Most toothed whales produce broadband clicks that spread energy across a wide frequency range. Harbour porpoises and their relatives instead emit narrow-band high-frequency clicks, with almost all energy concentrated in a band around 130 kilohertz. Several unrelated lineages, including porpoises, Cephalorhynchus dolphins and the pygmy sperm whales, have converged on the same design.
Killer whale hearing falls away sharply above about 100 kilohertz. A signal placed above that ceiling still works for echolocation over the short ranges a porpoise needs, while being poor material for an eavesdropping predator. The same species also lack whistles altogether, removing another source of detectable sound.
The convergence is strong circumstantial evidence, but it is inference from pattern rather than a tested result, and high-frequency clicks also attenuate quickly in seawater, which limits detection range. Porpoises compensate by hunting almost constantly: a small body in cold water gives little margin for missed meals.