Science Facts

Why Do Walruses Have Tusks? – Evolution Of Tusks

Walruses Tusks

Walruses can be found in the Arctic Circle with all of their friends, or sometimes in Alaska. They have groups that they hang out in that range from the tens to thousands. About 15 million years ago, the Central Valley of California was covered by a shallow inland sea. This sea was filled with exotic marine life – ancient seals, massive sea turtles, and giant megalodon sharks. There were also walruses and three different species of them, which, it turns out. Because at least one of them didn’t have tusks, the other two probably didn’t, either. 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?

Modern walruses are 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 vast diversity of extinct walruses, like 20 different species, stretching from Florida to Japan and dating back to the early Miocene Epoch. So, where did they all go? Why is the modern Arctic walrus the only one still around?

Why do walruses have tusks?

The rise and fall of ancient walruses, and how modern ones got their tusks is a story that spans almost 20 million years. While there are parts of the story that we’re still trying to figure out, it looks like tusks. These are just enlarged upper canine teeth that didn’t have anything to do with how 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. A couple of 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. Scientists find fossils of another two walrus species in Japan. They were both on the smaller side, and at least one of them had sharp teeth for eating fish, but again no tusks.

The presence of all these early walruses shows us that these marine mammals diversified quickly. 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. And this was just the beginning. Paleontologists think that these animals diversified so quickly because they were geographically isolated. 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 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 as 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? It is a question that has puzzled paleontologists for years. For a very long time, walruses were 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 useful for stabilizing a big skull.
  • The second hypothesis was that tusks evolved to help walruses to pick their way across the ice. After watching living walruses, scientists came up with this idea use their tusks to haul themselves onto ice floes.

But Gomphotaria lived in temperate, warm waters where there was no ice. And another species of extinct tusked walrus has also been found in warm and ice-free places in the late Miocene – in places like South Carolina, the Netherlands, and Morocco. So, tusks probably didn’t evolve as ice picks. That leaves the third hypothesis: That tusks were used in competition between males for mates. Walruses today spar with their tusks, competing to monopolize groups of females.

The males posture at each other, sometimes even drawing blood. So that made scientists wonder whether male competition was at work in early walruses, too. In animals, males compete for groups of mates. And looking back in the fossil record, we can find evidence that one of California’s Central Valley species 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 polygynously living in groups of one male and many females from the very start of their lineage. It suggests that competition between males was common and may have eventually driven the evolution of tusks. But here’s the thing: big tusks can be uto ed as weapons, but they’re also useful for display. 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 includes one sex preferring certain traits in the other sex. In other animals, sexual selection tends to produce big, showy structures in males – like deer antlers or peacock’s tails. In fact, the most well-studied example of sexual selection is probably in deer. The fossil record shows that, as deer diversified into many species, their antlers diversified, too.

In deer, paleontologists think antlers evolved as weapons for fighting at first but eventually took on a new, separate role as a sexual display. In walruses, evolutionary biologists see the incredible diversity of tusks over the last 8 million years as similar evidence of sexual selection. When tusks became indicators of fitness, the thinking goes, male competition for mates didn’t have to be lethal. Instead, they could just intimidate with displays.

So, the fact is, the most elaborate structures in males rarely inflict damage in the fighting. Instead, they serve as indicators of a male’s status, size, and health. But having these huge tusks came at a cost. Walruses with tusks don’t eat fish because their giant canines get in the way of catching them. So, these walruses shifted to another feeding mode as their tusks got bigger and bigger due to sexual selection.

We can see this, for example, in the Pliocene walrus known as Valenictus. It had no teeth except for its tusks. But the arched palate in its mouth allowed it to create a vacuum that it could use to suck up mollusks from the sediment. Today’s walrus does have teeth. But it doesn’t use them for feeding. It just noses through sediments for mollusks, like clams, feeling around with its sensitive whiskers and just sucking the animal right out of the shell.

So, walruses evolved from having sharp teeth for eating fish to having giant tusks for competition and display in their long history. And the evolution of tusks coincided with the incredible diversification of walruses. Over time, there have been at least 21 species, in at least 18 different genera, with today’s walrus being the only surviving representative of the group.

Now, there are still gaps to fill in the fossil record. But it’s clear that the diversity of these animals was a roller coaster rising and falling several times as climate and sea levels changed over millions of years. Walruses suffered their final dropoff in diversity after the close of the Pliocene Epoch, 2.6 million years ago. Changes in climate and sea level and the presence of humans converged to reduce them to one species.

The males can weigh up to like 3,700 pounds. Their skin makes up about 20% of that weight. And the blubber that’s underneath the skin is half a foot thick. They can grow to be about three feet long and canine teeth that grow throughout their lives. They’re found in both males and females. And they use them for a variety of reasons. Sometimes they use it to haul themselves out of water because they’re so big. They also use it to make breathing holes in the ice. So they can be underwater for about 30 minutes. The males can also use their tusks aggressively to maintain their territory or protect their harems. Harems are groups of lady walruses that all have sex with one.

To The walrus harems have a ratio of 1:23, give or take. But fortunately for the gals, they don’t start giving birth until they’re about seven or eight years old. They’ll only give birth every three years because the gestation period is 15 months. And once they have the babies, the babies need help from their mamas for over two years.

The walruses have mutches, but they’re not hairs. They’re very similar to cat whiskers, and they are functioning organs. And they’re called Mystacial vibrissae. So they will use them to find their favorite snacks, specifically shellfish, at the bottom of the seafloor. Also, they’ll use their little whiskers.


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Sources:

Lowry L (2016). “Odobenus rosmarus”. IUCN Red List of Threatened Species.
Wozencraft, W. C. (2005). “Order Carnivora”.
“Walrus: Physical Characteristics”.
Fay FH (1985). “Odobenus rosmarus”. Mammalian Species. 238 (238): 1–7. doi:10.2307/3503810.
“Etymology of mammal names”. Iberianature.com.
“Morse, n., the etymology of”. The Oxford English Dictionary (2nd ed.).
Allen JA (1880). History of North American pinnipeds, US Geological and Geographical Survey of the Territorie.

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