Analysis of the social behaviour in regard to gender in Tursiops truncatus (eastern Mediterranean Sea)

The bottlenose dolphin Tursiops truncatus (Montagu, 1821) is a cosmopolitan species; it can be found in tropical and temperate waters worldwide. Its accessibility in nearshore waters has made it one of the best-studied cetaceans. In the eastern Mediterranean Sea bottlenose dolphins have been studied since 2001 when the project “Delfini Metropolitani”, developed by Acquario di Genova, started; the study area runs from Genoa to La Spezia. The bottlenose dolphin population inhabiting the study area is exposed to potential impact from human activities; this makes important to gather information on its ecology, social structure, abundance and demography. The aims of the present study are to: - assess the distribution; - estimate the abundance using photographic mark-recapture technique; - analyse the association pattern (in relation to gender); - calculate the calving interval (the time elapsed between subsequent births). Surveys were conducted using two inflatable boats; survey tracks and sighting waypoints were recorded by GPS. Photographic data were collected during the sightings and analysed using the photo-identification technique. Data collected during this research provide that bottlenose dolphin distribution is confined to the continental shelf (concentrated within the 100 m isobaths) and that the highest amount of animals is located on the east side of the study area, as found by other researchers. According to our abundance estimates the population remained steady throughout the years. We discovered that calves significantly influence both group size and composition: groups are bigger when calves are present; larger groups contain a higher percentage of females while the percentage of males is lower. Analysing the social structure, we discovered that it’s characterized by: a good level of cohesion, weak and temporary bonds; long-term associations occur between members of the same sex; females live in bigger groups with other females and calves. These results are consistent with other studies. Data collected during this research provide information available for comparisons of social ecology and behaviour between this population and others, living in the Pelagos Sanctuary as well as around the world. Such information might help in devising management protocols for this cetacean species contributing to its protection