Image: Michael Valos / Getty Images
When most people think of noise pollution, the image conjured up is one of concrete buildings and crowded streets bursting with beeping car horns and screeching industrial tools. But there’s another flavor of noise pollution happening underwater, across the world’s oceans, and it’s taking an even worse toll on its inhabitants. The constant hum from human-made inventions (think massive shipping boats, deep-sea mining, and offshore oil rigs) has radically transformed the sound of the ocean………Continue reading..
By: Mack DeGeurin
Source: Popular Science
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Critics:
Whales are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are related to the Indohyus, an extinct chevrotain-like ungulate, from which they split approximately 48 million years ago. Primitive cetaceans, or archaeocetes, first took to the sea approximately 49 million years ago and became fully aquatic 5–10 million years later.
What defines an archaeocete is the presence of anatomical features exclusive to cetaceans, alongside other primitive features not found in modern cetaceans, such as visible legs or asymmetrical teeth. Their features became adapted for living in the marine environment. Major anatomical changes included their hearing set-up that channeled vibrations from the jaw to the earbone (Ambulocetus 49 mya), a streamlined body and the growth of flukes on the tail (Protocetus 43 mya).
The migration of the nostrils toward the top of the cranium (blowholes), and the modification of the forelimbs into flippers (Basilosaurus 35 mya), and the shrinking and eventual disappearance of the hind limbs (the first odontocetes and mysticetes 34 mya). Whale morphology shows several examples of convergent evolution, the most obvious being the streamlined fish-like body shape.
Other examples include the use of echolocation for hunting in low light conditions — which is the same hearing adaptation used by bats — and, in the rorqual whales, jaw adaptations, similar to those found in pelicans, that enable engulfment feeding. Today, the closest living relatives of cetaceans are the hippopotamuses; these share a semi-aquatic ancestor that branched off from other artiodactyls some 60 mya.
Around 40 mya, a common ancestor between the two branched off into cetacea and anthracotheres; nearly all anthracotheres became extinct at the end of the Pleistocene 2.5 mya, eventually leaving only one surviving lineage – the hippopotamus. Whales split into two separate parvorders around 34 mya – the baleen whales (Mysticetes) and the toothed whales (Odontocetes).
The whale ear has specific adaptations to the marine environment. In humans, the middle ear works as an impedance equalizer between the outside air’s low impedance and the cochlear fluid’s high impedance. In whales, and other marine mammals, there is no great difference between the outer and inner environments.
Instead of sound passing through the outer ear to the middle ear, whales receive sound through the throat, from which it passes through a low-impedance fat-filled cavity to the inner ear. The whale ear is acoustically isolated from the skull by air-filled sinus pockets, which allow for greater directional hearing underwater. Odontocetes send out high-frequency clicks from an organ known as a melon.
This melon consists of fat, and the skull of any such creature containing a melon will have a large depression. The melon size varies between species, the bigger the more dependent they are on it. A beaked whale for example has a small bulge sitting on top of its skull, whereas a sperm whale’s head is filled up mainly with the melon.
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