The giants that sing and hunt with bubbles

A humpback whale can be as long as a city bus and weigh up to about 40 tons, according to NOAA Fisheries. Yet much of what makes this animal remarkable is not its size. It is what humpbacks do: they cross whole oceans, they work together to trap fish with curtains of bubbles, and the males sing songs that slowly change from year to year.

Two homes, one ocean apart

Most humpbacks live a life split in two. In the colder, more productive waters at high latitudes they feed. In warm tropical waters they mate and give birth. Between the two lies a migration that NOAA Fisheries describes as one of the longest of any mammal: some populations swim about 5,000 miles from their tropical breeding grounds to their feeding grounds.

Some of those journeys are also fast. Humpbacks in the North Pacific can travel the roughly 3,000 miles between Alaska and Hawaiʻi in as few as 28 days.

The reason for this commute is a trade-off. The polar and subpolar seas are rich in food in summer, but they are cold. Tropical waters are poor in food but warm and calm — a safer place for a newborn calf. A humpback calf is born after an 11-month pregnancy and already measures about 13 to 16 feet (4 to 5 metres) at birth. Females usually have one calf every two to three years.

A net made of air

Humpbacks are baleen whales. Instead of teeth they have plates of baleen that work like a sieve. NOAA Fisheries notes that they filter-feed on small crustaceans, mostly krill, and on small fish.

Their best-known hunting technique is bubble-net feeding. A group of whales dives beneath a shoal of fish and releases air in a spiral or a line. The rising bubbles form a curtain that the fish are reluctant to cross, so the shoal is squeezed together and pushed towards the surface. Then the whales lunge upward through the middle of the net with their mouths wide open.

NOAA describes it as group-coordinated feeding. For an animal that weighs tens of tons, catching prey the size of a finger requires exactly this kind of strategy.

The song that keeps changing

Humpbacks make many kinds of sounds. Researchers distinguish between calls — shorter sounds used in many situations — and song.

According to the U.S. National Park Service, humpback song is a complex, repetitive and geographically distinct signal that is sung by males, mostly in a breeding context. What makes it extraordinary is that it does not stay the same. The breeding-season songs of male humpbacks change continually over time.

Calls are a different story. In Southeast Alaska, researchers compared recordings across 36 years and found that many call types were still being used decades later. Eight call types turned up in all four decades studied. So humpbacks combine two systems: a stable vocabulary of calls and a song that is always being revised.

Why males sing, and why the song changes, is still being studied. What is clear is that song is closely tied to breeding.

Back from the brink

Commercial whaling hit humpbacks hard. NOAA Fisheries states that all populations were greatly reduced, most of them by more than 95 percent.

Protection has made a real difference. Humpbacks are now managed as 14 distinct population segments. Nine of these have recovered enough that they are no longer listed under the U.S. Endangered Species Act. One is listed as threatened and four remain endangered: those breeding off Central America, in the western North Pacific, in the Arabian Sea and around the Cape Verde Islands and northwest Africa.

That mixed picture matters. "Humpbacks are back" is true in some oceans and not in others. NOAA Fisheries lists the main threats today as entanglement in fishing gear, vessel strikes, harassment by vessels and underwater noise — the last a particular problem for an animal that relies so heavily on sound.

Why it matters

A humpback’s year links the poles to the tropics. Its feeding connects the smallest animals in the sea — krill — to one of the largest. And its recovery in many regions shows that protecting a species can work, while its continued decline in others shows that the work is not finished.