Male dolphins seeking mates form social networks with ranges larger than those of any species except humans, a new study has found.  

An international team of scientists observed how unrelated male dolphins not only create small friend groups, but also combine these cohorts to forge larger alliances. The researchers published their findings on Monday in the Proceedings of the National Academy of Sciences. 

“Cooperation between allies is widespread in human societies and one of the hallmarks of our success,” co-author Stephanie King, of England’s Bristol University, said in a statement, acknowledging that such behavior “was once thought unique to our species.”  

Bottlenose dolphin alliances have reached such size because the animals combine multiple smaller friend groups, the researchers observed. 

Studying 121 male dolphins in Western Australia’s Shark Bay, the scientists found that “first-order” groups of two or three males come together to pursue females.

Second-order alliances of between four and 14 unrelated males then compete with each other for access to those females, while third-order alliances can form between second-order friend groups, according to the study. 

“Cooperative relationships between groups, rather than simply alliance size, allows males to spend more time with females, thereby increasing their reproductive success,” King said. 

The study also dispelled a long-held belief about “intergroup cooperation” in humans: that it emerged as a side effect of our evolution of pair-bonding and parental care by males — which humans do but chimpanzees, our nearest relatives, do not.