Observing the nature of relationships in male bottlenose dolphins

As long-term studies reveal, bottlenose dolphin communities comprise a complex network of individual relationships. Individuals form strong bonds (e.g., mother-calf or male partnerships), transient relationships, and also compete against each other for resources. Evidence of bonded partnerships is typically revealed by the years-long study of associations with repeated sightings. However, quickly determining which individuals have close affiliations would benefit both field researchers working to describe individual behavior as they engage in cognitive activities such as cooperative foraging as well as caregivers in zoos who must decide which individuals should be housed together. Observations in aquariums are well-suited for collecting long-term, detailed information on how pairs interact because subjects can always be found and their behavior both above and below the water can be seen well. These are conditions that are rare for most (but not all) ocean-based studies. We used multiple measures to detect affiliated behavior across several dimensions of pairwise affiliation. Specifically, we used association indices to measure the frequency of affiliative behavior, the symmetry of the partnership, the tenor of interactions, and the stability of which partners were strongly affiliated from year to year. Synchronous behavior and reciprocity in proximity-seeking are two examples of potential markers of an affiliative relationship where individual choices-to join, to move together, and to leave-are visible to observers. We found that the combined measures were effective at identifying one pair that maintained a strong, stable relationship across years, one individual that formed a moderately strong trio relationship with both members of the most-affiliated pair, and one individual who was more variable in his relationships. These social markers provide a means of rapidly identifying bonded males in both aquarium and ocean settings, particularly when long-term knowledge of individual histories is not available.