Researchers had long been thinking that Antarctic killer whales migrated to tropical waters once a year to give birth to their young. However, new research suggests that there may be another reason behind their annual voyage.
Most whale species move from cold waters to tropical waters once a year and the reasons for this long journey are still unclear.
Scientists have proposed that whales might migrate to a “friendlier” environment, similar to other species, with fewer predators and more readily available birth food.
However, the presence of small calves in Antarctic waters has been observed by marine ecologists, suggesting that whales may be able to give birth safely in icy waters.
If so, then they are not forced to travel thousands of kilometers and bear the dangers of calving into warmer climates.
So what is the real explanation for this conduct, then? Researchers from the National Marine Fisheries Service in La Jolla, CA and the Oregon State University’s Marine Mammal Institute in Newport may now have the answer.
Traveling for ‘skin maintenance’
The researchers explain in their new study paper, which appears in the journal Marine Mammal Science, that, between 2009 and 2016, they tracked 62 Antarctic killer whales (Orcinus orca) for about 7 years.
Of these, at least three whales made long-distance round trips of up to 11,000 kilometers (about 6,835 miles) which took them to complete 6–8 weeks.
These migrations have been from the initial habitat of whales in icy cold polar waters to lower latitudes and much warmer waters (20–24 ° C sea surface temperatures) and back again.
“Historically, large-scale whaling has been described as an annual, round trip between high latitude, summer feeding grounds and low latitude, winter breeding grounds— a feeding / breeding paradigm that has swayed for more than a century,” the researchers noted in the study paper.
Yet thanks to their size, even at low temperatures, killer whales and other large cetaceans are able to maintain their body heat and could give birth in polar waters.
The team behind the new study suggests that this way, whales 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 might otherwise affect the health.
“I don’t think people have given the skin molt due consideration when it comes to whales, but it’s an essential physiological need that can be fulfilled by migrating to warmer waters,” says lead author Robert Pitman, Ph.D.
Researchers claim that whales migrate to tropical waters to allow their skin metabolism to control molting without affecting body heat, which is easier to maintain thanks to tropical waters The authors write this in their paper on study:
“I[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 species, including snakes, regularly shed their entire outer skin, and many more, among them humans, constantly shed dead skin cells.
The researchers note why Cetaceans, like whales, both do. Sometimes, however, the environmental conditions may interfere with this maintenance process.
The team states that killer whales in the Antarctic frequently take on yellowish discolouration. It, they claim, is a result of the skin being coated in a film of diatoms, or microscopic algae, which means they don’t undergo their natural, “self-cleaning” skin molt.
The film of diatoms also falls away when they do molt, revealing once again the clean patches of white skin.
“Although Antarctic killer whales are often covered with a yellow diatom film, at other times the same individuals can be clean, without a hint of yellowing,” writes the researchers.
“[ W]hen killer whales migrated to the tropics and molded their skin, diatoms would also be shed and cleaned back to the Antarctic,” they added.
The researchers say that although calving may also usually occur when the whales arrive in tropical waters, this process may be coincidental in reality.
“The feeding is ultimately so effective in fertile Antarctic waters that a remarkable migration pattern has formed with the relatively small, warm blooded killer whale. It enables it to utilize these resources and still maintain healthy skin function,” says study co-author John Durban, Ph.D.
The authors note that the momentous annual migration of whales has a major impact on local habitats, allowing them predators and prey in various locations during different seasons.
Although they suggest that skin maintenance processes may be the main driving force behind migratory behavior of some large cetaceans, the researchers nevertheless point out that this hypothesis needs further study.
Future research, they claim, will aim at examining patterns of skin growth in both migratory and non-migratory whales, in polar and tropical waters, throughout the year, to check the present statement.