A social network of dolphins

Doubtful Sound is a very large and naturally imposing fiord in Fiordland, in the far south west of New Zealand. Doubtful Sound was named ‘Doubtful Harbour’ in 1770 by Captain Cook, who did not enter the inlet as he was uncertain whether it was navigable under sail. It was later renamed Doubtful Sound by whalers and sealers. The fiord is home to one of the southernmost population of bottlenose dolphins, the most common and well-known members of the family Delphinidae, the family of oceanic dolphins. The Doubtful Sound bottlenoses have formed a very insular sub-group of only about 70 individuals, with none having been observed to leave or enter the Sound during a multi-year monitoring regime. Their social grouping is thus extremely close, which is also partly attributed to the difficult and unusual features of their habitat, which is much colder than for other bottlenose groups and is also overlaid by the freshwater layer.

David Lusseau, a researcher at the University of Aberdeen, observed the group of dolphins of Doubtful Sound. Every time a school of dolphins was encountered in the fjord between 1995 and 2001, each adult member of the school was photographed and identified from natural markings on the dorsal fin. This information was utilised to determine how often two individuals were seen together. Lusseau defined social acquaintances in the network as preferred companionships, that is individuals that were seen together more often than expected by chance. He then built a social network with 62 dolphins and 159 undirected ties representing preferred companionships. In the dolphin network, the sex of dolphins was known for all but four individuals from direct observations of genitalia using an underwater camera.

The following are some visualizations and statistics of the dolphin social network that I made using R and igraph: