Callosities are areas of thickened, keratinised skin above the eyes, along the jaw and on the rostrum. The tissue itself is grey. Its white appearance comes from dense colonies of cyamids, small crustaceans commonly called whale lice, which live on the roughened surface and feed on skin.
The arrangement of callosities is fixed by about a year of age and differs between individuals, so aerial and vessel photographs can be matched against a catalogue. The North Atlantic Right Whale Catalog holds records reaching back to the 1930s and has followed named individuals across decades, allowing calving intervals, injuries and causes of death to be tracked animal by animal.
This is why the population figure is unusually firm. The estimate of roughly 370 whales is not a survey extrapolation but is anchored in a near-complete individual register. It also means entanglement scars can be counted directly: most living right whales carry evidence of having been caught in rope at least once.