Affiliation history and age similarity predict alliance formation in adult male bottlenose dolphins

Male alliances are an intriguing phenomenon in the context of reproduction
since, in most taxa, males compete over an indivisible resource, female
fertilization. Adult male bottlenose dolphins (Tursiops aduncus) in Shark
Bay, Western Australia, form long-term, multi-level alliances to sequester
estrus females. These alliances are therefore critical to male
reproductive success. Yet, the long-term processes leading to the
formation of such complex social bonds are still poorly understood. To
identify the criteria by which male dolphins form social bonds with other
males, we adopted a long-term approach by investigating the ontogeny of
alliance formation. We followed the individual careers of 59 males for 14
years while they transitioned from adolescence (8-14 years of age) to
adulthood (15-21 years old). Analyzing their genetic relationships and
social associations in both age groups, we found that the vast majority of
social bonds present in adolescence persisted through time. Male
associations in early life predict alliance partners as adults. Kinship
patterns explained associations during adolescence but not during
adulthood. Instead, adult males associated with males of similar age. Our
findings suggest that social bonds among peers, rather than kinship, play
a central role in the development of adult male polyadic cooperation in
dolphins.