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Bottlenose dolphins ( [link] ) are marine mammals named for their characteristic bottle-shaped, elongated snout (Gunter 1943). There are actually two species: the common bottlenose dolphin (Tursiops truncates) and the Indo-pacific bottlenose dolphin (Tursiops aduncus). Growing to an average adult length of 2.5 m and weight of 200-300 kilograms, bottlenose dolphins are adept, quick swimmers (Gunter 1943). They navigate by using echolocation to perceive their surroundings and use varying vocalizations to communicate with each other (Connor&Smolker 1996). Using their strong tail and aquiline shape, bottlenose dolphins are famous for their aerial and swimming displays.
A first-order (primary) alliance consists of two or three males that band together to consort a single female. A second-order (secondary) alliance consists of two or three primary alliances that work together to steal females from other alliances (Connor 1992).
First-order alliances vary in stability, remaining together anywhere from several days to 20 years. Stable primary pairs and trios spend the majority of their time together, regardless of whether they are actually herding females (Gero et al. 2005). This long-term association is thought to maximize their reproductive success because males in alliances are more successful in consorting unwilling females, confronting female alliances, and defending themselves against harassment by other male alliances. In a study by Krutzen et al., it was found that primary alliances are more closely related than expected by chance, which presents inclusive fitness benefits as a possible driving force for the formation of primary alliances (2003).
Second-order alliances experience frequent reshuffling of their primary alliance components (Connor et al. 1999). A study by Krutzen et al. revealed that secondary alliances are also closely related (2003). Second-order alliances are most likely to band together at the peak of mating season, when competition for females is most fierce. [link] shows bottlenose dolphins in close proximity.
An unusually large secondary alliance, called a “superalliance”, with 14 individuals (4-6 primary alliances) was observed in Shark Bay, Australia (Connor et al. 1999). The primary alliances in the super-alliance were quite labile (Connor 2007). Alliances were analyzed by calculating an association coefficient based on the cooperation observed in consorting a female (Connor et al. 1996). Males were considered to be members of the same party if there were within 10 meters of each other. The analysis gave a number between 0 and 100 (0 meaning a pair is never together and 100 meaning a pair that is always together). No stable alliances (association coefficient>25) were observed in the super-alliance. Connor et al. theorized that switching partners may serve to keep the stability of the super-alliance because dolphins are able to make a large number of affiliative bonds (1999) However, the lability of super-alliances could also be dictated by the equivalence rule , where animals group things into classes of equivalent value and all member of a certain class are interchangeable (Connor et al. 1999). In a super-alliance, first-order alliances may be fluid because individuals view each other as interchangeable members of the same equivalence class. However, Connor et al. found that bottlenose dolphins in Shark Bay show marked preferences and avoided certain members, suggesting that these dolphins do not follow the equivalence rule.
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