Bull sharks form social relationships with specific "friends", new research reveals.
Sharks are often viewed as solitary, but the study – carried out on the Shark Reef Marine Reserve in Fiji – found that rather than mixing at random, sharks have "active social preferences" and choose their social partners.
The research was carried out by the University of Exeter, University of Lancaster, Fiji Shark Lab , and Beqa Adventure Divers.
"As humans we cultivate a range of social relationships – from casual acquaintances to our best friends, but we also actively avoid certain people – and these bull sharks are doing similar things," said lead author Natasha D. Marosi, an Exeter researcher and founder of Fiji Shark Lab.
The study is based on six years of observations of 184 bull sharks in three age categories: sub-adult (not yet sexually mature), adult and advanced-adult (post-reproductive age).
Researchers examined both broad-scale "associations" – measured by individuals remaining within one body length of each other – and fine-scale social interactions such as "lead-follow" and parallel swimming.
Social ties were common between adult sharks, and sharks were most likely to interact with partners of a similar size to themselves.
"Contrary to commonly held perceptions of sharks, our study shows they have relatively rich and complex social lives," said Professor Darren Croft, from Exeter's Centre for Research in Animal Behaviour .
"We are only just beginning to really understand the social lives of many shark species. Just like other animals, they likely gain benefits from being social – this may include learning new skills, finding food and potential mates while avoiding confrontations."
The study found that both sexes preferred to socialise with females. However, males had more social connections on average than females.
"Male bull sharks are physically smaller than females, thus one potential benefit they may gain is by being more socially integrated; they are buffered from aggressive confrontations with larger individuals," said Marosi.
It was further found that adult sharks form the "core" of the social network, while the advanced adult and sub-adult sharks were generally less socially connected.
"This study capitalises on data and knowledge from one of the longest running shark ecotourism dive sites in the world. This offered a unique opportunity to observe the detailed behaviour of these individuals over many years, as they grow, develop and manage their social relationships," said Dr David Jacoby, from Lancaster University's Lancaster Environment Centre.
Marosi added: "The Shark Reef Marine Reserve is a protected area where large numbers of sharks gather year round, giving us the ability to study individual sharks repeatedly over time.
"Our results show that older sharks tend to be less social.
"These older individuals have many years of experience honing their skill sets, hunting and mating, and sociality may not be as integral to their survival as it is for an individual in their prime.
"Sub-adult bull sharks rarely visit the Reserve. Sub-adults usually occupy near-shore habitats, while juvenile bull sharks can be found in Fiji's river and estuarine systems.
"During these early life stages, there is a need to avoid predation – including the threat posed by adult bull sharks.
"We do have some bolder sub-adults at the Reserve, and they have established social ties with some of the adult sharks. These older individuals may act as facilitators for inclusion within the social network, and also possibly provide pathways for social learning."
Marosi stressed the importance of developing a deeper understanding of sociality within shark species, which she believes can help inform policy frameworks for their management and preservation. Fiji Shark Lab is currently working alongside the Ministry of Fisheries, Fiji to use the study's valuable information in joint conservation efforts.
The study was funded by Fiji Shark Lab, Hai Stiftung Shark Foundation and the Waitt Foundation.
The paper, published in the journal Animal Behaviour, is entitled: "Rolling in the deep: drivers of social preferences and social interactions within a bull shark aggregation in Fiji."
