Sharks may not be the lone hunters we've always imagined them to be. New research on sand tiger sharks, presented at this week’s Ocean Sciences Meeting in New Orleans, finds that the animals actually have complex social interactions. The interactions are similar to those seen in marine mammals like dolphins and whales.

A group of Delaware-based researchers have tagged more than 300 sand tiger sharks since 2007, tracking the marine animals' whereabouts and recording how often they bump into fellow sharks. They also recaptured two sharks and downloaded the data from their mobile transmitters. These two sharks alone had met more than 350 individuals—mostly other tiger sand sharks, but also different shark species and some fish.  

Overall, they found that sharks hang out in groups that change throughout the year. This “fission-fusion social behavior” is a complex form of social organization, previously observed in mammals like elephants [PDF], chimpanzees, and dolphins.

Depending on the time of the year and the location, sand tiger sharks live in groups that can morph into larger or smaller bands. In the late winter and early spring, the sharks tended to go off on their own, rejoining other sharks later in the year. This could be because they’re getting away from other sharks in order to mate and find food, but they find it advantageous to stick together later in the year.

But they’re not just taking up with random strangers. The sand tiger sharks ran into the same individuals over and over again throughout the year, some more than 20 times. However, the researchers are still not sure if these sharks are forming family groups or if they band together based on size or sex or something else entirely. They’ll need to keep stalking sharks to find out.