Humpback Whales Winter in Antarctica: Not All Cetaceans Prefer Warmer Waters

Not all humpback whales prefer warmer climes during the winter. It turns out that some of these behemoths prefer to winter in Antarctic waters. The surprising discovery reveals new insight into the behavior of these amazing creatures.

Southern Hemisphere humpback whales were thought to all migrate toward the equator at the end of the Antarctic summer. There, they thrive in the warmer waters until temperatures warm again in the colder reaches of the planet. Yet this assumption changed one April morning when a researcher turned on the live stream of PALAOA, the underwater acoustic observatory.

While the humpback whales should have been swimming about 7,000 kilometers further away in the warmer waters off of Africa, the researcher heard the calls of the humpback whales. This was completely unexpected considering the fact that they should have long since departed the area.

"I was totally surprised, because the textbook-opinion until that day was that humpback whales migrate to Antarctic waters only in the austral summer months," said Ilse Van Opzeeland, one of the researchers, in a news release. "And even then, stranding believes were that they would only be feeding on krill in the ice-free regions around 60 degrees south latitude. However, our PALAOA observatory monitors and area 70 degrees south--so, much further south than hitherto known feeding grounds. With this in mind, hearing the animals on a winter morning near our observatory was a double surprise."

Intrigued by this event, the researchers decided to see whether or not this occurrence was unique or if it happened yearly. They created a procedure for the automatic detection of humpback whale calls. Then, they analyzed all PALAOA recordings from 2008 to 2009 for acoustic signs from these animals.

In the end, the researchers picked up recordings of high-frequency calls in addition to stereotyped calls that sound a bit like a moan. They discovered that in 2008, humpback whales were present near the observatory with the exception of the months of May, September and October. This means that it's very likely that the whales spent the entire winter in the eastern Weddell Sea.

In fact, the researchers have an explanation for the absence of calls during some months. It could be the Antarctic sea ice itself, which shifts and changes position.

"Near the observatory, open water areas in the sea-ice, also known as polynias, regularly form during winter," said Van Opzeeland. "Such polynias form due to offshore winds which press the sea-ice off the continent out to sea. We suspect that humpback whales use these ice-free areas. When polynias close or change position, the whales may move with them and leave the recording radius of 100 kilometers, which our underwater microphones are monitoring. However, we do not yet have proof for this behavior."

The researchers believe that the whales that winter in these colder climes are probably young whale cows that are not yet pregnant. Since a humpback whale female loses up to 65 percent of her body weight when giving birth to and suckling a calf, it's advantageous for younger whale cows to remain in Antarctic waters during winter.

The findings reveal a little bit more about these humpback whales. This, in turn, could help researchers better target conservation efforts in the future. Currently, the scientists are planning on finding out to which population the humpback whales from the eastern Weddell Sea below.

The findings are published in the journal PLOS One.

Want to hear the whale calls for yourself? You can listen to them here.