Sharks have a reputation for being ruthless, solitary predators, but evidence is mounting that certain species enjoy complex social lives that include longstanding relationships and teamwork.
A new study, published in the latest Animal Behaviour, documents how one population of blacktip reef sharks is actually organized into four communities and two subcommunities. The research shows for the first time that adults of a reef-associated shark species form stable, long-term social bonds.
The image contrasts with usual reports on this species, which mistakenly sinks its sharp teeth into surfers and swimmers from time to time.
Lead author Johann Mourier told Discovery News that “other species, such as grey reef sharks and scalloped hammerheads form polarized groups where individuals have a specific place, and such species may also have complex social organization.”
Mourier, a scientist at the Center for Island Research and Environmental Study (CNRS-EPHE), and colleagues Julie Vercelloni and Serge Planes conducted the study at Moorea Island in the Society archipelago, French Polynesia. A total of seven sites were surveyed on a regular basis along just over 6 miles of the north shore of Moorea. The surveys included nearly hour-long dives at a depth close to 50 feet, with the diver photographing nearby sharks.
Analysis of the gathered data determined that the sharks were not within non-random collections, but rather had organized themselves into meaningful social groups.
“The four main communities are mixed-sex communities that use a specific home range, however, within these communities individuals tend to associate more often with others of the same sex and length,” Mourier said.
In a prior study, he determined that length is proportional to a shark’s age, with male blacktip reef sharks being mature at about the age of 7 and measuring around 3.6 feet long. Females are slightly larger than males.
Mourier suspects the sharks join together in communities for protection and to avoid aggression with each other. He and his colleagues also observed a remarkable feat, “when a group of about four or five blacktip reef sharks herded a school of fishes around a coral structure.” This suggests they can cooperate with each other to hunt as a team.
Yet another perk to organizing could be that each shark becomes a comforting landmark for others in the group. As Mourier said, “Using a home range and knowing all individuals may help individuals to have a better knowledge of their environment.”
The researchers point out that sharks’ relative brain mass-body ratios have been found to be comparable to those of mammals, indicating that they are capable of complex social behaviors on par with those demonstrated in birds and mammals.
It could just be that the highly mobile nature of sharks, combined with the difficulty of following individuals in the open sea, has kept their social interactions hidden away from human eyes until recent years.
In another study, led by Demian Chapman, researchers showed that lemon sharks at the Bimini islands, Bahamas, tended to stay near their coastal birthplace for many years.
"We were very surprised to see that many lemon sharks lingered for years around the island where they were born -- often more than half of their development to adulthood,” said Chapman, a shark scientist with the Institute for Ocean Conservation Science at Stony Brook University.
In both studies, age therefore seems to help shape a shark’s social life. Family ties may also be important to sharks, a possibility that Mourier and his colleagues are investigating now.
The scientists clipped the fins of 70 percent of the sharks involved in this latest study and are analyzing the bits for DNA.
He said, “This will soon reveal if they tend to group with relatives, as is the case in other social animals, such as for some mammals.”