[a] The blue whale's long and slender body can be of various shades of greyish-blue on its upper surface and somewhat lighter underneath.

In general, blue whale populations migrate between their summer feeding areas near the poles and their winter breeding grounds near the tropics.

It continues to face numerous man-made threats such as ship strikes, pollution, ocean noise, and climate change.

The name "blue whale" was derived from the Norwegian blåhval, coined by Svend Foyn shortly after he had perfected the harpoon gun.

[13] The earliest discovered anatomically modern blue whale is a partial skull fossil from southern Italy identified as B. cf.

Comparison with Antarctic blue whales showed that this hybridization began after the separation of the northern and southern populations.

However, the genetic distinction is less between them and the Eastern North Pacific blue whale, hence there might be gene flow between the Southern and Northern Hemispheres.

[32] A 2019 study by Luis Pastene, Jorge Acevedo and Trevor Branch provided new morphometric data from a survey of 60 Chilean blue whales, hoping to address the debate about the possible distinction of this population from others in the Southern Hemisphere.

[38][39][40] The underbelly has lighter pigmentation and can appear yellowish due to diatoms in the water,[7][35][36] which historically earned them the nickname "sulphur bottom".

[12][41] The male blue whale has the largest penis in the animal kingdom, at around 3 m (9.8 ft) long and 12 in (30 cm) wide.

[48] The longest scientifically measured individual blue whale was 30 meters (98 ft) from rostrum tip to tail notch.

[35][50] Hydrodynamic models suggest a blue whale could not exceed 33 metres (108 ft) because of metabolic and energy constraints.

[54] The weight of the heart of a stranded North Atlantic blue whale was 180 kg (400 lb), the largest known in any animal.

[4] During the harvest of a female blue whale, Messrs. Irvin and Johnson collected a fetus that is now 70% preserved and used for educational purposes.

[64] In addition, female blue whales develop scars or corpora albicantia on their ovaries every time they ovulate.

[35][72] Blue whales have been recorded making 180° rolls during lunge-feeding, possibly allowing them to search the prey field and find the densest patches.

Due to their size, blue whales have larger energetic demands than most animals resulting in their need for this specific feeding habit.

[76][87][88] In the Southern Ocean, baleen whales appear to feed on Antarctic krill of different sizes, which may lessen competition between them.

[89] Blue whale feeding habits may differ due to situational disturbances, like environmental shifts or human interference.

[93][94] Female pygmy blue whales are 21.0–21.7 meters (68.9–71.2 ft) in length[52] and roughly 10 years old at the age of sexual maturity.

[64] Blue whales produce some of the loudest and lowest frequency vocalizations in the animal kingdom,[27] and their inner ears appear well adapted for detecting low-frequency sounds.

Photograph-identification studies of blue whales have estimated that a high proportion of the individuals in the Gulf of California have rake-like scars, indicative of encounters with orcas.

A blue whale mother and calf were first observed being chased at high speeds by orcas off southeastern Australia.

[120] The first documented attack occurred in 1977 off southwestern Baja California, Mexico, but the injured whale escaped after five hours.

[27] Five deaths in 2007 off California were considered an unusual mortality event, as defined under the Marine Mammal Protection Act.

[150] In Sri Lanka, a blue whale was documented with a net wrapped through its mouth, along the sides of its body, and wound around its tail.

[156][157] Blue whales in the Southern California Bight decreased calling in the presence of mid-frequency active (MFA) sonar.

However, because blue whales feed low on the food chain, there is a lesser chance for bioaccumulation of organic chemical contaminants.

[160] Analysis of the earwax of a male blue whale killed by a collision with a ship off the coast of California showed contaminants like pesticides, flame retardants, and mercury.

Reconstructed persistent organic pollutant (POP) profiles suggested that a substantial maternal transfer occurred during gestation and/or lactation.