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The cost of vocal behaviour is usually expressed in energetic terms; however, many animals may pay additional costs when predators or potential prey eavesdrop on their vocal communication. The northeastern Pacific is home to two distinct ecotypes of killer whales, Orcinus orca, called residents and transients. Resident killer whales feed on fish, a prey with poor hearing abilities, whereas transient killer whales hunt marine mammals, which have sensitive underwater hearing within the frequency range of killer whale vocal communication. In this study, we investigated how the superior hearing ability of mammalian prey has shaped the vocal behaviour of the transient killer whale ecotype. We recorded pulsed calls and the associated behavioural context of groups of transient and resident killer whales in British Columbia and southeastern Alaska. Transient killer whales produced pulsed calls significantly less frequently than residents. Transient killer whales only showed significant amounts of vocal behaviour after a marine mammal kill or when the whales were displaying surface-active behaviour. Vocal activity of transients increased after a successful attack on a marine mammal. Since marine mammals are able to detect killer whale pulsed calls and respond with antipredator behaviour, the reduced vocal activity of transients is probably due to a greater cost for calling in this ecotype resulting from eavesdropping by potential prey. The increase in vocal behaviour after a successful attack may represent food callin