Why do whales migrate?
Whales migrate primarily for two reasons: to find food and to breed, but also for molting!
Whales undertake some of the longest migrations on earth, often swimming many thousands of miles, over many months, to breed in the tropics. But what drives whales to brave the high seas like this? This is a question that has long fascinated both the general public and the research community. Several studies have attempted to shed light on this phenomenon and three main conclusions emerge. Depending on the species, whales migrate for reasons related to reproduction, feeding or, more surprisingly, even for molting.
Migration is defined as a movement from one place to another taking place periodically. In many cases this periodicity is seasonal and involves regular outward and return movements on a geographical scale. Whales, dolphins and porpoises, the order Cetacea, form a group of highly specialized mammals living an entirely aquatic existence. It includes two suborders: the Mysticeti (known as baleen whales) and the Odontoceti (toothed whales). The mysticetes have no teeth and instead they carry a few hundred baleen plates in their mouths. These plates form a dense mat which acts as a filter for sifting food from seawater.
Differing from baleen whales, the odontocetes could have from one to a hundred teeth in their mouth and are designed for catching one food item at a time. Baleen whales are notable migrants while toothed whales can undertake seasonal migrations, but they are not comparable to that found in baleen whales. Most baleen whales are highly migratory, moving toward high-latitude (polar) feeding areas in the summer and toward low-latitude (tropical) calving areas in the winter.
Warmth and shelter
During winter months, many whale species migrate to warmer waters to breed and give birth. Calves are born with a thin layer of blubber, making warmer waters important for their survival.
Another selective advantage to migrant pregnant female whales is to reduce the risk of predation on their newborn calves. A calf is particularly vulnerable to attacks by predators. Killer whale abundance in high latitudes is substantially greater than that in lower latitudes, and most killer whales do not appear to migrate with baleen whales. This is why some cetaceans, including the grey whale, will instead migrate to refuges that are certainly warm, but more importantly, safe.
Food availability
During summer months, many whales often migrate to cold, nutrient rich waters, where food like krill, small fish and plankton is abundant. With the onset of winter, prey become scarce in these regions and the whales therefore move to lower latitudes. This cycle repeats itself every year.
Fasting allows whales to dedicate all its energy to its marathon migration. This is especially important slot deposit for pregnant whales. They will need every ounce of nutrition to sustain them during the rigors of the long migration, birthing and nursing their calves.
However, if a given summer range does not adequately meet the nutritional needs of the whales that frequent it, some individuals may refrain from migrating. This great annual marathon is very demanding and requires excellent preparation. During the winter migration, whales do not feed. If prey are not widely available, the energy reserves of some individuals could be insufficient to undertake the journey.
Shed skin
Recent studies have revealed a previously unsuspected motivation behind whale migration: molting. Some cetacean species, including killer whales, humpback whales and grey whales, molt when they reach warm, tropical regions after feeding in cold waters for part of the year. This phenomenon might be explained by the same mechanism that allows whales to maintain their body temperature. In polar climates, blood circulation is reduced in the outermost layers of skin, which helps minimize heat loss. Skin cells go dormant and stop renewing themselves. The molt is therefore altered.
Once the whales arrive in their wintering grounds, the warmer waters boost the metabolism of their skin cells and molting can resume. In addition to ridding the animals of their dead skin, this exfoliating treatment is also believed to help them shed colonies of diatoms, microscopic algae that accumulate on their skin over time.
Made possible in particular to drones and satellites, this recent discovery proves once again that when it comes to whales, we learn more every year!
More info:
• Gabriele, C.M., Straley, J.M., Herman, L.M., Coleman, R.J., 1996. Fastest Bebas88 documented migration of a North Pacific humpback whale. Mar. Mamm. Sci.;12, pp. 457-464.
• Davis, GE, Baumgartner, MF, Corkeron, PJ, et al (2020). Exploring movement patterns and changing distributions of baleen whales in the western North Atlantic using a decade of passive acoustic data. Glob Change Biol.; 26: pp. 4812– 4840.
• Modest, M., Irvine, L., Andrews-Goff, V. et al (2021). First description of migratory behavior of humpback whales from an Antarctic feeding ground to a tropical calving ground. Anim Biotelemetry; article 42.
• Andrews-Goff, V., Bestley, S., Gales, N.J. et al (2018). Humpback whale migrations to Antarctic summer foraging grounds through the southwest Pacific Ocean. Sci Rep; 8, article 12333.
• Pitman, RL, Durban, JW, Joyce, T, Fearnbach, H, Panigada, S, Lauriano, G. Skin in the game: Epidermal molt as a driver of long-distance migration in whales. Mar Mam Sci. 2020; 36: pp. 565– 594.