Playback experiments simulating the presence of intruder male lions elicited cooperative behaviour of male coalitions resident with prides of females. Resident males approached the broadcast roars on all occasions when more than one male was present for the experiment, and only failed to approach in three of eight cases when a single male faced the roars of three intruders. Three routes to cooperation were tested: kinship, reciprocity and mutualism. These results suggest that approaching the roars of strange males is a cooperative behaviour, and that this cooperation is not conditional on either the kinship or the behaviour of a male's companions. Cooperation in male lions instead appears to be based on mutualism.