For a long time, researchers had thought that Antarctic killer whales migrated once a year to tropical waters to give birth to their young. New research, however, suggests there may be a different reason behind their annual trip.
Many whale species migrate once a year from cold waters to tropical waters, and the reasons for this long journey have remained mysterious.
Researchers have hypothesized that similarly to other animals, whales might migrate to a “friendlier” environment with fewer predators and more readily available food to give birth.
However, marine ecologists have observed the presence of small calves in Antarctic waters, suggesting that whales may be able to give birth safely in icy waters.
If so, then they are not obliged to travel thousands of kilometers and endure the dangers of reaching warmer climates to calve.
So what, then, is the real explanation for this behavior? Researchers from the National Marine Fisheries Service in La Jolla, CA, and the Marine Mammal Institute at Oregon State University in Newport may now have the answer.
In their new study paper — which appears in the journal Marine Mammal Science — the researchers explain that they tracked 62 Antarctic killer whales (Orcinus orca) for approximately 7 years, between 2009 and 2016.
Of these, at least three whales made long-distance round trips of up to 11,000 kilometers (approximately 6,835 miles) that took them 6–8 weeks to complete.
These migrations were from the whales’ original environment in icy cold polar waters to lower latitudes and much warmer waters (sea surface temperatures of 20–24°C) and back again.
“Historically, large whale migration has been described as an annual, round trip movement between high latitude, summer feeding grounds, and low latitude, winter breeding areas — a ‘feeding/breeding’ paradigm that has held sway for over a century,” the researchers note in the study paper.
Yet, thanks to their size, killer whales and other large cetaceans are able to maintain their body heat even at low temperatures and could give birth in polar waters.
The team behind the new study argues that whales thus migrate not to give birth, nor even in search of food, but to take advantage of warmer waters to molt — or shed dead skin that could otherwise affect their health.
“I think people have not given skin molt due consideration when it comes to whales, but it is an important physiological need that could be met by migrating to warmer waters,” says lead author Robert Pitman, Ph.D.
The researchers argue that whales travel to tropical waters to allow their skin metabolism to regulate molting without affecting body heat, which is more easily preserved thanks to the warm waters. In their study paper, the authors write that:
“[I]nstead of whales migrating to the tropics or subtropics for calving, whales would be traveling to warm waters for skin maintenance and perhaps find it adaptive to bear their calves while they are there.”
Many animals, such as snakes, shed their entire outer skin periodically, and many more, humans among them, continuously shed dead skin cells.
Cetaceans, including whales, do both, the researchers note. However, environmental conditions may sometimes interfere with this maintenance process.
The team explains that Antarctic killer whales often take on a yellowish discoloration. This, they say, is a result of the skin becoming covered in a film of diatoms, or microscopic algae, which suggests that they are not experiencing their normal, “self-cleaning” skin molt.
When they do molt, the film of diatoms also falls away, revealing the clean patches of white skin once more.
“Although killer whales in Antarctica are often coated with a yellow diatom film, at other times, the same individuals can be clean, without a hint of yellowing,” the researchers write.
“[W]hen killer whales migrated to the tropics and molted their skin, they would also shed the diatoms and return to Antarctica clean,” they add.
The researchers say that, though calving may also usually take place upon the whales’ arrival into tropical waters, this process may actually be coincidental.
“Basically, the feeding is so good in productive Antarctic waters that the relatively small, warm blooded killer whale has evolved a remarkable migration behavior. This enables it to exploit these resources and still maintain healthy skin function,” says study co-author John Durban, Ph.D.
The momentous annual migration of whales, the investigators note, has an important impact on local ecosystems, making them predators and prey in different locations, during different seasons.
While they argue that skin maintenance processes may be the main drive behind some large cetaceans’ migratory habits, the researchers nevertheless point out that this hypothesis requires more investigation.
Future research, they say, should aim to analyze skin growth patterns in both migratory and nonmigratory whales, in polar and tropical waters, all year round, in order to verify the current argument.