Animals that live under water can use sound to find prey, escape from predators, and find their way around. For example, a fish in distress who is thrashing around can alert a bigger fish that a tasty morsel is nearby.
Sharks are one type of fish that can rely on sound to catch their next meal. As far as we know, sharks don’t make any sounds themselves (unlike, for example, dolphins, who communicate using clicks and squeaks and other sounds. Which is incredibly cool. I’ll have to blog about dolphins some time). Sharks do have an inner ear, though, and tests have shown that they can detect certain frequencies.
A couple of scientists in Australia decided to see whether sharks can learn where to get food based on music cues. Scientists have studied the behaviour of sharks for a long time, often using things that the sharks can see, rather than hear. These two Australian scientists realized that we don’t know much about how flexible sharks can be in terms of learning things about sound cues. So, they designed an experiment with Port Jackson sharks–a species of shark that we know relatively little about when it comes to cognition, or mental processes.
Some info about the sharks: Port Jackson sharks (we’ll call them PJs for short) live along the coast of Australia. Here is a map with orange dots showing where these sharks have been sighted:
PJs are benthic sharks, which means they hang out around the ocean floor. They eat sea urchins, molluscs, crustaceans, and fishes. As you can see from the photo below, PJs don’t have the big, ferocious-looking teeth that sharks are usually known for. But their bite can still pack a punch!
On the smaller side of sharks, male PJs generally grow to only 75 cm in length, and females grow 80-95 cm. (75 cm is roughly the length of a grown man’s arm.) PJs are pretty harmless to humans, as you can see from this short, up-close video that a scuba diver took of several PJs:
So, back to the music experiment. The scientists used eight young PJs (four male, four female) that had been raised at the Sydney Institute of Marine Science (SIMS) from wild-caught eggs. (Click here to learn more about SIMS.) The sharks lived in huge seawater tanks and once the scientists’ experiment was finished, all of the sharks were released into the wild where their eggs had been found.
The scientists put a special arena into a tank of seawater:
Only one shark was ever in the arena at a time, and they gave the PJs lots of opportunities to swim around the arena to get used to it. The scientists actually gave each of the eight PJs a “boldness score”: They placed the shark in the start box with the sliding door closed, and when they opened the door to give the shark access to the rest of the arena, they timed how long it took for the shark to come out and explore. It turns out the male PJs were more shy than the female PJs: they took longer to come out of the start box.
So, where does the music come in? As you can see in the diagram above, there was a speaker at the opposite end of the arena. The scientists played a 20-second clip of a jazz song after the shark was let out of the start box. (They were careful to select a clip that had sound frequencies that were within the sharks’ hearing range.) For half of the sharks, if they swam to the far right-hand corner of the tank when they heard the music, then they got a prize: a tasty piece of squid (the sharks’ favourite food!). For the other four sharks, if they swam to the far left-hand corner of the tank when they heard the music, they got some squid. So, the sharks had to learn that the music was a cue to swim to a specific corner of the arena.
(Click here to listen to the jazz music that the sharks heard.)
As a test, the scientists played the music a few times and did not give the sharks any squid, to see if the sharks would swim to the correct corner. Five of the eight sharks passed the test: they learned to swim to a specific corner of the tank when they heard the jazz music.
Next, the scientists added classical music to the mix. Again, they chose a 20-second clip that had sound frequencies that the sharks would be able to hear. This time, if the PJs heard the jazz music they were to swim to the same corner as before to get some tasty squid. However, if the heard the classical music, they had to swim to the opposite corner to get squid. In other words, the type of music that was played to the sharks told them which corner to swim toward.
(Click here to listen to the classical music that the sharks heard.)
Interestingly, the sharks did not do well at this task. They kept swimming to the far right corner of the tank, regardless of which music was played. Even the five sharks who, in the previous task, learned to swim to the left when jazz music was played, swam to the right. In other words, the five sharks who were successful before did worse in this task. Why did all the sharks favour the right side of the tank so much? We’re not sure.
So there was no evidence that the PJs were able to tell the two types of music apart. Or, if they could tell them apart, their behaviour certainly didn’t show it. And remember how the scientists scored the “boldness” of each shark at the beginning of the experiment? Well, it turns out “boldness” did not have an effect on the sharks’ learning ability (or lack thereof).
It is possible that the five PJs who originally swam to the correct corner of the tank learned the simple rule of “swim to the right (or left) corner of the tank,” and basically ignored the jazz music. They figured out how to get a squid meal using the simplest way possible. They didn’t need any music. Pretty bright if you ask me.
To learn more about PJs, click here to check out the Australian Museum.
Vila Pouca, C., & Brown, C. (2018). Food approach conditioning and discrimination learning using sound cues in benthic sharks. Animal Cognition, 21, 481-492. https://doi.org/10.1007/s10071-018-1183-1