Nobody likes an eavesdropper. But for humpback calves, being overheard can be a death sentence.
Sound travels four times faster in water than in air, and the creatures living below the waves have evolved highly sensitive hearing. Humpback whales use the water’s sound conduction properties to communicate with one another over long distances. But those wavelengths can also be intercepted by orcas, which have been known to hunt the calves of other whales.
A new study now shows that humpback calves may have a workaround: They “whisper” to their mommies. The sounds are actually more like squeaks and grunts, says lead author Simone Videsen, a marine biologist at Denmark’s Aarhus University.
Videsen listened in on these intimate conversations by attaching temporary, suction-cupped sensors to whale mothers and their calves in Australia’s Exmouth Gulf. Many aspects of whale migration remain a mystery, but it’s thought that the gulf’s warm waters provide sanctuary while the mothers spend a few months suckling the calves.
During this time, the calves must gain as much weight as possible to increase their chances of surviving the long swim back to colder waters where krill, sardines, anchovies, and other small prey exist in larger quantities. And since food is scarce in the Exmouth Gulf’s waters, mother humpbacks are essentially fasting while their calves are draining them of nutrients. Now, throw in orca pods on the prowl, and you’ve got a situation in which there’s very little room for error.
This many whales in one place, however, makes for good study material, and Videsen was eventually able to record whale communications that have never been heard before by scientists. (She and her team were very careful not to hurt the whales’ chances of survival. Once the recorders were in place, they observed the whales from more than two football fields away with their boat engine off. Even the stick-on recorders were programmed to pop off after 22 hours.)
At this point, it’s unclear what the whales are saying, but Videsen suspects their utterances help mother and baby stay close to each other in the murky waters of Exmouth Gulf.
“We don’t know exactly what would happen in the case of a separation or whether they are able to compensate by producing louder signals,” says Videsen. “However, if they do resort to louder calls, then that could potentially make them more detectable to predators.”
And it’s not just orcas that the duos would like to avoid. Male humpbacks also patrol these waters in search of females. The males don’t actively attack calves―behavior you might see in lions or polar bears―but they do get in the way as they “escort” the females back to the feeding grounds.
“The presence of male escorts can create disturbance and a higher activity level, thereby disrupting the calm behavior between mother and calf,” says Videsen. It also leaves less time for the calves to nurse.
Killer whales, meddlesome bachelors . . . And as if the mothers didn’t have enough disrupting their nursery, human noise pollution is also causing a ruckus. From the whir of a whale-watching boat’s rotor to the steady churn of a freighter, humans create a cacophony below the waves. Seismic testing, drilling, exploratory air guns . . . study after study has shown that the noise we make has serious consequences.
Videsen says all of this noise pollution “could mask the communication calls between mother and calf and reduce the distance they can hear each other.” She estimates that the whale calves’ whispers travel no more than a football field in any direction. This makes the sounds super quiet compared with everything else going on below the waves.
What’s more, Jasny worries about what will happen if the Trump administration gets its wish to open up the Atlantic Ocean to seismic blasting, which could flood our coastal waters with noise at frequencies that critically endangered whales use to communicate.
That’s right, choices we make on our shores can affect humpbacks, too. From Alaska to California and Maine to Florida, these whales can be found in most patches of the earth’s oceans.
“The ocean is a world of sound,” says Jasny, “and this latest research gives a sense of the fragility of that world.”
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