A new level of complexity in the male alliance networks of Indian Ocean bottlenose dolphins (Tursiops sp.)

Abstract

Male bottlenose dolphins in Shark Bay, Western Australia form two levels of alliances; two to three males cooperate to herd individual females and teams of greater than three males compete with other groups for females. Previous observation suggested two alliance tactics: small four to six member teams of relatives that formed stable pairs or trios and unrelated males in a large 14-member second-order alliance that had labile trio formation. Here, we present evidence for a third level of alliance formation, a continuum of second-order alliance sizes and no relationship between first-order alliance stability and second-order alliance size. These findings challenge the 'two alliance tactics' hypothesis and add to the evidence that Shark Bay male bottlenose dolphins engage in alliance formation that likely places considerable demands on their social cognition.

Figure 1.

Sociogram showing preferred third-order alliance associates based on permutation tests in socprog 2.4 (RHP and PD, PD and KS, CB and FCB) and other groups with between-group association coefficients in the same range (10–17) as those with significant associations (XF and BL, PB and HH, CB and PHG).

Figure 2.

Cluster diagram shows associations among 34 males in the WC (green), KS (brown), PD (blue) and RHP (purple) groups from 2001–2006. These groups were involved in six third-level alliance interactions (see text). The Average-linkage Cluster Diagram was generated in socprog 2.4 using half-weight association coefficients restricted to social, resting and travelling groups (WC n = 11–24; KS n = 24–77; PD n = 43–75; RHP n = 71–81).