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In our last report, we heard from Yves Lefevre who is based on Rurutu, French Polynesia. Scientists think that what Yves says is right, that they are going south to the Antarctic when they leave Rurutu, but actually there is no evidence of this! We don’t know where they go to exactly.
How do Humpbacks know where to migrate to? The first migration for a young calf, from the feeding ground and then back to the tropical breeding ground is shown to it by its mother. This path is learned by the calf, and remembered. It is then unlikely to deviate from this path, except for one-off ‘holidays’ for the rest of its migratory life – some 60 to 70 years.
Scott and his team tested the Humpback’s migration patterns using mitochondrial DNA. They found young male Humpbacks received their migratory destination from their mothers but it wasn't hardwired. Without the mother physically showing the way, they would not take the right route. The migration is, essentially, learned.
You might think that, having learned from their mother where to go when migrating, and sticking to that path, they might stick with their mother. But they don’t. They tend not to adhere to family groups, although they do travel in small units, of 3, 4 or 5 whales. They’re not like elephants for example who live in matrilineal groups - Humpbacks usually detach from their mother's company after a year when they are adults and can use the migratory route on its own.
This maternal fidelity exists in other species but is very strong in Humpback Whales. Their ability to learn from their mothers has lasted tens of thousands of years, in a habitat with no geographical barriers. To the whales themselves, the oceans are structured, but without physical barriers. They use various mechanisms to get from A to B. They have some magnetic sense, and some scientists are researching whether the whales also use celestial/astrological navigation, the sun, the moon, the stars. Following the topography of the seabed is an important mechanism for navigation. They are able to envisage a map of the seabed by analysing the sound signatures of certain locations.
Sound is therefore important in migration. The Humpbacks use aural cues as well as physical ones. And they are making sound as they move. It was first thought that whales only sang when they were on the breeding grounds, but since the 80s, we’ve known that they also sing as they migrate. The function of this migratory singing is unclear, or at least disputed among whale experts.
Some experts think that the boundaries are unclear in some cases between the migration path and the breeding ground, so what we’ve recorded and think is migrating sound is actually breeding sound. Some think that the breeding is a bit of an ongoing event, and is still happening as the whales migrate. Scott doesn’t think so – for a start, the female cycle is over by the time they’re migrating. And the females don’t respond when they hear the song as they move. The song ends at the feeding ground, when the whales make different noises, calling noises, without the song.
The audio of the Humpback's song was recorded off the SW coast of Madagascar on July 18, 2007.
Further Reading:
Last report: Swimming with Humpbacks
Audio by Salvatore Cerchio for Wildlife Conservation Society
Images from Marine Mammal Institute