In need of a spa day? Why whales migrate to tropical waters

Pitman, Robert et al. (2019) Skin in the game: Epidermal molt as a driver of long-distance migration in whales. Marine Mammal Science,

Why the long migration?

For many whale species including humpbacks (Megaptera novaeangliae), right whales (Eubalaena spp) and grays (Eschrichtius robustus), their migration to warm tropical waters coincides with giving birth to their young. In fact, at 14,000 miles, the gray whale undertakes the furthest annual roundtrip of any mammal living today. However, while the warm waters may sound nice to us as we long for a summer vacation, these waters are actually devoid of food and force adult whales to go without eating for up to five months. During this time, fatty tissues from the blubber provides the whales the energy they need, but eventually they must return to the polar arctic waters where food is plentiful.  As their parent’s need for food grows stronger, every young whale calf, must begin an epic trans-planetary journey when it is just a few months old.

Figure 1: A humpback whale (Megaptera novaeangliae) and her calf begin their trans-planetary migration. Source: Flikr

So why all that risk and effort? Surprising findings from a recent paper led by Robert Pitmann of Oregon University now suggests that the whales complete this migration in an effort to look after the condition of their skin. Many animals regularly shed their skin in a process called moulting. For humans, we moult or shed skin continuously, whereas other creatures, such as snakes, remove all their skin in a single effort. Moulting rids the individual of damaged or contaminated skin exposed at the body’s surface and replaces it with healthy, newer skin underneath. For whales, taking care of their skin also makes them more streamlined, which reduces the effort they need to swim.

Shed that skin

For the moulting process to occur in mammals there must be a plentiful supply of blood to the surface of the skin. Swimming in the freezing conditions of polar waters, whales face a difficult choice. If they choose to increase the flow of blood to their extremities they can slough off skin that becomes clogged with microscopic algae called diatoms. But in ridding themselves of diatoms, whales potentially put themselves at risk of cooling their core body temperatures. When allowed to accumulate, diatoms create a yellow film over the bodies of whales. This can change the appearance of whales so much, that early whalers referred to these whales as “sulphur-bottoms”, in reference to the colour of their normally white underbellies which showcase this change the most.

Figure 2:  An orca (Orcinus orca) covered in diatom microalgae. Source: R. Pitman et al. (2019)

The air temperature of the seas surrounding Antarctica and in the Arctic circle often drops below -40oC. While their blubber offers a great amount of insulation to these frigid conditions, whales cannot risk losing precious heat to the sea. Instead, they prefer to undergo their mega-migrations, travelling thousands of miles to bask in warm tropical waters and slough their skin in more amenable temperatures. You could say, they travel all that way for a spa day.

It’s more than just for whales

It is important to note that whale skin is rich in key marine nutrients, meaning that in looking after their own health, whales also help fertilise coastal ecosystems that are home to a multitude of other species. Given whale migrations are known to have occurred for thousands, if not millions of years, this vital flow of nutrients may have had a significant impact in shaping the diversity of life across the tropics.  

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Reviewed by: Jessica Wright and Laura Schifman

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Andrew Abraham

I am currently a PhD student at Northern Arizona University and University of Oxford. My research investigates the role of animals as nutrient arteries, quantifying the extent to which they transport vital minerals across landscapes in their flesh and dung. My work spans both terrestrial and marine environments and I have ongoing field projects in southern Africa, Amazonia and Scotland. I integrate this empirical data into ecological models to understand the collective impact of all animals in altering global nutrient cycles. My passion for the natural world ultimately stems from a lifetime immersed in wild places. Twitter: @EcologyRoo

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