What is a blue whale?

The blue whale (Balaenoptera musculus) is not only the largest living mammal. It is the largest animal that has ever existed on Earth, across all eras, larger than the most massive dinosaurs, larger than anything palaeontology has ever documented. This extreme size reaches proportions that go beyond human intuition. And yet, despite its almost mythological stature, it is an animal of surprising fragility in the face of modern pressures.

Larger than the dinosaurs

The largest known dinosaurs, giant sauropods such as Argentinosaurus or Patagotitan, are estimated at about 70 to 90 tonnes for 35 to 40 metres in length. The blue whale is well beyond them: a large adult female can reach 30 to 33 metres and weigh 130 to 180 tonnes. Exceptional records have approached 200 tonnes.

To give a concrete scale: a blue whale of 30 metres is the equivalent of three buses end to end. Its weight exceeds that of 25 adult African elephants. Its tongue alone weighs as much as an elephant, about 3 tonnes. Its heart, which can weigh 700 kilogrammes, is the size of a small car. With each beat, about 220 litres of blood are driven into its arteries, the aorta being of a diameter wide enough for a child to crawl along.

An interesting feature: females are on average larger than males, a sexual dimorphism reversed compared with most mammals. This difference is probably linked to the energetic demands of gestation and lactation in animals of such mass.

A misleading coloration

Despite its name, the blue whale is not really blue. Its skin shows a mottled blue-grey coloration, darker on the back and lighter on the belly. It is the effect of light filtered by the water that gives it that characteristic blue appearance when it is seen from the surface. In photographs of stranded animals, or of animals breaking the surface, the real colour looks duller.

Its skin is often covered with scars and marks left by marine organisms: sucker marks from attacking killer whales (rare but documented), bites from cookiecutter sharks (Isistius brasiliensis), traces of parasitic barnacles. These marks allow individuals to be recognised by photo-identification, as with humpback whales.

A diet made up exclusively of krill

The largest animal in the world feeds almost exclusively on the smallest: krill, those tiny crustaceans a few centimetres long that swarm in gigantic shoals in polar waters. An adult blue whale can consume up to 4 tonnes of krill a day in the feeding season, that is several million individuals.

It practises lunge feeding on a grand scale: it accelerates towards a shoal of krill with its mouth open, stretches its expandable throat pleats, and takes in up to 80,000 litres of water and krill in a single mouthful. It then pushes the water out through its baleen, retaining the krill. A single mouthful can hold the energy of a whole day of human activity.

The most powerful songs

The songs of blue whales are the most intense vocalisations ever measured in the animal kingdom. Emitted at very low frequencies (10 to 40 Hz), they can reach 188 decibels at the source. Thanks to the SOFAR channel (an ocean layer between 600 and 1,200 metres that acts as a natural waveguide), these sounds could travel thousands of kilometres in the pre-industrial ocean.

Today, the noise of shipping traffic in the same low-frequency band has considerably reduced that effective range. Communication between blue whales, once trans-oceanic, has become largely compromised in areas of heavy traffic.

A species rebuilding slowly

Decimated by industrial whaling in the 20^th century, the world population fell from several hundred thousand individuals to about 5,000 in the 1970s, and the blue whale is rebuilding extremely slowly. The current estimate is 10,000 to 25,000 individuals worldwide, still far below pre-whaling levels.

The reproductive cycle is particularly slow: sexual maturity at around 5-15 years, gestation of 11 months, a single calf per birth, an interval of 2 to 3 years between births, longevity of about 80-90 years. Each breeding female produces only some twenty calves over her whole life. This “K” strategy makes it extremely vulnerable to cumulative pressures: ship strikes, ingestion of microplastics (up to 10 million particles a day for a blue whale of the North-East Pacific according to recent studies), ocean noise, climate change. The largest animal of all time remains, paradoxically, one of the most fragile in the face of our civilisation.

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