What sounds can animals hear?
Many species differ in their ability to hear at any one frequency. Scientists and audiologists often display hearing abilities as a curve or audiogram, a graph of intensity vs. frequency. The figure below displays audiograms for several marine mammals and humans. You will notice that there are two y-axes. This is because we measure hearing in air (humans) differently than under water (marine mammals). Read here for more information on how hearing is measured, what humans can hear, or hearing threshold studies in marine animals.
All species of mysticetes (baleen whales),such as blue, fin, and humpback whales produce low frequency sounds. There are no direct measurements of hearing thresholds for baleen whales, but anatomical evidence and their vocalizations imply that all mysticetes hear well at low frequencies. Some species may be able to hear infrasonic or seismic signals (below 20 Hz). For example, measurements of the ears of thirteen North Atlantic right whales estimated that they can hear from 10 Hz to 22 kHz.
Direct studies have been performed on several odontocete (toothed whale) species including dolphins, beluga whales, and harbor porpoises. All species of toothed whales hear best at higher frequencies (10 kHz to 110 kHz). New technologies, including portable systems, are allowing scientists to measure an animal’s hearing in as little as 45 minutes. Studies can now be done on animals in the wild. For example, two white-beaked dolphins were captured while bow-riding off a fishing vessel. Their hearing sensitivities were measured onboard the vessel in a small plastic research tank, demonstrating the most sensitive high frequency hearing yet measured for any dolphin species. Similarly, an entire population of captive bottlenose dolphins was examined, including 42 individuals ranging in age from 4 to 47 years old. Scientists can see the change in hearing with age of this population.
Pinnipeds (seals, sea lions, and walruses) have increasing sensitivity from low to high frequencies. Several species of pinnipeds, including harbor seals and elephant seals, have been directly studied. Recent studies of harbor seals conducted in a pool specially designed for hearing studies (located in the Netherlands) have allowed scientists to more accurately measure hearing sensitivities. Because of the very low background noise, they discovered the lowest hearing sensitivity yet measured in harbor seals.
Studies on other marine species are just beginning to be conducted. A recent study using auditory evoked potential (AEP) techniques with sedated polar bears shows that they have good in-air hearing over a wide frequency range. They hear best between 11.2 kHz and 22.5 kHzNachtigall, P. E., Supin, A. Y., Amundin, M., Roken, B., Moller, T., Mooney, T. A., … Yuen, M. (2007). Polar bear Ursus maritimus hearing measured with auditory evoked potentials. Journal of Experimental Biology, 210(7), 1116–1122. https://doi.org/10.1242/jeb.02734.
Most data regarding sea turtle, shark, and skate hearing come from studies using auditory brainstem responses (ABRs). From this work, scientists know that sea turtles can hear low to mid-frequency sounds underwater, but with poorer sensitivity than mammals.
Sea turtles, in general, hear best between 200 and 750 Hz and do not respond well to sounds above 1,000 Hz (1 kHz). The graph shows that younger sea turtles of any species hear higher frequencies better than older turtles of the same species. As turtles grow older and larger, their hearing sensitivity shifts to lower frequencies.To determine the hearing capabilities in an adult green sea turtle, researchers obtained a behavioral audiogram for an older female green sea turtle, “Myrtle”, housed in the New England Aquarium’s Giant Tank exhibit. Based on her responses, it was concluded adult green sea turtles hear best between 200 Hz and 500 Hz.
Elasmobranchs, including sharks, skates, and rays, appear to respond best to low frequency sounds. The hearing of the little skate was tested using both behavioral and auditory brainstem