跟读练习: The Insane Biology of: The Narwhal - 通过视频学习英语口语

For centuries, humans have been entranced by the mystical powers and ethereal beauty of a mysterious, spiraling horn.

A horn that was thought to possess magical qualities, from purifying water to counteracting poisons and deadly diseases if ingested.

Its powers were so sought after that Queen Elizabeth paid 10,000 pounds for a horn in the 1500s.

6.3 million pounds in today's money.

But Queen Elizabeth and all the other purveyors of this magic horn got certifiably bamboozled.

The horn they so coveted was a real thing, and it probably inspired the unicorn legends.

But the actual creature is not quite so ethereal.

Maybe you're thinking, that's still a majestic creature even if it's not a unicorn.

Yeah, maybe you would think that, until you learn that that spiraling horn is actually a tooth.

extremely long, weird buck tooth.

The narwhal somehow gets away with this, but any other animal with a single, three meter long tooth would be immediately cancelled.

And things don't get better when you learn the name narwhal comes from Old Norse meaning corpse whale, because the color of their skin resembles a drowned sailor.

Gross.

And honestly, they just keep getting weirder the more you learn.

Scientists attached satellite trackers and cameras to narwhals and observed them spending the majority of their hunting time swimming upside down.

In rare cases, belugas and narwhals have even mated to create the hybrid narluga.

Narwhals are also one of the few cetacean species that polar bears will actively hunt, rather than waiting for a dead carcass to wash ashore,

which means they have to be especially careful about where and when they come up to breathe.

But even despite this, they have an incredible lifespan, with estimates that they can live up to 100 years,

because narwhals are not just weird, but also incredibly hardcore.

Narwhals are one of the deepest divers, swimming as far as 1,800 meters underwater to hunt for food, and going especially deep in the winter.

They manage 18 to 25 dives per day thanks to their thick blubber that provides insulation and energy, collapsible ribs that bend under high pressures,

and an incredible ability to store 70 liters of oxygen in their lungs, blood, and muscles.

Their bodies don't have the fast-twitch skeletal muscles that give dolphins such propulsive speeds,

and instead have muscles better able to handle endurance swimming, requiring less oxygen.

But the biggest question about them is, what's the deal with that horn?

Or rather, that big ol' tooth?

It's certainly fun to try to guess.

Multitool?

Intergalactic antenna?

Supersonic speed?

Kebabs?

Predator evasion?

A meter of its lies?

But jokes aside, scientists still don't really know the real answer.

Some past theories about its purpose included sound transmission, thermal regulation, breaking ice, and even a breathing organ.

But since it's usually only males who grow the horn, it can't be critical for survival.

And yet some females do also have the horn too, so it also likely isn't just to be for mating.

So what the heck is going on here?

What on earth are these long protruding teeth horns for?

Narwhals belong to the Odontoceti, or toothed whale order.

Like their distant relatives, the sperm whale and the orca, narwhals are endowed with teeth, but only two of them.

And in their case, the teeth are actually known as tusks.

Female narwhals have two tusks embedded in their jaw, which rarely become visible.

In about 1.5% of females, one of those tusks will grow into a huge spear, just like the males.

And in some cases, males will have no tusk, though sometimes they'll also grow two, for for reasons we don't really understand.

Even though the horns are called tusks, they have a very similar anatomy to other mammalian teeth.

The outer layers of the tusk are hard material to protect the nerve at the interior.

The outermost layer is cementum, a calcified material to protect the tooth.

Immediately beneath it is the dentin, which is more bone-like and immediately surrounds the pulp tissue.

What's unusual about narwhals is that they have no enamel covering to protect their tusk,

and the cementum and dentin actually have fluid-filled channels called dentinal tubules

that interact with the environment and pass messages onto the nerve, which goes directly to the brain.

All mammalian teeth have these channels, but they're generally covered by enamel.

Otherwise the tooth becomes hypersensitive to stimuli and painful.

Think about what happens when you bite directly into a popsicle.

I can't even watch people bite into an ice cream without cringing.

Many people are like me and experience discomfort in their teeth, because our teeth are sensitive to the sudden temperature change.

And that's with a coating of enamel protecting our dentinal tubules.

Yet narwhals have a 2 to 3 meter projecting tooth without any enamel, which is nearly always immersed in arctic ice-cold water.

We don't know for sure that they get toothaches, but it certainly seems uncomfortable to have a fully innervated and sensitive tooth constantly exposed to icy water.

