Here we report that synchronous surfacing in male bottlenose dolphins is associated with alliance membership and that synchrony between members of cooperating alliances is more common during social behaviour than during nonsocial behaviour, especially during bouts of 'intense' social behaviour (e.g. mounting, displays, chasing) with female consorts. Alliances of three males varied in the degree to which all members participated in synchrony, possibly reflecting differences in alliance unity and the extent to which alliances consort with females. The lack of a discontinuity between the elaborate displays that males perform around females and the synchronous surfacing of alliance members reported here suggest that synchrony is an adaptive signal. If so, we have a remarkable convergence in the use of synchrony as an alliance signal in humans and bottlenose dolphins. However, whether an adaptive signal or a correlate of proximity, synchrony will be a useful tool in the study of delphinid social relationships
