Sharks get by with a little help from their friends

Gray reef sharks in the Pacific Ocean form their own social networks, with some bonds lasting years.

Yannis Papastamatiou — an FIU marine scientist in the Institute of Environment — and a team of researchers from UC Santa Barbara, University of Hawaii and University of Exeter set out to investigate the social behavior of reef sharks around the Palmyra Atoll, using both acoustic tracking tags and animal-borne cameras. They discovered the sharks are living as groups and actually form stable social bonds. Of the 41 sharks tracked, some pairs even stayed together for the entire length of the four-year study.

Papastamatiou teamed up with David Jacoby from international conservation charity ZSL (Zoological Society of London) to make sense of the tracking data. Jacoby and his colleagues then developed a new analytical model to identify social patterns. The result is a color-coded web called a “social network.” It clearly identified a very tight, tangled web of sharks spending their mornings together as large groups, dispersing throughout the day and into the night only to reconvene with the same group members later on.

Although these gray reef sharks have an entire atoll to use, the data shows certain groups spent most their time in smaller regions, with one group only using the southwest side and another only the north. As Papastamatiou points out, there’s no known reason the sharks would have to return to the same exact place. After all, these sharks never stop swimming so there’s no time to rest. They could easily move throughout the atoll, but that might make it difficult to stay together.

“Using one part of the reef helps you find your friends, so to speak,” Papastamatiou said. “It can be hard to maintain social bonds when you live in the ocean, but if sharks all routinely return to the same spot on the reef then that will help them maintain their group structure. 

Proving these relationships exist wasn’t easy because the social structures of sharks don’t follow the rules people are accustomed to with animals. The sharks don’t vocalize or call to one another. They aren’t affectionate. They don’t care for or raise their young. But sharks do socialize in their own, cryptic ways.

The tracking receivers detected when tagged sharks would come and go and log the dates and times. Through Jacoby’s analysis, the research team was able to identify which sharks were together during those daily movements and whether individuals were associating more than expected from random arrival and departures. The animal-borne cameras provided an intimate, sharks-eye view confirming how the sizes of the groups fluctuated throughout the day.

“For some time now, we have known that sharks are capable of having distinct social preferences for other group mates,” Jacoby said. “We had no idea, though, that these social bonds could last for multiple years or that in the absence of reproduction or parental care that such communities might function as areas to exchange information.”

In some ways, the social groups of sharks resemble those of seabirds that leave their colony to forage for food. However, there are some distinct differences. For one, bird communities tend to be seasonal, but these sharks stayed together year-round. Also, sharks don’t nest. But Papastamatiou believes being in a group makes it easier for the sharks to share information, and specifically, where to find food. If a shark notices a fish and goes after it, another shark could see that shark take off and decide to follow it in case it’s chasing food. Another shark would likely see that shark and follow it, ultimately sparking an inadvertent chain reaction of sharks on the hunt.

This rare insight into the social behavior of sharks has left the scientists with many more questions they plan to explore. Papastamatiou says animals that live in groups can be more vulnerable to other predators as well as to over-fishing. Knowing some sharks thrive in social groups means that any major losses to their community could have devastating consequences for those that remain, especially when they could possibly rely on their social network for finding food, as this study suggests. That is why Papastamatiou and the team are continuing his research into the cryptic behaviors of the ocean’s great predators. 

The latest findings were recently published in Proceedings of the Royal Society.

FIU has been ranked No. 9 in the world for positive impact on life below water by The Times Higher Education Impact Rankings. The university ranked third in the United States and is the only institution in the state of Florida to make the list.