But maybe there's a good reason for such a sensitive tooth.

So researchers devised a fascinating test to see what it might be useful for.

They decided to measure how well a tusk can detect changes in water salinity.

This is potentially a very important adaptation

because one of the risks of living in the Arctic is getting stuck in what's called an entrapment.

Narwhals breathe air like all mammals, but they live in an environment that's absolutely covered in ice.

Of the 25,000 square kilometer waterways they traverse, there's often less than 3% open water,

and sometimes as little as half a percent of open water, and they absolutely can't break through the ice with their tusks.

And if the weather changes suddenly, with a burst of cold and wind, ice can form incredibly rapidly,

closing off the areas that narwhals need to surface to breathe.

The whales can become stranded without enough breathing holes, and hundreds will squeeze into shrinking openings in the solid ice.

Many unfortunately die if they can't get to the small ice gap in time.

And the ones that do manage to battle for space to breathe become easy prey for polar bears

that gather around the breathing holes.

Between 2008 and 2010 alone, locals reported four entrapments of large groups of narwhals up to 600 individuals,

due to the sudden growth of sea ice.

So how does salinity factor into the dangers of entrapment?

Because the ice contains very little salt, only the water freezes.

So the water around newly formed ice becomes much saltier than it is in other locations.

That means being able to sense a sudden increase in salinity would be very helpful for narwhals, because they'd know it was time to book it and avoid getting trapped in the ice.

Now back to the study.

Researchers captured six male narwhals and hooked them up to electrodes to detect their heart rate,

then made a jacket for the tusk so that they could fill it with fresh water and then a high salt solution.

The researchers found that narwhals' heart rate fell with fresh water and rose dramatically with saltwater,

probably because the narwhals were panicking about being trapped in ice, though that's not something we can ask them directly.

What the test did confirm for researchers is that the tusks have sensory capabilities, and that if the tusk is damaged, as was the case for two of the males,

they have a harder time detecting salinity changes.

And we know narwhals travel in pods, sometimes with hundreds of members, so maybe those who don't have a tusk are just taking signals from the ones that do.

But that doesn't explain what happens with an all-female pod when none of them have tusks.

So it seems like the tusk can't just be for this.

There's got to be more going on with this protruding tooth horn.

Despite the fact that narwhals only have two teeth, and neither are adapted for chewing, They actually eat fairly sizable fish,

including halibut, cod, and gonadus squid.

They find their prey using echolocation, then swallow them whole using a strategy called suction feeding.

Basically they create a vacuum in their mouth by retracting the tongue and just slurp up the fish.

Although narwhals are very hard to follow in the wild, we've learned a good amount about their diet thanks to their long history of being hunted.

Scientists have studied preserved remnants of their stomach, as well as stable isotopes of carbon

and nitrogen in their skin to understand what they're eating and how deep they're going to get it.

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In the past, researchers thought narwhals were picky eaters

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and would therefore be more likely to suffer from the combined effects of climate change and overfishing.

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But a recent study compared the world's three main narwhal populations from the Baffin Bay, the Northern Hudson Bay, and East Greenland.

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The scientists were surprised to discover that their diets were quite varied from one place to another.

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In East Greenland, the narwhals seemed to feed more in a pelagic food web, especially on capelin fish.

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The North Hudson Bay narwhals did the most benthic feeding, partly explained by the fact that they live in shallower water.

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They largely ate benthic shrimp.

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The Baffin Bay narwhals had the highest levels of nitrogen-15 isotopes, which means they feed at the highest trophic level of the three groups,

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including a lot of halibut.

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But possibly the most interesting of all was the differences between male and female diets.

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Males are significantly larger than females, and it seems that they make more frequent and deeper dives, so they can feed on benthic species more easily.

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It's possible that their greater size allows them to hold more oxygen for these dives.

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And then there's the presence of the tusk.

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Using it to stab prey, kebab style, seems like a bad idea, because the narwhals then wouldn't have any way of getting it off their tusk.

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But in 2017, scientists captured footage of narwhals using their tusk to smack fish, which seemed to momentarily stun them immobile.

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And from there, the narwhals could more easily hoover up the fish.

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This behavior has only been seen once, so it's impossible to say if it's a widespread practice.

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We also don't know if it makes males better hunters than females, but that certainly doesn't seem to be the case.

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So it seems like there still must be something more to this tooth horn.

