Whether in the open sea or in rivers – bull sharks inhabit a wide variety of habitats and are at the top of the food chain. The predatory fish were previously considered solitary creatures. However, a long-term study now provides evidence that the animals have a lively social life and even maintain something like friendships.
As coastal residents, bull sharks (Carcharhinus leucas) occur in tropical and subtropical waters worldwide. They can live in both salty and fresh waters and often swim far up rivers. The two-meter-long bull sharks owe their name to their stocky build, wide, blunt snout and behavior that is considered comparatively aggressive and unpredictable.
In order to study their social behavior in more detail, a research team led by Natasha D. Marosi from the University of Exeter in Great Britain has now observed the animals for a longer period of time. The study is based on six years of observations of a total of 184 bull sharks in Fiji, an island nation in the South Pacific. The sharks were divided into three age groups: immature juveniles, adults and older adults after the reproductive phase. The researchers examined how often two animals were within a body length of each other and what social interactions occurred, such as following a fellow animal and swimming parallel to each other.
Who swims with whom
The result was surprising: bull sharks have an astonishingly complex social life and prefer certain members of their species. According to the observations, the sharks did not choose their social partners at random. They preferred to interact with conspecifics of similar size. It was also noticeable that both males and females sought proximity to female animals more often. “As humans, we maintain a variety of social relationships – from casual acquaintances to our best friends. But we also actively avoid certain people – and these bull sharks behave similarly,” explains Marosi.
The analysis also showed that males were, on average, more connected and had more social contacts. One possible reason for this: Male bull sharks are usually smaller than females. “A possible advantage of their greater social integration could be that they are better protected from aggressive confrontations with larger individuals,” says Marosi.
Differences between young and old
It also showed that adult animals form the “core” of the social network, while very old and immature sharks are significantly less integrated. Overall, older bull sharks are less socially active. “These older individuals have spent many years honing their hunting and breeding skills. For them, social behavior may be less crucial for survival than for midlife animals,” explains Marosi.
Younger animals also often live in different habitats than their older counterparts – for example in coastal areas or river systems – and tend to avoid larger counterparts because they can pose a danger to them. However, some particularly courageous young animals venture close to the adults and make contact with the adult sharks. These could make it easier for them to enter the social structure – and perhaps even help them learn important behaviors.
What’s the point of friendship in the sea?
However, exactly what role these social bonds play in the animals’ everyday lives has not yet been conclusively clarified. “We are only beginning to truly understand the social lives of many shark species. Like other animals, they probably benefit from being social – for example, by learning new skills, finding food or potential mates, and avoiding conflict,” says Marosi’s colleague Darren Croft.
Marosi emphasizes the importance of better understanding the social behavior of shark species. This knowledge could help to develop better framework conditions for their protection and management. One of the study’s sponsors, the “Fiji Shark Lab”, is currently working with the Fiji Ministry of Fisheries to incorporate the study’s findings into joint conservation measures.
Source: University of Exeter; Specialist article: Animal Behavior, doi: 10.1016/j.anbehav.2026.123511