Complex Social Structure of an Endangered Population of Bottlenose Dolphins (Tursiops truncatus) in the Aeolian Archipelago (Italy)

Abstract

We investigated social structure and association patterns for a small population of Mediterranean bottlenose dolphins, Tursiops truncatus, inhabiting the Aeolian Archipelago (southern Italy). Specifically we evaluate the role of sex and age composition, residency patterns and interaction with trammel nets on this social organization. Association data for 23 regularly sighted individuals were obtained from summer photoidentification surveys collected from 2005–2012. Using a combined cluster and social network analysis approach, we found associations between dolphins were hierarchically structured, where two mixed-sex social units were subdivided into smaller temporarily dynamic groups. We found non-random and long-term preferred associations in the population; however, the degree of social cohesion, residence pattern and interaction with trammel nets differed considerably between the two social units. Six of eight females occurred in the more resident social unit-1; in addition, social unit-1 individuals had significantly stronger associations, higher preferred associates, lived in larger groups and occurred less frequently with trammel nets. Nine of eleven males were clustered in social unit-2 and five of these males, interacting with trammel nets, formed small groups and preferred associations. We propose that female and male groups associate in the study area during the breeding season and that some males choose to interact with reproductive females forming a distinct but interrelated social unit. Other males may be associating in a larger fission-fusion network, which consists of dolphins that appear to temporarily join the network from the coastal population. We cannot exclude that some males specialized in trammel net foraging, suggesting that this foraging technique may favor a solitary lifestyle. Large group sizes and high degree of social cohesion for females could be an indication of greater protection and more efficiency in detecting, deterring or repelling anthropogenic pressures. Most likely dolphins' social organization depends on a combination of socio-ecological, demographic and anthropogenic factors.

Bottlenose dolphin (Tursiops truncatus) social structure in the Shannon Estuary, Ireland, is distinguished by age‐ and area‐related associations

“…The structure of many resident populations has been described including in Sarasota Bay, Florida (Wells ), Shark Bay, Australia (Connor et al . ), the St. Johns River, Florida (Ermak ), the Aeolian Archipelago, Italy (Blasi and Boitani ), Laguna, Brazil (Daura‐Jorge et al . ), Port Stephens, Australia (Wiszniewski et al .…”

Influence of seasonality on cetacean diversity, abundance, distribution and habitat use in the western Mediterranean Sea: Implications for conservation

“…During recent decades, an increasing amount of research has enhanced knowledge about cetacean populations living in the Mediterranean basin, although most research focused only on the summer season and on the most common species. The spatial coverage of most studies was on the highly productive area of the Pelagos Sanctuary, a large international Special Protected Area of Mediterranean Importance (SPAMI), (UNEP‐MPA‐RAC/SPA, 2010), in the north‐western Mediterranean (Azzellino, Gaspari, Airoldi, & Nani, ; Cominelli, Moulins, Rosso, & Tepsich, ; Fiori, Giancardo, Aïssi, Alessi, & Vassallo, ; Gnone et al, ; Notarbartolo di Sciara, Agardy, Hyrenbach, Scovazzi, & Van Klaveren, ; Panigada et al ., ; Tepsich, Rosso, Halpin, & Moulins, ), in the southern Mediterranean latitudes, especially on potential fin whale wintering areas in the Sicilian channel and Messina Strait (Canese et al ., ; Celona & Comparetto, ; Sciacca et al ., ), on specific cetacean hot spot areas, such as the Cuma Canyon (Mussi, Miragliuolo, De Pippo, Gambi, & Chiota, ) and the Alboran Sea (Cañadas, Sagarminaga, De Stephanis, Urquiola, & Hammond, ), or on more coastal areas focusing on bottlenose dolphins (Blasi & Boitani, ; Diaz López, ; Gnone et al ., ). A few large‐ scale surveys have also provided information on wider portions of the western Mediterranean basin (Aissi et al ., 2008; Cotté, Guinet, Taupier‐Letage, Mate, & Petiau, ; Gannier, ; Panigada et al ., ), but these occurred over a short time frame and predominantly focused on only one species.…”

Establishing the reliability of rhesus macaque social network assessment from video observations

“…Environmental factors can alter the roles of individual mice within a group, which in turn alter the social structure (Shemesh et al, 2013). Dolphin social organization depends on environmental and demographic factors (Blasi & Boitani, 2014). Collective behaviour of schooling fish both influences and is influenced by individuals’ decisions (Miller, Garnier, Hartnett, & Couzin, 2013; Sumpter, Krause, James, Couzin, & Ward, 2008).…”