Group size influencing factors and social structure

of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the Northern Egyptian Red Sea

As part of his Master’s thesis in animal behaviour at the University of Zurich, and in collaboration with Dolphin Watch Alliance, Marc Frei collected research data on Indo-Pacific bottlenose dolphins in the Egyptian Red Sea in February/March and August/September 2020. He investigated what factors influence group size. Behavioural data was collected from the boat, as well as whilst diving and snorkelling. He was able to demonstrate that behaviour influences not only the size of the groups, but also that of the smaller subgroups. For example, it was found that the dolphins can temporarily split into small subgroups whilst playing, whereas they usually gather in large numbers and swim closely together when they are sleeping.

When calves are part of a group, the group size increases almost linearly with the number of calves. Further analysis would be needed to determine whether the increase in group size when calves are present is solely attributable to the number of sexually mature females, or whether it could be explained, for example, by babysitting or increased protection from predators due to the greater number of individuals. As larger groups are more likely to contain sleeping individuals and to include calves, this is also relevant to dolphin tourism and conservation. Calves are the most vulnerable individuals, also because they are lactated almost exclusively whilst sleeping. If they are constantly disturbed while doing so, they become weakened, which can have a negative impact on the health of the population. In spring and summer, during mating peak season, where more individuals gather at the same time, the groups are larger than in autumn and winter.

As a second aspect, he examined the complex social structure of the Indo-Pacific bottlenose dolphins. Photo-identification data on the animals (primarily images of the dorsal fin, but also of the pectoral fins and fluke) were collected and used to create social networks (social network analysis). He found that sex and degree of kinship influence the strength of the dolphins’ social bonds. Thanks to the stable alliances formed by adult males, male–male bonds were the strongest, whilst male–female bonds were the most unstable. It was observed that adult daughters frequently remain with their mothers even after weaning and presumably also act as babysitters for younger (half-)siblings. In conclusion, whilst females do form more flexible bonds with other females than the alliances formed by males, friendships and preferred associations do exist here too.

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