Name: How the Walrus Got Its Tusks
Uploaded: 2021-05-03T13:20:46.000Z
Duration: 8 min 17 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

About 15 million years ago, the Central Valley of California was covered by a shallow inland

And this sea was filled with exotic marine life - ancient seals, massive sea turtles,

and giant megalodon sharks, which, I want to remind you, are super duper extinct.

And there were also walruses - three different species of them, which, it turns out, were

Because, at least one of them definitely didn't have tusks.

And the other two probably didn't, either.

Like, if they didn't have tusks, how did scientists know these animals were walruses?

Aren't they a key feature of what we consider to be … walrus-ness?

And, if you can be a walrus without tusks, why does the only species of walrus alive

Modern walruses are literally named for these features; their genus name means “tooth-walking,”

because they've been observed pulling themselves out of the water with their tusks.

But there used to be all kinds of walruses in the oceans!

The fossil record has revealed a huge diversity of extinct walruses - something like 20 different

species - stretching from Florida to Japan and dating back to the early Miocene Epoch.

Why is the modern Arctic walrus the only one still around?

The rise and fall of ancient walruses, and

how modern ones got their tusks, is a story that spans almost 20 million years.

And while there are parts of the story that we're still trying to figure out, it looks

like tusks - which are literally just enlarged upper canine teeth - didn't have anything

to do with how or what these animals ate.

Instead, the walrus probably got its tusks because of sex.

The earliest known fossils of walruses have been found in Japan, Oregon, and California,

and they're from the early Miocene epoch, around 17 million years ago.

They were all smaller than their modern relative, and none of them had tusks.

And a couple million years later, when those three species were swimming around the Central

Valley, walruses had already started to diversify, developing different body sizes and diets.

But again, at least one of them definitely didn't have tusks.

So paleontologists had to use other features to recognize them as walruses.

It turns out, tusks are not one of these animals' defining features.

Instead, they can be identified by a wide bone on the upper jaw that's found only

in walruses, as well as a characteristic ear bone, and really big molars.

Now, fast-forward to the late Miocene, about 10 million years ago, and we find fossils

They were both on the smaller side and at least one of them had sharp teeth for eating

The presence of all these early walruses shows us that these marine mammals diversified really

By 10 million years ago, six species had already become extinct, but three more species had

shown up in the fossil record to take their place.

Paleontologists think that these animals diversified so quickly because they were geographically

As sea levels rose and fell over millions of years, different populations

became isolated from each other and started to evolve independently.

Now, looking forward another couple million years, to around 8 million years ago, we find

fossils of an enormous walrus in Oregon and California.

Pontolis was the size of a modern elephant seal, with a big, long skull.

But, while its canines were sharp and fang-like, they didn't stick out of its mouth.

It would take another million years before the first tusked walrus appeared.

It was also pretty huge, and it had short, stout tusks on both the upper and lower jaws,

as well as these weird, tusk-like third incisors.

And it didn't eat fish, like its ancestors did.

Instead, it fed on shellfish - judging from the extreme wear that's been found on its

teeth and tusks, it looks like it just crushed them up, shells and all.

So, what drove the evolution of tusks in walruses?

This is a question that has puzzled paleontologists for years, because, for a very long time,

walruses were obviously doing just fine without them.

The first hypothesis was that tusks evolved to stabilize a walrus's head while it trawled

the seafloor for food, kind of like the runners on a sled.

But Gomphotaria's tusks were short and stout, and their shape made them not really useful

The second hypothesis was that tusks evolved to help walruses to pick their way across

Scientists came up with this idea after watching living walruses use their tusks to haul themselves

But Gomphotaria lived in temperate, warm waters where there was no ice.

And another species of extinct tusked walrus has also been found in places that were warm

and ice-free in the late Miocene - in places like South Carolina, the Netherlands, and

So, tusks probably didn't evolve as ice picks.

That leaves the third hypothesis: That tusks were used in competition between males for

mates - but not always in the way you might think.

Walruses today spar with their tusks, competing to monopolize groups of females.

The males posture and jab at each other, sometimes even drawing blood.

So that made scientists wonder whether male competition was at work in early walruses,

Because, in animals where males compete for groups of mates, you tend to also see differences

in body size between the sexes, with males being bigger than females.

And looking back in the fossil record, we can find evidence that one of the species

from California's Central Valley had bigger males than females.

In fact, if you look even further back in their history, you'd find this pattern of

having larger males in the ancestors of walruses that lived more than 20 million years ago.

So it looks like walruses were polygynous -  living in groups of one male and many

females - from the very start of their lineage.

And this suggests that competition between males was common, and may have eventually

But here's the thing: big tusks can be used as weapons, but they're also useful for

And traits like these can be acted on by sexual selection.

Sexual selection is a type of natural selection that's driven by competition for mates.

It includes males fighting other males over females, and also includes one sex preferring

As I'm sure you've noticed in other animals, sexual selection tends to produce big, showy

structures in males - like deer antlers or peacock's tails.

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How the Walrus Got Its Tusks

literally

stick

eventually

incredible