Though many species of shark have long been considered solitary creatures, new research suggests that their social networks may in fact be surprisingly sophisticated. After tracking the movements of sand tiger sharks living along the eastern coast of the U.S., scientists have found that their interactions with one another display a level of complexity normally associated with mammals and rarely seen in fish.
Presenting the research at the Ocean Sciences Meeting, Danielle Haulsee explained how more than 300 sand tiger sharks had been fitted with tracking devices since 2007, enabling her and her team to follow their movements and detect which other animals they encountered. After recapturing two of these sharks and analyzing the data recorded by their trackers, the researchers discovered that they interacted with around 200 other sharks during the course of a year, and even identified a number of “best friends.” This status was afforded to those sharks that met the animals in question more than 20 times over a 12-month period.
While this finding is quite surprising in itself, the researchers were even more intrigued by the seasonal variation of the sharks’ social behavior, which seemed to peak in the summer before dropping off in late winter and early spring. During this time, the creatures barely interacted with any other members of their species, which Haulsee says is evidence of fission-fusion social behavior.
This type of social organization has previously been reported in several species of mammal, and involves large numbers of individuals forming fluid groups. These fluctuate in membership throughout the year, depending on the seasonal availability of resources as well as mating needs, although at this stage it's difficult to pinpoint what is causing these patterns in the sand tiger sharks.
Until now, little had been known about the social behavior of sand tiger sharks along the east coast of the U.S. outside of the summer months, when they gather in large numbers around Delaware Bay. Typically, they stick relatively close to the shore and prowl the seabed in search of prey, giving rise to their name.
Researchers mapped the social networks of two sand tiger sharks. American Geophysical Union
Despite being apex predators – meaning they themselves have no natural predators – sand tiger sharks are a threatened species, largely due to the fact that they have extremely low reproduction rates. Because of this, populations are highly vulnerable to even the smallest environmental pressures, which is why scientists now want to learn more about their behavior in order to better understand how to protect them.
Accordingly, Haulsee hopes to see her work contribute to the implementation of more effective conservation efforts. For instance, she explained in a statement that “if we know there are certain times and places where breeding females, or even more importantly the pregnant females, are aggregated together, we can devote resources into those areas to protect those sharks.”
