Communication range predicts dolphin alliance size in a cooperative mating system.

It is well known that communication range, often termed active space, varies with habitat structure, and this variation can influence individual vocal behavior across taxa. While theoretical predictions imply that communication distances can drive the evolution of mammalian alliance sizes, empirical tests of this hypothesis are currently lacking. In Shark Bay, Western Australia, unrelated male bottlenose dolphins form multilevel alliances, where males work together in pairs or trios to herd single estrus females. Here, we use empirical measures of male dolphin vocalizations, ambient noise levels, and high-resolution bathymetry data to estimate variation in active space across the study site. We combine this with long-term data on male alliance behavior to determine how active space influences alliance group size and mating success. We show that the active space of vocalizations used by allied males in a reproductive context predicts the number of preferred alliance partners with whom individuals cooperate over the longer term, ultimately contributing significantly to male access to mating opportunities. These results reveal that variation in sensory ecology driven by heterogeneous habitat influences optimal cooperative group size and mating success within a single population of wild animals.