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Each spring, eastern Australia’s humpback whales migrate south toward their summer feeding grounds in Antarctica. Like humpback whales around the world, this is a talkative bunch. The males sing out—haunting melodies that pierce through thousands of kilometers of seawater. Yet these songs are just one type of humpback communication. In addition, humpbacks will grunt and groan. They’ll bark and snort. They’ll use their bodies as mallets, slapping their tails or fins against the sea’s surface. Even the crash after a breach is thought to be a way of reaching out.
It’s an impressive communicative repertoire, but over the past century or so, whales have had an increasingly difficult time being heard. With their loud, banging motors, and twisting, thrumming propellers, ships generate noise that makes it difficult for these leviathans to keep in touch as they swim, forage, and mate.
Humpbacks have established techniques for dealing with some ocean noise, of course. Battering winds and breaking waves mean that the ocean has always been loud. To be heard over this natural noise, the whales modify their communication in one of two ways: by changing the acoustic properties of their calls, or by using one type of communication (say, tail slapping) over another (calling). For cetacean biologist Rebecca A. Dunlop, the next question became: which of these techniques do humpbacks use to overcome human noise pollution?
The answer, as it turns out, is “none of the above.”
During the 2002–2004 and 2008 migrations of the eastern Australian humpbacks, Dunlop worked with a small army of volunteers to track the whales’ locations and behaviors. At the same time, she used an array of hydrophones to record both the whales’ sounds and the background noise, anthropogenic or otherwise.
When it was windy, Dunlop found that the whales modified their calls by increasing their “vocal source level”—that is, they got louder, “shouting” to be heard through the din. They also switched from relying primarily upon vocal sounds to surface-generated percussive sounds.
But when faced with boat noise, the whales didn’t use either of these techniques. In fact, their vocal source levels were actually lower than predicted, meaning that the sounds of puttering boats probably masked their calls to all but their nearest companions.
“They don’t seem to be coping with vessel noise,” says Dunlop. “As the vessels pass, they just don’t see the point [of communicating] since it only lasts a few minutes.”
Dunlop admits that it’s surprising that humpbacks don’t try to compensate for anthropogenic noise when they do it so readily for natural noise. She thinks it might be because, in eastern Australia, boat density is still fairly low. “Maybe in twenty years time we’ll do this study again, there will be heaps more vessels, and we’ll find that they do tend to emit these sounds louder.”
But for now, when a boat or ship cruises by, these whales’ songs are silenced.