- a migratory baleen whale, Balaenoptera musculus, mostly of oceans and seas in the Southern Hemisphere, the largest mammal ever known, growing to a length of 100 feet (30.5 meters) and having a furrowed, slate-blue skin mottled with lighter spots, in some seas acquiring a yellowish coating of diatoms on the underside: now classified as endangered, it was once hunted nearly to extinction, but international conservation efforts begun in the 1960s have enabled the blue whale population to rebound significantly.
- the largest mammal: a widely distributed bluish-grey whalebone whale, Sibbaldus (or Balaenoptera) musculus, closely related and similar to the rorquals: family Balaenopteridae
