found that the young whales communicated with their moms using quiet grunts and squeaks much different than the long, haunting songs heard in previous humpback recordings.

"We know humpback whales are known for their long songs. These are short and sporadic compared to these long songs," said Videsen in an interview with CBC News. 

The calls are used to keep the mother and calf together in the murky waters of their breeding ground in the Exmouth Gulf in Western Australia, where visibility is only two to three metres, she said.

Videsen said the quiet calls may be used to prevent detection by predators.

Killer whales hunt young humpback calves in this region. "One hypothesis could be that they produce weaker signals to avoid predation by killer whales," she said.

Dead-beat whale dads

The low-volume communication may also help the mother-and-child pairs avoid another problematic interruption of baby's nursing time: the approach of male humpbacks who want to mate with the nursing females.

Male humpback whales are opportunistic breeders who compete for mating partners and don't play a role in the lives of their young. In fact, they get in the way of newborn whales who need to suckle.

Humpback whales spend their summers in the food-rich waters of the Antarctic or Arctic. In the winter they migrate to the tropics to breed.

The migration out of the tropics is demanding for the young calves, who must travel more than 8,000 kilometres through rough seas.

Tracking the behaviour patterns of the newborns, particularly their nursing relationships with their mothers, will help scientists to better target conservation efforts, says Videsen.

humpback-whale-whisper

Shipping traffic in busy seaways can disrupt whale migration and potentially separate calves from their mothers by masking the sounds they use to stay together, according to the research by ecologists in Denmark, Australia and Scotland. (Mathieu Belanger/Reuters)

"From our research, we have learned that mother-calf pairs are likely to be sensitive to increases in ship noise. Because mother and calf communicate in whispers, shipping noise could easily mask these quiet calls."

Humpbacks are slow to reproduce. Pregnancy lasts for about a year and calves stay with their mothers for their first year of life.

'It's crucial for them to gain a lot of weight to be able to survive the migration back.' - Simone Videsen, marine biologist

While in tropical waters, the babies must gain as much weight as possible — growing as much as a metre per month — in order to endure their first long swim to cooler waters.

"It's crucial for them to gain a lot of weight to be able to survive the migration back," said Videsen.

There are two major humpback whale populations, one in each hemisphere.

Shipping problems

The humpback whale population that feeds in North Atlantic waters each summer was removed from the Endangered Species Act last year.

Still, the whales remain vulnerable to boat traffic.

U.S. government scientists launched an investigation on Thursday into an unusually large number of humpback whale deaths from North Carolina to Maine, the first such "unusual mortality event" declaration in a decade.

Forty-one whales have died in the region in 2016 and so far in 2017, far exceeding the average of about 14 per year, said Deborah Fauquier, a veterinary medical officer with National Oceanic and Atmospheric Administration Marine Fisheries in Maryland.

Ten of the 20 whales that have been examined so far were killed by collisions with boats, something scientists are currently at a loss to explain because there's been no corresponding spike in ship traffic.

The investigation will focus on possible common threads like toxins and illness, prey movement that could bring whales into shipping lanes, or other factors, officials said.

Videsen said that moms and their calves often lie on the surface of the water where they can be prone to ship collision, adding she hopes research like theirs can be used to help inform the shipping industry.

With files from Associated Press.