Zoologger: Social shark that doesn't look like a lemon

Loving each other’s company

Species: Negaprion brevirostris
Habitat: coastal waters of tropical Africa, and North and South America

The lone shark prowls through the water, hunting for prey. It rarely encounters other members of its species, and when it does it either ignores them or drives them away. Other than the occasional mating, it is solitary.

Or not. The popular image of sharks is that they are mindless antisocial killers, but the reality is different. Many species gather into groups, often because several sharks have been drawn to the same food source. Hammerhead sharks regularly spend the daytime in large shoals before dispersing at night to hunt alone.

Lemon sharks are one of the more social species. Unlike whale sharks, which form groups but then ignore each other, lemon sharks actively prefer the company of their peers. A new study suggests they can learn from each other, a skill that only evolves in truly social animals.

A shoal of lemons

Lemon sharks are so called because they sometimes appear yellow, depending on the light conditions. In many photos, their colour is somewhere between brown, white and grey. The yellowish colouration helps them to blend in above sandy sea beds.

While they’re not as well-known as great whites or megamouths, lemon sharks are one of the best-studied sharks because they thrive in captivity. As a result, we have a wealth of information about them.

Tristan Guttridge of the Bimini Biological Field Station in Miami, Florida, recently discovered that juvenile lemon sharks prefer group living to the solitary life when given the choice. This suggests they aren’t just gathering around food: they are actively social. Guttridge says they often spend four or five hours a day socialising, with groups of adults resting together on the sea bed.

But they don’t hang out with just anyone: they prefer to associate with lemon sharks of about the same size as themselves. That might be because swimming in a shoal of near-identical animals makes it difficult for predators to target them.

Shark mimicry

Guttridge and colleagues have now found evidence that the sharks aren’t just gathering together for protection: they are learning from other members of the group. He captured 18 juvenile lemon sharks and gave them a learning test.

Pairs of sharks were placed in a holding pen containing a black-and-white banded target. If a shark swam up to the target and touched it, it was rewarded with a piece of food.

In some cases, a shark that had never seen the target before was paired with a “demonstrator” that had already been trained. In other cases, the naïve shark was paired with a “sham demonstrator” with no advance knowledge.

Sharks that had an experienced demonstrator were quicker to start touching the target, and touched it more often. When they were tested on their own, they performed better than sharks that had been paired with sham demonstrators. That suggests the sharks watched what the demonstrator did, and worked out that touching the target produced food.

Personable shark

Guttridge emphasises that the evidence is “preliminary”. Nevertheless, it is unprecedented. Although there is ample evidence that fish learn from each other, all the evidence comes from studies of bony fish. Guttridge’s study is the first to look at a cartilaginous fish, which is on a lower branch of the vertebrate evolutionary tree.

Guttridge is now trying to find out if lemon sharks have personalities. Many other animals do – including such seemingly simple creatures as sea anemones – and anecdotal evidence from divers suggests that individual lemon sharks show consistent, idiosyncratic behaviour.

“Most people associate social learning and personality with non-human primates,” Guttridge says. By contrast, sharks aren’t thought of as sociable individuals but rather as “death fish from hell”. They are certainly ferocious predators, but it seems they might also be community-minded.

Journal reference: Animal Cognition, doi.org/h9f