Bottlenose dolphins are very social animals who live in complicated social systems known as fission-fusion societies. Much like humans and chimpanzees, who also live in fission-fusion societies, all individuals in the dolphin community do not stay together; the number of individuals in groups changes over time; and each animal has certain individuals he or she prefers to associate with. The complexity of fission-fusion systems lies in the fact that information about who is associating with whom is not always available. Since relearning social standing every time dolphins encounter each other would presumably waste time and energy, dolphins would benefit from remembering individuals and interactions between individuals not seen on a regular basis. The fact that dolphins live in the water where it is relatively easy to move from place to place means that individuals can interact with larger numbers of other dolphins on a less regular basis than animals that live on land. The average group size in Shark Bay is 4 – 5 dolphins, however this number may depend on the individuals present in the group and what they are doing at the time (Smolker et al. 1992).
Overall, adult male and female dolphins lead fairly different social lives. Male bottlenose dolphins in Shark Bay form hierarchical alliances that cooperate to obtain and sequester females for mating (see section on male alliances). This nested nature of male bottlenose dolphin alliances is more complex than any non-human mammal (Connor et al. 1992). While adult male Shark Bay dolphins are almost always with their alliance partners, female bottlenose dolphins do not form alliances and vary widely in their degree of sociality. These females form loose social networks with their number of associates ranging anywhere from 0-139 (Smolker et al. 1992; Gibson & Mann 2008b). While females do appear to have certain individuals they prefer more than others, they typically spend less than 30% of their time with these top associates (Smolker et al. 1992). In a recent examination of the possible reason for female bottlenose dolphin groups in Shark Bay, researchers found that mothers with calves in their first year of life tend to form larger groups, suggesting that the groups provide these vulnerable calves with some form of protection. The researchers also found evidence that female grouping allows calves, particularly males, to develop social skills before a lack of social savvy negatively influences reproductive opportunities (Gibson & Mann 2008a). Bottlenose dolphins show bisexual philopatry, meaning that as adults, both sexes stay in the general area of their birth. Therefore, social relationships have the potential to form early in life and last into adulthood, however the first in-depth investigation into bottlenose dolphin calf social development was only recently published (Gibson & Mann 2008a). Not surprisingly, calves increased the time spent separated from their mothers as they approached weaning. Additionally, male calves increased the time they spent in groups during separations from their mothers, while female calves decreased their time spent in groups. Researchers also found that the number of associates a mother had was reflected in the number of associates her calf had (Gibson & Mann 2008a).
Few species inhabit social systems as complex as the fission-fusion system of bottlenose dolphins and it is presumed that this complexity persists because it increases survival and reproduction. Research in Shark Bay is addressing this assumption by investigating bottlenose dolphin sociality using a method known as social network analysis.
A social network is defined as a set of actors linked by relationships. Facebook is the most common example of a social network in which people are linked by bonds of “facebook friendship.” Social networks provide more realistic representations of dolphin society and allow us to look at multiple levels of the society by making available information on individuals, subgroups, and the network as a whole. We can look at how connected a dolphin is to others in the network and how tightly groups of dolphins are connected to each other. These different social network measurements offer different information about the same entity and provide us with data not accessible using traditional methods. We are currently using social network analysis to investigate topics including calf social development, potential fitness consequences of sociality, and the transmission of foraging strategies.
