Author Carson McCullers famously wrote about the heart as "a lonely hunter," but her description seems to apply just as well to a shark. They cruise the ocean on their own and generally have little contact with other sharks — or do they? While these apex predators were typically thought to lead mostly solitary lives, a new study finds that sand tiger sharks may be a lot more social than scientists had suspected.
Sand tiger sharks (Carcharias taurus) go by a number of common names, including grey nurse shark, spotted ragged-tooth shark, slender-tooth shark and ground shark. They swim in coastal waters in the western and eastern Atlantic, the Mediterranean, and near Australia and Japan, and measure about 10 feet (3 meters) long.
During the summer months, sand tigers that inhabit the waters off the coast of the eastern United States migrate to Delaware Bay, where they are in close contact with one another. However, scientists were uncertain about whether the sharks continued to interact socially when they dispersed back to the open ocean. [See Photos of a Sand Tiger Shark Nursery]
Previously, researchers had explored shark interactions in controlled environments like pens or laboratories, but this was the first study to investigate social behavior in sharks swimming in the open ocean. For the study, scientists attached acoustic tags to more than 300 sand tiger sharks, tracking their movements and recording interactions among the sharks for nearly a year.
The scientists conducted initial data analysis from two individual animals, and found that the sharks enjoyed an active social life year-round. They registered almost 200 encounters with other sand tiger sharks, and interacted repeatedly with the same individuals. The sharks also formed groups that varied in size depending on their location and the time of year.
And during late winter and early spring, the sharks took a break from their socializing and hardly encountered any other sharks, the scientists discovered. Danielle Haulsee, one of the researchers and a doctoral candidate in oceanography at the University of Delaware in Lewes, suggested in a statement that sharks might self-regulate their time in a group, depending on individual needs for certain activities that are best done alone, like finding food or mating.
Discovering that sharks are capable of making decisions associated with social networking casts these former "loners" in a new light, Haulsee said. "Our research shows that it is important for the scientific community to not rule out these types of behaviors in nonmammalian species," she said.
The findings were presented Feb. 22 at the 2016 Ocean Sciences Meeting, and the abstract was published online.