Yannis Papastamatiou, International University of Florida
Sitting anchored to the rocky reef 70 feet (21 meters) below the ocean’s surface, hundreds of scalloped hammerheads swam above me at once, moving as one. When most people think of sharks, they don’t think of them as social creatures. The schooling hammerhead sharks overhead were a striking example of shark social groups, a subject that has not been much researched.
I am a marine biologist studying predator behavior. For the past 22 years, my research has focused on sharks.
Biologists have long known that some sharks — such as hammerhead sharks — are social creatures, but whether great white sharks interact while hunting, and if so, how, is still a mystery. Since 2014, my colleagues and I have been visiting the beautiful island of Guadalupe off the coast of Mexico to find out. Using state-of-the-art technology, we have been able to gain a better understanding of the secretive social lives of these apex predators.
What makes a social animal?
A social animal is an animal that interacts and spends time with other individuals of the same species. While almost all animals exhibit some degree of sociality — when they mate, for example — social behavior can range from solitary snow leopards to highly sociable ant colonies.
When people think of social predators, they most likely imagine a pack of wolves hunting in an organized, cooperative group. But social behavior can be much simpler than that. An animal may simply decide to stay close to another individual because it has learned that if its “colleague” finds prey, its own chances of getting a meal increase.
Sharing information – the location of the prey – in this example is unintentional. The first predator did not purposely warn the second predator of the presence of a meal. But under the right circumstances, this kind of basic social interaction can increase the hunting success of both animals.
Hints of a social shark
White sharks travel to seal colonies during the seal breeding seasons in the summer and fall. Sharks generally hunt by patrolling the waters adjacent to seal colonies and ambush seals on the surface.
In 2001, researchers in California published a paper describing how white sharks patrolling a seal colony on Año Nuevo Island would stay within “eavesdropping distance” of each other. The biologists suggested that if a shark killed a marine mammal, other sharks nearby would record this information and quickly approach the scene of the kill, perhaps hoping to eat the remains of the prey. While the sharks may not work together, they may still benefit from interacting with each other.
Further research into the behavior of white sharks in Australia went a step further. Researchers found that great white sharks often turn up over and over at cage dive sites with the same individuals. The fact that white sharks not only stay close together, but also have preferred mates made me wonder if these animals were perhaps more sociable than people thought.
How to tag a great white shark
The island of Guadalupe is located about 150 miles west of the Baja Peninsula in Mexico. Each fall, at least 100 white sharks arrive on the island to feed on Guadalupe fur seals, California sea lions, squid, and tuna. In 2014, I contacted my friend and colleague, Mauricio Hoyos-Padilla, who has been tagging white sharks in Guadalupe for over 15 years, to see if he was interested in studying the social behavior of white sharks.
To do this, we have developed a new electronic tag that we call the ‘social tag’. It has sensors that can detect simple acoustic transmitters that we have attached to other sharks so that we can see which sharks hang out with each other and for how long. The social tags also include a video camera and motion sensors that can track how fast the shark is swimming and how deep it is.
Starting in 2017, I went to Guadalupe for about a week every year to try and tag sharks with Mauricio and his students from the nonprofit research organization Pelagios-Kakunja. Sometimes we tagged sharks from the safe cages, but more often we went freediving with them. We used bait to lure a shark to the boat and when one appeared, three or four taggers would jump into the crystal clear waters. We would then wait for one of these large sharks to become curious and swim within a few feet of us. When that happened, we used a long stick to clamp the tag onto the shark’s dorsal fin.
After three years of successful tagging, we deployed our social tags on three male and three female sharks and tagged an additional 37 individuals with acoustic transmitters. The tags would last for one to five days before falling off and floating to the surface for the team to recover. In total, we collected more than 312 hours of data from the six social tags.
When sharks hang out
During the many hours of data we collected, the sharks often came within 100 feet of other individuals. Many of these encounters were brief and seemingly random — a bit like crossing paths with someone in a grocery store — but a few lasted longer and seemed to be genuine social interactions. We recorded five cases of these longer interactions, one of which lasted more than an hour.
We also found that individual sharks behaved very differently from each other. Two of the tagged sharks were particularly sociable and associated with 12 and 16 other individuals, while two others seemed much less sociable, crossing paths only with only four and six other sharks, respectively. The tags used on the last two sharks didn’t have working sensors, so we couldn’t measure interactions.
Another interesting behavioral difference was that some sharks hunted in shallow waters and others hundreds of meters deep.
Our new evidence suggests that white sharks are indeed social animals. As previous research suggested, our results align with the idea that the benefit of great white shark sociality is that they can “eavesdrop” on other sharks. They can quickly acquire information, such as a seal killed at depth by another shark, and this can lead to an easy meal. However, there is still so much more to learn.
Measuring sociality over months and over a year, as opposed to just days, would yield much deeper insights. When the sharks leave the island of Guadalupe in the spring, they travel long distances across the open ocean — some even swim as far as Hawaii. Do they travel together or alone?
The social life of white sharks has been a secret hidden from researchers for decades. It took new technology and new research methods to see it.
Yannis Papastamatiou, professor of biological sciences, International University of Florida
This article is republished from The Conversation under a Creative Commons license. Read the original article.