Sharks too may be social animals: study

Sharks, which are historically seen as solitary animals, may have complex social networks that are typically seen in mammals but rarely observed in fish, a new study has found.

Using tracking devices to trace the movements of individual animals in the open ocean, researchers found that Sand Tiger sharks form more complex social structure than previously thought.

"Higher-order decision-making processes are often associated with mammals, or species that we think of as really smart – dolphins, elephants, or chimpanzees," said Danielle Haulsee, a PhD candidate at the University of Delaware in UK.

Sand Tiger sharks, top predators that live in coastal waters off the Eastern US, have experienced drastic population declines over the past several decades.

They are important regulators of marine food webs but have been historically understudied, Haulsee said.

Researchers used acoustic tags to track the movements of over 300 individual Sand Tiger sharks and record shark-shark interactions over the course of a year.

Previous studies have looked at shark interactions in laboratories or species contained in pens, but this the first study to record interactions for almost a year in free-swimming sharks, Haulsee said.

Initial data from two individual sharks show they encountered nearly 200 other Sand Tigers throughout the year, as well as several individuals from other shark species.

The sharks show fission-fusion social behaviour, meaning that the number of sharks in a group and the individuals that are part of the group change by location and time of year.
Researchers found that groups of Sand Tigers stay together for certain times of the year and fall apart during other times. They also found that Sand Tigers re-encounter the same sharks throughout the year.

One surprise was a sudden lack of encounters with other Sand Tigers in the late winter and early spring, Haulsee said.

Up until that point, both Sand Tigers were encountering other sharks regularly, but in the late winter, both seemed to enter a dispersal phase where they encountered very few other sharks.

This could be related to other aspects of the sharks' lives, such as mating and searching for food, which suggests that they could be performing a kind of social cost-benefit analysis, researchers said.

"If you're living with a group, there could be some kind of protection or information sharing that comes with being in that group," Haulsee said.

"But if there's a lot of competition for food resources or mating resources, then it's not beneficial anymore to be in a group, and you might swim away from your group and go off on your own," she said.
The researchers hope to use their results to answer questions about whether Sand Tigers form family groups or whether sharks of similar size and sex form distinct groupings.