Off the coast of Fiji, a team of researchers spent six years watching bull sharks swim circles around each other. What they found challenges everything people thought they knew about these fearsome predators.
Bull sharks — large predators that can grow up to 11 feet long and are known for swimming far inland up rivers — actually prefer hanging out with their relatives. Scientists from the University of Exeter, Fiji Shark Lab and Lancaster University made this discovery after watching sharks at the Shark Reef Marine Reserve. They published their findings in the journal Ecology and Evolution.
Lead researcher Natasha D. Marosi, who founded the Fiji Shark Lab, had already shown in earlier work that bull sharks are not solitary animals. They actively choose their friends based on factors like sex, age and size. Now, her team has discovered another layer to shark social life.
They call it the "kinfluence" effect — the pull of family ties on who sharks choose to spend time with. While most of the sharks studied were not closely related, individuals still showed a slight preference for swimming near relatives rather than strangers.
The researchers measured two types of social behavior. The first was simple proximity — sharks staying within one body length of each other. The second was active, intentional interactions: swimming in parallel lines, following another shark as a leader, or joining another shark to swim in sync. Family connections showed up in both types of socializing, especially between pairs of male sharks.
"To discover that some of these large, apex predators choose to socialize with their relatives is truly exciting," Marosi said. "It's something never found before in an adult population of sharks."
Dr. Andrew Griffiths from the University of Exeter said the results are surprising because sharks still carry a reputation as cold, solitary hunters. "This paints these animals in a completely new light," he said.
Scientists still do not fully understand how sharks recognize their relatives. Marosi suggests that young bull sharks may learn to identify family members during their early years in rivers and estuaries. Half siblings and cousins likely spend time together as juveniles. Sharks may carry that recognition into adulthood, even after traveling vast distances.
The researchers believe family ties might play an even bigger role in bull shark society than the study shows. The problem is that few young sharks survive to adulthood. Illegal fishing in rivers and coastal areas kills many juveniles before they can grow up and form their own social circles.
The study adds bull sharks to a short list of animals known to prefer relatives as social partners — a behavior more commonly studied in birds, monkeys and social insects. That such a massive, powerful predator operates this way suggests that family bonds may be more widespread in the animal kingdom than scientists ever imagined.