Move to the Music
Every now and then scientists complete some research which makes the whole world smile, just because it seems so delightfully random. For example, recently a group of researchers studied whether sharks could tell the difference between genres of music! You might assume this is all just fun and games, to settle an argument between dorky scientists with different tastes in music, but, in fact, such studies can teach us an awful lot about the cognitive capabilities of these fish and could even be important for saving endangered species from extinction...
It might seem peaceful to us, but the underwater world buzzes with a myriad of different sounds: the crackling of shrimp, popping of air bubbles, the chirps of fish, the motion of waves above... These can be both "biotic" sounds (make by living animals) or "abiotic" (sounds caused by non-living parts of the natural environment, like waves or wind). All these noises come together to form what is known as a "soundscape"; the background sounds that are present within the environment all the time. These noises are actually very important "cues" used by marine animals, which can be used to find mates, communicate with other animals, find food sources, avoid predators and even to navigate through the habitat (Vila Pouca & Brown, 2018).
Many people are surprised to learn that sharks have ears and very good hearing! Just like us, sharks' ears are located on either side of their heads. The ear canal is filled with water and transmits sound vibrations into the inner ear, which are transmitted directly to the shark's brain. Whatsmore, the "lateral line" running along the side of their body, which can sense water displacement, is also involved in the detection of different sounds (Vila Pouca & Brown, 2018).
As sound travels further in water than in air, sharks are able to sense noises over quite long distances; potentially even from over 1.5 km away. They are sensitive to sounds up to 1,000 Hz, but are most sensitive to sounds which are below 100 Hz (Vila Pouca & Brown, 2018).
It is thought that sharks use sounds as "acoustic cues" to find food and for navigation. For instance, scientists believe lemon sharks (Negaprion brevirostris) hone in on the sounds of sick or struggling fish when they are foraging (Banner 1972, Vila Pouca & Brown, 2018).
So, because they are so sensitive to sounds, researchers wondered whether sharks could be taught to respond to music as a sound cue and whether they could tell different genres of music apart...
To test this, scientists trained sharks to associate jazz music playing into their tank with a food reward. They released Port Jackson sharks (Heterodontus portjacksoni) into a laboratory tank with music playing at one end and only gave them a food reward when they swam to a certain side of the tank (either left or right). They then counted how many times the sharks went to the correct side of the tank to look for food when the music was playing. This type of experimental set-up is known as "food conditioning" (Vila Pouca & Brown, 2018).
The researchers found that the Port Jackson sharks were very good at learning where to find food when the music was playing. However, the sharks were not able to discern between different genres of music. When they tried to train the sharks to go to opposite corners of the tank when different types of music were playing (jazz versus classical), the sharks could not tell the difference and just randomly looked around for food (Vila Pouca & Brown, 2018).
These findings suggest that the Port Jackson shark uses acoustic signals in their environment for foraging. Because they sharks are "nocturnal" (more active at night), it makes much more sense that these shark would rely on auditory cues, rather than their vision, for hunting in the dark. The scientists concluded that these sharks can hear the sounds of other life on the ocean floor and probably use these sounds to navigate their environment; waves lapping on a shallow area of reef could be used as a navigational "landmark", so they can figure out where they are, for instance, or the sounds of an octopus struggling with fish might alert them to the presence of prey nearby (Vila Pouca & Brown, 2018).
Whilst this research seems very cute and fun, it actually has a very serious and important scientific purpose... We know very little about the cognitive abilities of these little sharks, but this research shows us that, on top of both biotic and abiotic sounds, these sharks are capable of learning to use man-made sounds as foraging signals. This means they are capable of learning to recognise new sounds and to remember this information. In turn, this suggests they are able to adapt to a changing environment (this is known as "behavioural plasticity") (Vila Pouca & Brown, 2018).
As the oceans are now filled with anthropogenic sounds, (from drilling, seismic surveys, boat traffic, construction etc. (To learn more you can check out, Keep the Noise Down!)), it is very important that scientists understand how acoustic signals affect animals in the marine environment. We are only just beginning to understand how our "noise pollution" can impact upon marine animals like sharks, so work like this is very important! These findings teach us that these sharks are certainly sensitive to man-made noises and our activities in their habitats can have an impact on their behaviour. Thankfully, the Port Jackson shark is not considered threatened with extinction (IUCN, 2021), but, sadly, there are many, many species of sharks which are endangered. So whilst they may seem frivolous, studies like this one are vital for us to better understand how we are affecting endangered species, and to figure out how to minimise our impact on their natural habitat (Vila Pouca & Brown, 2018).
To find out more about learning in sharks, you can check out Making Memories.
Banner A (1972) Use of sound in predation by young lemon sharks, Negaprion brevirostris (Poey). Bulletin of Marine Science, 22:251–283.
Casper BM (2006). The Hearing Abilities of Elasmobranch Fishes. Graduate Theses and Dissertations, University of South Florida, USA. Access online.
IUCN (2021). The international Union for the Conservation of Nature Red List of Threatened Species. Access online.
Vila Pouca C & Brown C (2018). Food approach conditioning and discrimination learning using sound cues in benthic sharks. Animal Cognition, 21, 481–492.