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Humpback whales in the North Pacific Ocean spend their summers in cool northern feeding grounds like Alaska and off the coast of Russia, and every winter, different populations migrate to distinct warm areas off Hawai‘i, Mexico, the Philippines, or Japan, where they mate, breed, and raise their calves. At least, that’s what scientists thought.
Two new research papers challenge the idea that unconnected populations of humpback whales live separate lives. The new discoveries suggest that Pacific humpbacks move between these breeding grounds and that population groups—once thought to be discrete—actually mix and mingle far more than previously believed. The evidence? Their songs.
“It’s yet another piece of information telling us the whales aren’t staying in the boxes we’re drawing on the map,” says Jim Darling, lead author of both papers and a biologist with Whale Trust, a humpback research and education nonprofit based in Maui, Hawai‘i. The findings could have an impact on conservation policies, which are currently designed around the assumption that the whales gathering at each location are separate groups.
While male humpback whales are mostly quiet in the summer, during winter migrations and breeding activities they sing long, complex patterns of sounds. The songs last between five and 20 minutes and are repeated note for note. What’s more, all male whales in a group sing the same song when they’re together. When one whale changes its tune, the rest of the group changes to match.
In one paper, Darling used songs to identify different populations. He and his colleagues collected and analyzed recordings of whale songs in the Philippines, Japan, Hawai‘i, and Mexico over three winters from 2011 to 2013. They confirmed previous research that showed each group had its own song.
However, they also found similarities between the groups’ songs, with whales repeating some of the same musical phrases even in regions thousands of kilometers away. In some years, the groups’ songs were very different from one another, but in other years they were very similar. Researchers have compared regional whale songs before, but never over multiple seasons.
“In 2013, there was remarkable similarity across the whole ocean,” Darling says. “There’s no chance the phrases in the songs are the same by accident.”
“Something had to occur that caused the whales to produce the same sequence of sounds. That led us to the conclusion that there must be a lot of mixing in the North Pacific,” he says.
In the second paper, Darling collaborated with the Jupiter Research Foundation of Hawai‘i using their autonomous, self-propelling Wave Glider to survey whale songs on a 100-day round trip between Hawai‘i and Mexico in the winter of 2018. Surprisingly, the glider recorded whale songs midway between the two gathering areas—a place where the whales were not thought to be. According to previous research, scientists assumed whales migrate directly to their breeding grounds in the fall and returned along the same path in the spring. So what were they doing out there, singing halfway between their breeding and feeding grounds? Darling says it’s possible their work uncovered an undocumented migration route, a previously unknown breeding ground, or a clue that the whales were traveling between different breeding grounds.
“It’s very tantalizing,” says Christine Gabriele, a wildlife biologist at Glacier Bay National Park and Preserve in Alaska. While she thinks the possibility of an offshore breeding area is unlikely, “it makes you think whales might be moving between Mexico and Hawai‘i during the winter.”
The research is important for conservation, Gabriel adds. The US Endangered Species Act (ESA) recognizes whales in Hawai‘i and Mexico as separate populations and gives each different levels of protection. In 2016, humpbacks in Hawai‘i lost their ESA protections, while those in Mexico are still considered threatened. “It’s awkward because these population segments may not be as distinct as people thought when they were defined,” Gabriel says.
Both Darling and Gabriel say more research is needed to better understand humpback movements and behavior. “This is saying the model we’ve been working from is probably not very reflective of what actually goes on out there,” Darling says. “But if you ask me what does go on out there, I can’t tell you, we’re just getting glimpses of it.”