The scientists, with colleagues from the University of Zurich and University of Massachusetts, analyzed association and consortship data to model the structure of alliances between 121 adult male Indo-Pacific bottlenose dolphins at Shark Bay in Western Australia. Their findings have been published today in the Proceedings of the National Academy of Sciences.

Male dolphins in Shark Bay form first-order alliances of 2–3 males to cooperatively pursue consortships with individual females. Second-order alliances of 4–14 unrelated males compete with other alliances over access to female dolphins and third-order alliances occur between cooperating second-order alliances.

Co-lead author Dr. Stephanie King, associate professor from Bristol's School of Biological Sciences, said, "Cooperation between allies is widespread in human societies and one of the hallmarks of our success. Our capacity to build strategic, cooperative relationships at multiple social levels, such as trade or military alliances both nationally and internationally, was once thought unique to our species."

"Not only have we shown that male bottlenose dolphins form the largest known multilevel alliance network outside humans, but that cooperative relationships between groups, rather than simply alliance size, allows males to spend more time with females, thereby increasing their reproductive success."