Social Structure of Tursiops truncatus

Bottlenose dolphin communities are composed of several small subpopulations. A subpopulation is defined as a group of dolphins that inhabit the same coastal area (Pryor and Norris 1991). The structure of the population is often found to be characterized by the sex and age of the dolphins within the group.

From observations of bottlenose dolphins in Sarasota Bay, it was found that four main structural units of the population were present. The structural units included mother-calf pairs, subadult groups, female bands and adult males (Pryor and Norris 1991).

Mother-Calf Pairs

Calves normally stay with the mother for three to six years. Separation of the calf from the mother may be induced earlier by pregnancy (Pryor and Norris 1991).

Subadults

As soon as calves separate from their mothers, they join the subadult groups. Observations have revealed that this group type is segregated by age. Ages of subadults were estimated using body length as an indicator of age (Pryor and Norris 1991). It was found that similar-aged dolphins tended to make associations. Also, subadult groups in Sarasota Bay tended to be skewed with respect to composition of males and females. More males were found on average than females. This sex difference in numbers was attributed to differences in life history patterns between the sexes. It could be that a greater number of males than females are born, or that the survival rate is greater for males than females, however evidence is lacking in this area. It is known however, that females leave the subadult groups earlier than males. Therefore, the more frequent turnover of females from the subadult groups could contribute towards the sex composition differences.

Behaviour of the two sexes within subadult groups is also different. While the males form associations only within the subadult groups, it was observed that females would interact more frequently with adults than would males (Pryor and Norris 1991). While females begin to visit the female bands, males begin to form paired associations.

Observations of the Sarasota Bay subadult dolphin groups reveal that subadults tend to be the most active. Leaping, chasing, and socializing as well as a range of close physical contacts (rubbing, stroking, pushing, homosexual and heterosexual interactions) are performed. Unlike the human perception of dolphins being strictly passive, violent behaviour has also been observed (Pryor and Norris 1991).

Female Bands

Female bands are not static; composition may change, as female bands intermingle. However, preferences have been observed for certain female bands. This change in composition may change as bands move through other bands’ area (Pryor and Norris 1991).

Observation of female bands in the Sarasota Bay community indicate that numbers within the band are usually small. The largest Sarasota Bay female band was composed of 13 females in addition to offspring (Pryor and Norris 1991).

As with male-male associations, females tend to associate with other females of the same reproductive status (for example, females without calves swim together; females with newborn calves swim together).

Evidence indicates females from subadult groups are recruited into existing female bands, instead of forming their own band. Females leaving subadult groups most likely return to the band from which they came. The degree of relatedness of females in Sarasota Bay female bands is calculated from genetic analyses (Pryor and Norris 1991).

Adult Males

Upon reaching sexual maturity, at approximately 10 – 15 years, male pairs depart from the subadult group, and begin mating with individuals from the female bands (Pryor and Norris 1991).

Although the majority of male bottlenose dolphins have been known to form paired associations, solitary males within the Sarasota Bay community have been observed (Pryor and Norris 1991). Solitary males tend to have a small home range compared to that of paired males. Alliances are thought to be important in herding females to mate (Connor, Smolker and Richards 1992).

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