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We've seen that narwhal tusks are sensitive to the environment around them, and useful for bopping fish on the head.

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But are these the real purposes of narwhal tusks, or just a nice side benefit?

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When a specific trait appears in the males and not the females of a species, scientists look at a couple of clues to try to figure out whether

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that trait is the result of sexual selection or not.

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One of the first things to examine is fitness.

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Not in the Jimbro way, but in the context of being able to survive the environment.

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In other words, are female narwhals without a tusk capable of thriving in the Arctic?

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The answer is yes, which suggests the tusk is not a crucial element of survival.

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So is this horn perhaps something more to do with mating, or sexual selection?

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To understand this, it's important to consider hyperolometry, which is common in sexually selected traits.

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It means that for the organism's body size, the sexually selected traits are larger than would be expected.

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Take the claw of a fiddler crab, for example, an important tool for aggressive posturing and for attracting mates.

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As a male crab's body gets larger, the claw doesn't grow in size with it in a 1 to 1 ratio.

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It grows even more.

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This steep line has a slope of 1.56, meaning the relationship between these beefy claws and body size is hyperallometric.

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And hyperallometry is common in sexually selected traits, because more exaggerated traits can more easily signal your genetic quality to your mates,

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even from a distance.

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So do narwhal horns fall into this category?

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To test the hypothesis that narwhal tusks are sexually selected,

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researchers compared the body and tusk size of 245 adult male narwhals collected from 1983 to 2018.

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Sure enough, they found the relationship between tusk length and body length to be hyperallometric.

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And another study found that a larger tusk size correlated to a larger pair of testes.

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And the tusk keeps growing for the entirety of the narwhal's life, so that thing is getting longer and longer the older they get.

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The real question about the narwhal is whether the visual of a big tusk alone is enough to attract a female, or if the males have some kind of dominance display as well.

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Researchers have found that 40-60% of narwhal males have damaged or broken tusks, and many of them have scarring on their heads,

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and even bits of tusk embedded in their jaw.

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The tusks can't withstand direct ramming forces, but they are resilient enough for lateral strikes.

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This might explain why we sometimes see narwhal males crossing their tusks above the water.

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Maybe they're doing some kind of jousting performance for the females.

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We just don't know, because their breeding period tends to be in March and April, when they're still living in the dense ice of winter.

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You might also be wondering, if these tusks are solely for the purpose of finding a mate,

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how come narwhals also have the ability to stun fish and sense how salty the ocean is?

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There is a couple of theories about that too.

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Maybe those traits have been retained from the evolutionary ancestor of narwhals.

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Maybe the tusks were used solely for sensory purposes until some

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of the females started selecting males for the size of the tusk.

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On the other hand, the tusk might be currently evolving into a sense organ.

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Maybe its extra abilities are nice little add-ons.

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We do know that crustacean claws aren't just about getting a mate, but can also be used as weapons, for hunting prey, and for chemical communication.

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Who knows, maybe someday all narwhals will end up with horns, male and female alike.

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For now, scientists aren't sure why some females end up with tusks, or why some males have two of them.

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The answer may be revealed in time, depending on which whales seem to be gaining or losing tusks over the generations.

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For now, narwhals remain one of the ocean's most mysterious animals.

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And even though I'm not a fan of a singular protruding tooth horn on most animals, it does suit the narwhal quite nicely.

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There's something truly beautiful about pondering the complexity of evolution throughout the billions of years life has existed on this planet.

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Whether it's wondering about the existence of a strange whale tooth, or pondering how a single cell becomes many,

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reproducing, mutating, giving rise to the sponges, worms, jellyfish, the invertebrates and the vertebrates,

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the cephalopods, birds, reptiles, mammals, and, eventually, us.

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跟读法 (Shadowing) 是一种有科学依据的语言学习技巧，最初开发用于专业口译员的培训，并由多语言者Alexander Arguelles博士普及。这个方法简单而强大：您在听英语母语原声的同时立即大声重复——就像是一个延迟1-2秒紧跟说话者的影子。与被动听力或语法练习不同，跟读法强迫您的大脑和口腔肌肉同时处理并模仿真实的讲话模式。研究表明它能显着提高发音准确性，语调，节奏，连读，听力理解和口语流利度——使其成为雅思口语备考和真实英语交流最有效的方法之一。

跟读练习: The Insane Biology of: The Narwhal - 通过视频学习英语口语

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