The secret social lives of sharks

Far from being solitary hunters, some shark species are social. In fact, some sharks like to ‘hang out’ with the same group of animals for years in a relatively small location. Adam Murphy had a chat with Yannis Papastamatiou to find out what happens when sharks socialise, and if they actually make friends.

Adam - For some, the stereotypical image of a shark is of a lone predator sneaking its way through the ocean with no company, not at all like a pack of wolves. But science is starting to challenge that assumption.

Yannis - At least for some species we have evidence that they do like other individuals would like to spend time with other individuals, and how stable that is can vary. It may be that they just like to hang out with a certain individual for, let's say, one evening. And then the next evening they'll be hanging out with somebody else. But in some cases we have found some sharks which seem to hang out with each other, not continuously on and off, for up to four years.

Adam - That's Yannis Papastamatiou, who's been studying sharks off the coast of the Palmyra Atoll, which is a tiny 12 kilometre long island in the middle of the Pacific ocean. They've been tagging them to see where they go around the ocean and it seems like around this Atoll at least, there are little shark gangs forming.

Yannis - So for example, we know that they show strong residency to the Atoll, and all of them seem to be living there for multiple years, if not much, much longer. We also unexpectedly found that sharks only use small portions of the Atoll. So the Atoll is not very long, it's about 12 kilometres in length, so you're not talking about a very big place. But we found that sharks hang out in small sections. So a shark, for example, that lives on the west side of the Atoll only really stays on the west side of the Atoll. Whereas one that's on the east side of the Atoll only stays on the east side. And that's surprising because a shark could pretty much navigate the Atoll in a day, if it wanted to, that's not a great distance for a shark, but they choose not to. And we also then found out they kind of hang out in these groups together within their various regions and hang out with other individuals within those groups. And sometimes, as I was saying before, those social associations could last many years.

Adam - So why might such long sharp partnerships form up? What's in it for the sharks?

Yannis - Animals sharing information is quite common, and information could be things such as the location of prey. It could also be the arrival of a predator, things like that. The information isn't something that the animal may necessarily want to share - not saying that they purposefully share it with each other, but it's shared nonetheless. And in the case of sea birds, for example, you may have birds that head out to sea to forage, and you may have some birds that will follow other birds because they have a good idea that that bird knows where the good prey can be found. This may be beneficial for everybody in the group because overall your foraging success or hunting success goes up.

Adam - Because this hasn't been studied in detail, we don't know a lot about why sharks do this. And most of our best guesses have to be based off seabirds. But it does have implications for where the concept of being sociable came from in the first place.

Yannis - Sharks in some ways simplify things because they don't have parental care, for example. So a lot of the more sort of complex social interactions you might see with other animals like mammals, you don't have. So it means that we can sort of narrow down the potential drivers of sociality in these animals. There also are some conservation implications because if you have, for example, animals that have evolved behaviours that involve, let's say group foraging, what happens if those groups are reduced in size? So as we know many shark populations are facing threats from overfishing, and it may be more detrimental than we realise because when you make these groups smaller, now these animals that may have evolved mechanisms that involve group hunting can no longer do that. So what does that mean for the forging success for those individuals that are still around?

Adam - And this partnering up? It's not just a good offensive strategy. It's a pretty good defensive one too.

Yannis - Sometimes it may be related to hunting. Sometimes it's going to be related to defence. Again, we like to make the claim that sharks are top of the food chain, top predators, but that's only true for a few species. Most sharks are not at the top of the food chain and they actually have predators. Being in a group can actually have a defensive function as well, almost like a fish school. So that's something we see off our coast here in Florida, where we have, in the spring, tens of thousands of Blacktip sharks, which aren't small, they're about 1.5 - 1.6 metres long, but they are hunted by Great Hammerheads, which are also quite abundant around here. And so you can see Great Hammerheads trying to stalk the group, but once individuals in the Blacktip group have caught wind of a Great Hammerhead, you could see that information spread because suddenly everybody starts to flee as that hammerhead tries to catch its prey.

Adam - So why does it matter then if our view of sharks isn't quite the one we thought it was?

Yannis - We tend to think of sharks as just mindless predators. And the more we find out about them, the more we realise that that's really not the case. In terms of sociality, I like to kind of use the term that this is the secret social lives of sharks. And it's not because they want to keep it a secret, it's more that up until recently, we didn't really know it was there.