A new study into the hearing of great cormorants shows that they have a better sense of hearing in water than they do above the surface. It is another example of how underwater sound may be much more important to seabirds than previously thought.
Only recently the first ever recorded vocalisations of seabirds made underwater was captured by researchers studying penguins in South Africa. Now researchers have also shown that great cormorants have a superior sense of hearing underwater than they do above it. Due to the nature of how cormorants live, and in particular hunt, it suggests that this may be true of a lot more seabirds as well. This new field of avian-audio studies is becoming increasingly important considering human noise pollution in our oceans is greater than ever and seabirds are in decline globally. So just how do cormorants manage to perfect their listening skills underwater and how many other seabirds can do the same?
Comparing types of hearing
This new discovery into the hearing of great cormorants was made by a team of researchers from the University of South Denmark, led by Ole Næsbye Larsen, and was published last week in the Journal of Experimental Biology. You might wonder how the researchers were able to accurately study hearing in a diving bird. Of course it would be nearly impossible to do in the wild, so the team had to catch great cormorants and study their auditory responses in the lab under anaesthetic. The unconscious cormorants were exposed to both airborne and underwater sounds and the resulting neural activity was measured using electrodes under the skin. This is obviously not an ideal study method for the birds, but it actually has a surprisingly small effect on them when they are released and the benefits of making discoveries that could protect them in the future were deemed to out-weigh any potential impacts.
What this method revealed was that neural responses to sounds in water and air proved to be very similar, with almost the same sensitivity to sound pressure in the two media. You might initially think this means their hearing above and below the water is equally strong, but this is actually not the case because of how sound waves move through air and water. Although sounds travel faster and further through a liquid, because the particles are much more close together than a gas, the intensity of sound waves is actually reduced. This means that for most animals that live above the surface neural responses to sound underwater are much weaker and sounds are often more distorted. So by having equally good hearing above and below the surface means that the cormorant’s ears are much better at picking up sounds underwater than above it.
Adaptations & trade-offs
The reason that great cormorants are able to hear so well underwater is because their ears have evolved in a similar way to other air breathing aquatic animals like turtles and frogs. This means their eardrum and middle ear have adapted to allow for much larger vibrations than most terrestrial birds. Their eardrums are also much stronger than most terrestrial animals to withstand the pressures of diving deep into the ocean. However as we have already seen these adaptations come at a cost, as their ears are less well adapted to listening to sound through the air. These types of trade-offs, between gaining the ability to do one things and losing the ability to do another, are very common in nature. However this particular trade-off is surprising considering that unlike turtles and frogs, cormorants actually spend very little time underwater.
An evolutionary edge
On average cormorants spend as little as 30 seconds underwater when diving for fish, of which they will only do a few times a day. So a majority of their lives are actually spent on land. Airborne hearing is important for seabirds to find their mates and chicks amongst their colonies and for social interaction. So why have the cormorants sacrificed such a big part of their lives for the ability to hear better underwater for a few fleeting moments? The answer like a lot of evolutionary mysteries revolves around food.
Most seabirds including cormorants will locate their prey by smell, following chemical trails like dimethyl sulphide produced by plankton to find areas of high productivity, but after they have found the fish they must next accurately pinpoint them whilst diving. Despite this most seabirds actually have a rather poor sense of sight underwater, which isn’t much better than our own. So for the cormorants having good underwater hearing means that they are likely to hear the fish they are trying to catch, instead of seeing them. So their hearing trade-off has therefore secured their ability to successfully feed themselves over evolutionary time.
Exception or the rule?
So given what we know about cormorants and seabirds in general, what are the chances that other species share their incredible audio adaptations? In all likelihood it is probably fairly common. Most other species of diving seabirds such as gannets and puffins spend much longer periods underwater when searching for fish, as well as much deeper into the water column where light starts to become a limiting factor. So if sound is important to cormorants, it is likely to be important to others as well. Recently it was also revealed for the first time that seabirds can also make sounds underwater, after the first underwater vocalisations of penguins were caught by researchers in South Africa. The calls produced by penguins were short in length, high in frequency and made whilst diving for fish, which suggests they were using echolocation much like a dolphin. This implies that they too probably have much better underwater hearing than we previously realised.
However until we start to test the underwater hearing of more seabird species in the same way as the great cormorant, then there will be no way to tell for sure. Unfortunately this is likely going to be an issue in the future, because of increased noise pollution from humans. Shipping, deep-sea mining, renewable energy and military sonar can all be a very disruptive influence for a lot of marine animals. “We need more knowledge about how seabirds are affected by this noise” says Jakob Christensen-Dalsgaard, one of the authors from the new paper, speaking to Science Daily. It is likely that noise pollution could be interfering with the seabirds diving behaviour and resulting in a subsequent lack of food, which coupled with a lot less fish left to find in the first place could result in some dire consequences for the aquatic aviators.