Sexual displays enriched by object carrying serve to increase individual male fitness, yet are uncommon phenomena in the animal kingdom. While they have been documented in a variety of taxa, primarily birds, they are rare outside non-human mammals. Here, we document marine sponge presenting associated with visual and acoustic posturing found in several, geographically widespread populations of Australian humpback dolphins (Sousa sahulensis) over ten years of observation. Only adult males presented marine sponges, typically doing so in the presence of sexually mature females, although social groups predominantly consisted of mixed age and sex classes. Male humpback dolphins appear to be using sponges for signalling purposes in multi-modal sexual displays. Further, based on limited behavioural and genetic data, we hypothesise that pairs of adult male Sousa form at least temporary coalitions or alliances. The use of objects in sexual displays by non-human mammals is rare and, moreover, cooperation between males in the pursuit of an indivisible resource is an evolutionary hurdle relatively few species have overcome. These findings suggest a hitherto unrecognised level of social complexity in humpback dolphins. © 2017 The Author(s).