Porpoises are smaller marine mammals that inhabit coastal waters and estuaries globally. They are characterized by compact bodies, measuring between 1.5 to 2.5 meters in length, and short, rounded snouts. Unlike their dolphin relatives, porpoises possess spade-shaped teeth instead of conical ones. Sound plays a central role in their existence, allowing them to navigate their underwater environment, find prey, and interact with other porpoises.
The Distinctive Sounds of Porpoises
Porpoises primarily produce narrow-band high-frequency (NBHF) clicks. These clicks have a peak frequency around 125-140 kHz and a relatively narrow bandwidth of 11-20 kHz. Human hearing ranges from about 20 hertz to 20 kilohertz, making porpoise clicks largely inaudible to us. These sounds are also short in duration, lasting less than a millisecond.
The high-frequency nature of these clicks allows porpoises to create a focused sound beam, reducing echoes and improving their biosonar system’s resolution. This enables them to detect small objects, including prey. Porpoises also use these high-pitched clicks for communication, varying the repetition rate to convey different signals.
Echolocation and Communication: How Porpoises Use Sound
Porpoises produce sound through specialized structures in their nasal passages known as phonic lips, located just below the blowhole. Air is forced through these phonic lips, causing them to vibrate and generate clicks. The sound then travels through a fatty, rounded tissue in their forehead called the melon, which focuses the sound into a narrow, directional beam.
When these clicks are emitted, they travel through the water and bounce off objects, returning as echoes. Porpoises interpret the delay time between the emitted click and the returning echo to determine an object’s distance. Their sensitive hearing also allows them to discern the direction of prey or objects. This biological sonar system, or biosonar, allows porpoises to navigate and locate prey. When searching for prey, harbor porpoises may emit clicks at a rate of 20 times per second, increasing to several hundred clicks per second during a “terminal buzz” phase as they approach their target.
Porpoises and Dolphins: A Sound Comparison
While both porpoises and dolphins are toothed whales and use echolocation, their acoustic strategies show distinct differences. Porpoises are known for their specific narrow-band high-frequency (NBHF) clicks, which are above 100 kHz. This specialized acoustic niche may help them avoid detection by larger predators like killer whales.
Dolphins, in contrast, produce a broader range of sounds, including whistles, burst-pulsed sounds, and clicks. Whistles are continuous tonal sounds used for communication within their social groups. Dolphin clicks, also used for echolocation, can cover a range from tens of kilohertz to 150-170 kHz and are described as broadband. These acoustic differences reflect their evolutionary paths and roles in the marine environment.
Underwater Noise: A Threat to Porpoise Soundscapes
Human-generated underwater noise poses a threat to porpoises and their reliance on sound. Sources of this noise include shipping traffic, military sonar, seismic surveys, and offshore construction. This increasing noise interferes with porpoises’ ability to use echolocation for navigation and foraging, and to communicate.
The masking effect of noise can reduce their hearing range and ability to detect acoustic cues, leading to stress, displacement from habitats, and reduced feeding efficiency. High-intensity noise sources, such as seismic air guns, can cause temporary or permanent hearing damage, leading to stranding or death. Efforts to mitigate these impacts include reducing vessel speeds, modifying ship designs, and implementing quieter operational practices.