1.2 Alliance formation in bottlenose dolphins (tursiops sp.)  (Page 3/7)

Marked paternity advantage for males in alliances

The prevalence of alliance formation for consortships can support the idea that individuals in alliances get to mate more, but a question remains: is it essential to be in an alliance for reproductive success? Once a primary alliance captures a female, the fertilization is not divisible (Krutzen et al. 2004). Connor et al. observed simultaneous mounting of the female by two males, but mating was difficult to attribute to a single individual because of difficulty in underwater observation (1992). In a study done in Shark Bay, Australia, Krutzen et al. found that the vast majority of offspring were sired by males in alliances (2004). Moreover, it was observed that 75% of paternities were achieved by 2 out of 15 males. Additionally, 2 of 6 observed alliances procured 62.5% of paternities, suggesting that some individuals (and alliances) are more successful than others (Krutzen et al. 2004).

Female dolphin alliances are usually related

Female dolphins in the same reproductive state find it advantageous to associate because they have similar requirements for food and defense against male alliances (Gero et al. 2005). In a study by Duffield and Wells, it was found that female alliances are usually composed of related individuals (1991).

Bottlenose dolphin females are often observed engagng in contact swimming , where one dolphin’s pectoral fin rests near the other’s dorsal fin and the pair swims in synchrony. Because of the aggressive nature of male harassment and the fact that contact swimming is increasingly found in male-biased populations, it is hypothesized that contact swimming may help females fend off males. In a survey taken of participants in contact swimming, estrous females were overrepresented and lactating females were underrepresented, supporting the idea that females support each other against males through contact swimming when they are most likely to be accosted.

Female bottlenose dolphins maintain bonds with their mothers throughout their adult life while males do not. It has been observed that the majority of contact swimming occurs between females ( [link] , p<0.001), especially in male-biased groups. Female-female swimming accounted for 83.3% of all observed events. Moreover, when juvenile contact swimming was excluded and only adult contact swimming was analyzed, 96.4% of the pairs were female-female (Connor et al. 2006).

Being in an alliance is costly

Dolphins, as well as other organisms, choose groups based on the efficient number of individuals for tasks such as hunting, foraging, consorting, and raising young. An individual will remain in a group only if group living provides a greater advantage than living alone (Brager 1994).

Alliance formation in dolphins is an effort to exploit resources more efficiently (e.g. food for females or estrous females for males) (Connor&Whitehead 2005). Connor&Whitehead observed that alliances are more readily formed in groups of higher density, perhaps because density increases the competition for scarce resources (2005). High densities of males with fewer females increase the competition for female, prompting males to form alliances to partition the limited amount of females. A high density of females increases competition for resources and alliances form for foraging (Connor&Whitehead 2005).