These marine mammals spend time both on land and at sea, and need to balance life in both worlds, including their hearing. Known as\u00a0amphibious hearing<\/a>, this ability is unique to seals, and therefore quite the superpower.<\/p>\n While we know that seals can hear amphibiously, what\u2019s long remained a mystery is exactly how they do so, and when this ability evolved. In our new paper, published today in\u00a0Proceedings of the Royal Society B: Biological Sciences<\/a>, we investigated amphibious hearing in seals to find answers to these questions.<\/p>\n One or the other<\/p>\n Theoretically, being able to hear well in both air and water should be impossible for mammals.<\/p>\n That\u2019s because for animals like humans, who live on land, sounds travel through the air in our ear canals to the middle ear. It then transmits sounds to the inner ear for processing.<\/p>\n In water, which has a similar density to our bones, sound travels four times faster. This means sound travels through muscles and bones directly to the inner ear, bypassing the middle ear.<\/p>\n This auditory shortcut means our middle ears can\u2019t filter out unnecessary sounds, muddling the signal \u2013 this is why things sound muffled underwater to us.<\/p>\n So how do seals manage both? And how long have they been doing it for?<\/p>\n Special tissue in the ears<\/p>\n To find out, we 3D scanned the ears of seal specimens from museum collections around the world. These scans revealed that the ear canals and middle ears of seals contain something called cavernous tissue.<\/p>\n This type of tissue is \u201cspongy\u201d and contains a lot of blood vessels. In seal ears, it fills with blood and expands as the seal dives, allowing them to\u00a0equalise the air pressure<\/a>\u00a0inside their ears.<\/p>\n Importantly, the blood has a similar density to water and can fulfil a second function: conducting underwater sounds to the seal\u2019s middle ear.<\/p>\n As a result, underwater sounds travelling through the cavernous tissue lose less than 1% of sound signal, as opposed to 99% if the ear was still filled with air.<\/p>\n Next, we wanted to know how and when this remarkable adaptation evolved. This required some comparisons.<\/p>\n Seals are carnivorans \u2013 they\u2019re part of the major group of mammals that includes dogs, bears and cats. We measured the ears of seals and of some modern land-dwelling (terrestrial) carnivorans, as well as some extinct fossil relatives of seals.<\/p>\n We then used several statistical analyses to estimate whether these extinct seals could hear amphibiously or not.<\/p>\n What we found was that the earliest relatives of seals, including the\u00a0extinct freshwater ancestor Puijila<\/a>, could only hear in the air, like us. Their ears are most similar to their terrestrial relatives.<\/p>\n By contrast, the earliest marine seals, including\u00a0Enaliarctos, evolved amphibious hearing 26 million years ago. But this came at a cost for these early seals; their hearing was initially poorer in both environments.<\/p>\n This issue was eventually solved in the earliest relatives of modern seals, who evolved modified middle ears capable of acute hearing in both environments.<\/p>\n Crucially, the true seals (such as harbour, leopard and elephant seals) became very sensitive to underwater sounds.<\/p>\n Remarkable seal acoustics<\/p>\n Seals have other remarkable acoustic abilities too. True seals\u00a0sing<\/a>\u00a0underwater, sometimes at pitches\u00a0beyond the range of human hearing<\/a>. Seals can also outperform humans in\u00a0tests of rhythm<\/a>, a rare ability\u00a0in the animal kingdom<\/a>.<\/p>\n They can also learn to produce new sounds via vocal mimicry. A famous example is\u00a0Hoover the talking seal<\/a>\u00a0in the US, who mimicked some phrases from a Maine fisherman.<\/p>\n Seal acoustics are indeed special, and they are at the forefront of research into the\u00a0evolution of speech<\/a>.<\/p>\n The problem of noise pollution<\/p>\n The ocean is becoming increasingly\u00a0noisy<\/a>, and human-made\u00a0noise pollution<\/a>\u00a0is an emerging threat to marine mammals.<\/p>\n Just like how we have difficulty hearing a mate in a noisy pub, marine mammals\u00a0struggle to hear each other<\/a>\u00a0over noise from ships, construction, and sonar.<\/p>\n Ever been to a concert and your hearing is temporarily worse? That\u00a0happens to marine animals too<\/a>, except it\u2019s from the din of our ocean activities. Loud underwater sounds even have the potential to\u00a0cause harm<\/a>.<\/p>\n Marine life, including seals, need to hear each other. How do we keep the racket down? Simple solutions, such as\u00a0slowing down ships around marine wildlife<\/a>, are proving remarkably effective. By lowering our acoustic footprint, we can better protect the superpowered hearing of our aquatic neighbours.<\/p>\n This article, co-authored with Ian G. Brennan of the Australian National University, is republished from The Conversation<\/a> under a Creative Commons license. Read the original article<\/a>.<\/p>\n <\/p>\n This article was first published on Monash Lens<\/a>. Read the originalThe specialised anatomy of a seal\u2019s ear, as demonstrated by a 3D scan of a southern elephant seal.\u00a0Image credit: James RuleHow did seals evolve amphibious hearing?<\/p>\n
Early freshwater relatives of seals, such as\u00a0Puijila, could only hear in the air, like most mammals.\u00a0Art by Jaime Bran<\/p>\n
An early marine seal,\u00a0Enaliarctos, chases some fish. Its limited underwater hearing means it can\u2019t hear an echolocating toothed whale approaching from behind.\u00a0Jaime Bran<\/p>\n
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