A new study of 41 grey reef sharks shows that they spend their days together on coral reefs, and then swim out to the open ocean at night to hunt, Christopher Intagliata and Apoorva Mittal report for NPR.
The study, published on August 12 in the Proceedings of the Royal Society B, gives new insight into shark social dynamics. Far from the isolated predators of popular imagination, tracking tags and sharks-eye view footage recorded by cameras strapped to the sharks’ fins shows that sharks tend to spend their time with the same group of peers every day. This grouping suggests that the animals can not only recognize each other, but also choose to keep in touch.
“They are not friends in the sense of having any emotional bond with each other,” says Yannis Papastamatiou, director of the Predator Ecology and Conservation Lab at Florida International University, to New Scientist’s Michael Le Page. Instead, the researchers prefer to say the sharks who group together are “associates.”
To track the sharks’ daily routines, the researchers caught 41 sharks near Palmyra Atoll, an island in the central Pacific surrounded by coral reefs. Each shark was outfitted with a small location transmitter, which sent an identifiable signal if a shark approached one of the 65 receivers scattered strategically around the reef.
The researchers also attached cameras to two of the sharks. The study tracked the sharks for four years, about the battery life of the transmitters.
The team found that some of the sharks maintained a stable social group for the entire four-year duration of the study. The groups ranged from just a couple sharks to larger crews of 20. (A group of sharks is called a shiver, by the way.) Each night, the sharks swam into the open ocean to hunt, away from the receivers that tracked their locations. Then, the sharks returned each morning, and the size of their group grew as they congergated at the reef until they dispersed again the next night.
The researchers suspect that by spending their time in groups, the sharks are improving their chances of a successful hunt.
The sharks strategy might look something like this: "If we hang out together and I see something, then you can come and try and take advantage of that," Papastamatiou tells NPR. "And alternatively, if you see something, then I can try and take advantage of that."
The sharks didn’t show signs of actively cooperating. But if one shark misses its target, another shark could have a better chance of eating if it followed the first shark to its prey. Even if a shark loses its meal to one of its associates, it would be able to follow along and get another chance at dinner later in the night.
Grey reef sharks aren’t the first shark shown to have a social side. Researchers previously found that Port Jackson sharks, a relatively small species found near Australia, also spend their times in groups, Laura Parker reported for the New York Times in 2016. In that case, the team worked with the Taronga Zoo to show that the sharks were grouping together on purpose, and not just because they enjoyed the scenery.
But other species, like the hammerhead shark, prefer to be alone, per New Scientist.
“Not all sharks are social and some are likely solitary.” Papastamatiou tells Reuters’ Will Dunham. But new research techniques are allowing people to study shark social behavior more closely than before. “I like to talk about their ‘secret social lives’ not because they want it to be a secret, but because only recently have we developed the tools to start seeing and understanding their social lives.”