Have you ever stuck your head under water when swimming in a pool with friends and tried to communicate with each other? The sounds you can hear underwater seem dampened and warped when your friend shouts at you. But is this the same for animals adapted to a life at sea?
A new study has researched the hearing of great cormorants (Phalacrocorax carbo sinesis) to determine whether they have a better sense of hearing above water or below the surface. Until recently, researchers were not able to record underwater vocalizations of seabirds and the underwater hearing capabilities of seabirds were a big unknown. Researchers have however now been able to prove that great cormorants have a superior sense of hearing underwater compared to in the air. It is possible that this is true for many other seabird species that hunt for fish underwater. Studies into the hearing of seabirds are important, as human-caused noise pollution in the ocean – through shipping, dredging, exploration and fishing activities – is increasing and has already shown to have detrimental impacts on marine mammals that depend on echolocation and vocalizations for communication.
The Hearing Of Cormorants
Living an amphibious life above and below water means that senses, including hearing, need to function well both in the air and underwater. Researchers from the Sound and Behaviour Group, Department of Biology, University of South Denmark made the discovery into the underwater hearing of great cormorants, and published their findings in the Journal of Experimental Biology. Due to the difficulties of studying the hearing of birds in the wild, the team captured five great cormorant fledglings to study their hearing in the lab.
The fledglings were approximately 4 – 6 weeks old and were close to leaving their nests when captured. The cormorants were anesthetized, and once unconscious, were exposed to airborne and underwater sounds. First, they were exposed to clicks and tone bursts in an anechoic box in air (i.e. a box free from echoes and reverberations), and then their heads and ears were submerged 10 cm below the surface of a water-filled tank. Their neural activity was then measured using electrodes under the skin. Despite the invasive-sounding procedure, the study appeared to have little effect on the birds, who were successfully released.
The results revealed that the neural responses to both underwater and airborne sounds were very similar, with almost the same sensitivity to sound pressure in the two mediums. However, this does not mean their hearing above and below water is equally strong. Sound waves move through air and water at different speeds, with sound waves moving much faster through a liquid due to the particles in water being much closer together than in a gas. However, the intensity of sound waves is reduced in a liquid, despite the increased speeds. This means that animals that live above the water will experience weaker, distorted sounds underwater (like humans in a swimming pool). Therefore, the results showing that the cormorants have equally good hearing above and below the water means their ears are actually much better at picking up sounds under water than above it.
Adapted To Hearing Underwater
Great cormorants are able to hear so well underwater due to how their ears evolved – which is in a similar way to other air-breathing aquatic animals, like turtles and amphibians. Their eardrums and middle ears have adapted to allow for much larger vibrations than compared to terrestrial birds. Their eardrums are also much stronger, being able to withstand large pressures when they dive into the ocean in search of food. Unfortunately this has a trade-off, as this adaptation means they are not well adapted to listening to sound through the air. This is a surprising trade-off as cormorants still spend the majority of their lives above water (unlike turtles and frogs), and only spend about 30 seconds under water each time they dive for fish.
It is important for cormorants to still be able to hear above water, as they use sound to locate their mates and chicks during breeding season, and they also use noise for social cues at their colonies. So the question remains – why did cormorants (and possibly other seabirds) sacrifice their above-water hearing when they only spend such a small amount of time underwater?
The answer is food. Cormorants locate their fishy prey by using smell, following chemical trails like dimethyl sulfide produced by plankton to find areas of high productivity. Once they locate these areas, they need to be able to locate the fish with pinpoint accuracy when diving into the water. Flying seabirds have poor eyesight underwater, which means that the cormorants are possibly using their refined hearing to listen for the fish they are trying to catch, instead of seeing their prey. Their hearing trade-off therefore possibly allowed them to successfully hunt over evolutionary time.
Hearing In Other Seabirds
Although still to be studied, it is hypothesized that other seabirds have similar hearing adaptations to great cormorants. Other diving seabird species, such as gannets and puffins, spend longer periods of time underwater when diving for fish, and are also able to dive deeper underwater where there is less light. It can thus be assumed that hearing is equally important to these birds as it is to cormorants for finding food. For now this is only a theory, and would need to be tested and studied similarly to the great cormorants.
It will become increasingly important to understand the underwater hearing capabilities of seabirds, as marine pollution continues to pose a threat to many other marine animals, and the impacts of marine noise on seabirds are not well understood. It remains a possibility that noise pollution could be interfering with the diving behavior of seabirds and their ability to locate fish underwater. Marine noise may also be impacting the behavior of the fish. This, coupled with overfishing and a reduction in prey fish populations, could result in catastrophic consequences for various seabird species as there will be a continued lack of food.