While parsing through years of recorded blue whale songs looking for seasonal patterns, researchers were surprised to observe that during feeding season in the summer, whales sing mainly at night, but as they prepare to migrate to their breeding grounds for the winter, this pattern reverses and the whales sing during the day. This finding, published October 1 in the journal Current Biology, may explain known inconsistencies in whale song patterns.
"We found that during their feeding season, whales tended to eat during the day and sing at night, but after they had decided to begin their migration, they inverted this pattern and started to produce song primarily in the day, what we refer to as an acoustic signature of migration," says first author William Oestreich, a PhD candidate in biology at Stanford University.
Blue whales have distinct breeding and feeding seasons, separated by a long migration in most populations. For the whales in this study, feeding takes place off the west coast of North America in the summer, after which they swim hundreds to thousands of miles to the Pacific coast of Central America to breed in the winter. During feeding season, they bulk up on krill - up to 6 tons a day. This krill binge is essential to their survival, giving them the energy to make their migration, reproduce, and nourish their young.
"You can imagine the amount of energy required to make that journey is pretty great," says Oestreich. "That's compounded by the fact that they're the largest animal ever to live on Earth. So it makes the timing and intensity of their feeding season critical for their survival."
The discovery of this acoustic signature in their song was a happy accident. The research team noticed it only after analyzing data taken from recordings deployed and maintained by the Monterey Bay Aquarium Research Institute. The equipment had been continuously recording the sounds of a whale population off the coast of California since 2015. Because this group of whales makes the same migration each year, the team was also able to tag individual whales with data-tracking devices to monitor their movement, vocalization, and feeding behaviors.
"We were originally interested in trying to characterize the seasonal pattern of their presence and different types of sounds that they make, but as we started to dig into the data analysis, we noticed this dramatic separation of the song production during the daytime versus the nighttime," says Oestreich.
Oestreich and his colleagues are not the first researchers to notice whales singing at different times of day. However, this is the first time these differences have been connected to a broader pattern in their life cycle by tagging and monitoring individual animals. "This is something that's been described before in blue whales, but in some studies, they report the day/night difference and in others, there's no differences. So there's this discrepancy in the literature about when and why it happens," says Oestreich. The tag-derived data allowed the research to solve this puzzle, connecting the day-night difference to seasonal transitions between foraging and migrating.
This study is also a departure from the usual assumption that whale songs are strictly used as mating calls. "An unexpected part of this work is that whale songs are typically thought of only in a mating and reproductive context," Oestreich says. "We don't dispute that, but it's interesting that there's this kind of secondary utility of song in terms of detecting foraging or migration based on the time of day the sounds are being produced."
While there is still much to learn about the song and migration of blue whales around the world, the researchers are optimistic that this research can help in the tracking and protection of endangered whale populations.
"A lot of folks are interested in this kind of work in an applied sense - how this can help the management of marine habitats and mitigating risks of ship strikes on these animals in crowded shipping lanes," Oestreich says." This certainly isn't a silver bullet for that, but you could see it contributing to it."
This work was supported by the National Science Foundation, the David and Lucile Packard Foundation, the Office of Naval Research, and the Office of Naval Operations/Living Marine Resources
Current Biology, Oestreich et al.: "Animal-borne metrics enable acoustic detection of blue whale migration" https://www.cell.com/current-biology/fulltext/S0960-9822(20)31331-2
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