As further proof that whales can be enormous, covered in barnacles, and yet remain exceedingly cute, a new study has shown that newborn humpback whales “whisper” to their mothers to avoid being hunted by killer whales.
The new research by Aarhus University in Denmark documented two humpback mothers and eight of their calves in the Exmouth Gulf off western Australia at the time of their southbound migration. Using non-invasive tags attached to the individuals by suction cups, they managed to document the acoustic behavior between mother and calf. The study was published this week in the journal Functional Ecology.
“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," lead author Simone Videsen of Aarhus University explained in a statement. "We also 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.”
During the first weeks of life, the calves can grow by up to 1 meter (around 3 feet) every month, and as adults can eventually grow to be as large as 16 meters (52 feet) in length. Each year in winter, the humpbacks make this big migration to the tropics to mate and breed, and then in summer, they seek calm waters for their young to avoid predators and head for the food-rich waters of the Antarctic or Arctic.
However, taking this journey in itself can be a risk.
Aerial shot of a humpback whale mother and calf. Fredrik Christiansen.
"This migration is very demanding for young calves,” Videsen said. “They travel 5,000 miles [8,046 kilometers] across open water in rough seas and with strong winds."
"We know next to nothing about the early life stages of whales in the wild," she added, "but they are crucial for the calves' survival during the long migration to their feeding grounds."
The tags managed to document an intimate form of grunts and squeaks between humpback mothers and calves. Importantly, the calves' vocalizations were notably quieter than the noises typically made between humpback peers and could only be heard within a distance of fewer than 100 meters (330 feet). By humpback standards, that’s a mere whisper, as their calls can travel for tens and tens of miles.
They also think the rubbings are a silent way of informing the mothers the calves wanted to suckle, without drawing audio attention.
The findings of this research will go on to help the conservation of humpback whales. Perhaps most crucially, it also highlights the need for their nursery waters to remain quiet places, far away from shipping, human activity, or the use of sonar.