Why do whales migrate every year?

Every year, several million tonnes of cetaceans move across the oceans along precise routes, sometimes covering more than twenty thousand kilometres in an annual cycle. The Pacific grey whale holds the known record for a mammal: its migrations between the Bering Sea and the Mexican lagoons of Baja California can reach twenty-two thousand five hundred kilometres there and back. The humpback whale is not far behind. This planet-wide synchronisation is no accident: it answers a precise biological trade-off between two contradictory constraints, and that trade-off shapes the whole ecology of the great whales.

The biological calendar of a humpback whale

To make this migration tangible, let us follow a female humpback whale of the South Pacific through her annual cycle. It is January. She is off Antarctica, in a sea scattered with icebergs where krill swarm in enormous shoals. She feeds voraciously: up to a tonne and a half of krill a day, filtered through her baleen in mouthfuls of ninety thousand litres. For three or four months of the austral summer she does almost nothing else. Her layer of blubber thickens, her weight rises by several tonnes. This is her fuel reserve for the following eleven months.

In April and May, as the days shorten and krill becomes less accessible, she sets off on her migration north. She works her way up the Australian or New Zealand coasts, covering eight to ten thousand kilometres in two to three months. During this journey she barely feeds at all: she lives on her reserves. Her route follows learned paths, shared within her population, sometimes marked by known staging points.

In July and August she reaches the warm waters of Polynesia, New Caledonia or the Cook Islands. The surface temperature there hovers around twenty-five degrees. If she is pregnant, she gives birth on arrival. The calf, weighing a few tonnes and measuring four to five metres, has almost no fat at birth. It could not survive in Antarctic waters at zero degrees. For two or three months the mother stays in this warm, stable water, suckling intensively (the milk contains forty per cent fat), and accompanying her young through its first lessons. Still she eats nothing, or almost nothing.

If she is not pregnant, this is where she is mated by a male for the following season. The complex songs of the males around the breeding grounds are among the most emblematic behaviours of the species, and are heard only during this period: see our page on whale song.

In October and November she sets off back to Antarctica with her calf (which now weighs several tonnes). By January she is back at her starting point, ready to begin another cycle. The tally: sixteen to twenty thousand kilometres covered, ten months without feeding seriously, one calf raised. It is an energetic feat.

The fundamental trade-off

Why such an investment? Because polar waters and tropical waters offer two resources that are complementary but geographically separate.

Polar waters are extraordinarily productive in summer. The lengthening day, the vertical thermal mixing of the water and the upwelling of nutrients from the depths create perfect conditions for an explosion of phytoplankton. That in turn feeds the krill, which swarm in their thousands of billions. This is the place where a whale can, in a few months, build up enough blubber to live for a year. But it is also a freezing place. At zero degrees a newborn calf, still without insulating blubber, would lose its body heat within hours.

Tropical waters offer the opposite thermal environment: stable at around twenty-five degrees, where a newborn can survive without spending all its energy fighting the cold. But productivity there is low: an adult that spent the year in those waters would starve.

Migration is the solution. The whale builds up its reserves at the pole, gives birth in the tropics, and makes the round trip. The energy cost of the journey is high, but lower than the cost of the neonatal mortality it would face without migrating.

Complementary hypotheses

Several further hypotheses have been put forward to explain aspects of migration that the thermal trade-off alone does not account for.

The predator refuge hypothesis proposes that tropical waters would hold fewer killer whales preying on calves. The Antarctic killer whale ecotypes (in particular type A, which specialises in hunting the common minke whale) would represent a strong selective pressure pushing whales to give birth out of reach. This hypothesis is consistent with the heightened vigilance of mother whales on the migration back to the pole with their young.

The moult hypothesis, formulated more recently (Pitman et al., 2020, Marine Mammal Science), suggests that whales head for the tropics in part to shed their skin more efficiently. Epidermal moult would be inhibited by freezing temperatures, and whales would accumulate yellowish “patches” and skin parasites (in particular Cyamus, the whale lice) that they can only remove in warm water. If this hypothesis is confirmed, it would explain why migrations take place even in whales that are not breeding in the year concerned.

The competition hypothesis stresses that the concentration of whales in tropical water makes it easier for competing males to meet one another and favours sexual selection. The complex songs of humpback whales, for instance, only make sense in a context of close competition.

The atypical case of the sperm whale

Not all cetaceans migrate on this classic pattern. The sperm whale illustrates a remarkable case: it practises segregation by sex and by age. Females and young stay all year round in temperate and tropical waters, between forty degrees of latitude north and forty degrees of latitude south. Adult males, by contrast, migrate alone to high latitudes to feed in cold waters rich in giant squid, and return to warm regions only to breed. This split creates a particularly complex geography for the species.

See also our page on cetacean orientation to understand how these long-distance journeys are possible without GPS.

Sources

• Corkeron, P.J. & Connor, R.C. (1999). “Why do baleen whales migrate?”. Marine Mammal Science, 15.
• Pitman, R.L. et al. (2020). “Skin in the game: epidermal molt as a driver of long-distance migration in whales”. Marine Mammal Science, 36.
• Mate, B.R. et al. (1999). “Movements of North Pacific blue whales during the feeding season off southern California and their southern fall migration”. Marine Mammal Science, 15.
• Stafford, K.M. et al. (2007). “Long-range acoustic detection and localization of blue whale calls in the Northeast Pacific Ocean”. JASA, 122.
• NOAA Fisheries, Whale migration patterns.

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Further reading