Methodological inconsistencies hinder comparative studies of alliance in the genus Tursiops

Males of the genus Tursiops generally form alliances to cooperate for access to females, with variations in formation and complexity across regions. Given the variation in alliances, it may be possible to identify the factors that favor alliance formation through a cross-population comparative analysis. However, the association criteria that determine alliance formation in these previous studies are inconsistent, which has prevented researchers from deciding whether the variation in alliances reflects the inherent characteristics of each population or is simply pseudo-variation caused by criteria differences. To ensure the validity of future comparative analyses, we investigated the extent to which inconsistencies in association criteria among studies affect the variation through a literature review. We reviewed 50 studies from 23 regions and quantified criteria inconsistencies. We found discrepancies between the alliance statuses reported in the original studies and the outcomes of analyses in which criteria were exchanged within each study. Our results suggest that although inconsistencies in association criteria currently have little influence, the possibility that inconsistencies in criteria account for pseudo-variations in the alliance formation cannot be excluded. Based on this suggestion, we recommend that alliances should be determined by criteria that integrate behavioral data in addition to association data.