Free-swimming sharks detected a sound emitted from more than 70 meters away and were able to locate its direction.
Sharks have an inner ear, even though no external ears are visible on their heads. It allows them to perceive sound vibrations traveling through the water. Until now, the distance at which a shark could actually detect and locate a sound in its natural environment had remained poorly measured.
A great white shark.
Image Wikimedia
Researchers from Florida Atlantic University studied blacktip sharks, Carcharhinus limbatus, in Florida waters. Rather than placing the animals in a tank, they worked with individuals free to move around. An underwater loudspeaker then broadcast a controlled signal at different distances.
The scientists observed reactions up to 74 meters from the sound source. The sharks changed their behavior and could orient themselves toward the signal's origin. This distance is roughly equivalent to the length of three 25-meter swimming pools placed end to end.
Hearing is not enough to find a source. Underwater, sound travels quickly and reaches both sides of the head almost simultaneously. Sharks must therefore use small differences in the movement of water particles to determine where the signal comes from.
The experiment provides data specifically on this directional ability in the open sea. The animals were not merely sensitive to the presence of the sound: their movements indicated that they could identify its origin. The researchers were thus able to connect a behavioral response to a distance measured in the field.
This ability may have several uses in a shark's life. Low-frequency sounds are produced by fish, moving animals, and various underwater activities. They can provide information about the presence of prey, other animals, or certain environments, complementing smell and the other senses.
The results also interest researchers studying noise produced by humans. Ships, engines, and underwater construction add many sounds to coastal environments. Knowing the actual range of a shark's hearing helps estimate the distances at which these noises can alter its behavior.
The team now wants to measure responses to other frequencies and under different natural conditions. The range of a sound depends on its frequency, intensity, and the environment through which it travels. These new trials will determine whether the observed 74 meters represents a common distance or merely a particular case.