(host) Just a few million years ago, on what are now the Great Plains of North America, there lived some very weird rodents.

At their largest, these animals were only about the size of a small marmot, so it wasn't their size that made them so strange.

It was what they were sporting on their heads.

These odd rodents belong to a genus known as Ceratogaulus, but they're more commonly called horned gophers because-- you guessed it-- they had horns.

Specifically, they had a pair of horns that sat side by side on their nasal bones, and these horns could get pretty big relative to their body size.

And they're the only rodents we know of to ever have had horns, which is pretty incredible considering that almost 40% of mammal species alive today are rodents.

So what would these ancient rodents have needed horns for?

Paleontologists have been puzzling over that question ever since the first horned gopher was described in 1902.

That paper even described its anatomy as being absurdly like that of a miniature rhinoceros, which, yeah, it kind of is.

Hypotheses about the function of these horns have included digging, competing for mates, recognizing other members of their species, and defense against predators.

And it would take just over 100 years and a grad student bringing all of the evidence together to finally settle the debate.

It turns out the horns probably did have a purpose-- one that rodents would likely benefit from today.

But unfortunately for modern rodents, it looks like that group only got one evolutionary shot at horns-- eh, at least that we know of.

The horned gophers are part of a family of squirrel-like rodents called mylagaulids that have no living members.

Their closest relatives that are still around today are these kind of ugly-cute rodents known as sewellels-- also called mountain beavers, even though they aren't actually beavers.

But the mylagaulids first appear in the fossil record in western North America during the Oligocene epoch around 30 million years ago, and diversified into many different species of fossorial-- or burrowing--rodents.

The first two species of horned gopher show up around 16 million years ago in the Middle Miocene epoch.

They were the smallest of the horned gophers and had small nasal horns.

Then, around 12.5 million years ago, one of these little guys disappears from the fossil record, and four new, larger species of horned gophers appear, three of them with taller horns.

The largest of these was Ceratogaulus hatcheri.

It had nasal horns that were 33 millimeters tall, over 1/3 the total length of its skull.

And it stuck around until about 5 million years ago before becoming extinct, possibly due to the rise and spread of new groups of carnivores throughout North America at the end of the Miocene.

The point is, it's clear from the fossil record that the horned gophers got bigger and their horns got taller over time.

And this would be an important clue for paleontologists trying to figure out how they developed the horns in the first place and what they used them for.

So what drove this odd evolutionary innovation?

Well, one of the first ideas about the function of those nose horns was that they were used for digging.

We know that the mylagaulid ancestors of the horned gophers were burrowers.

These extinct rodents used their noses to excavate dens and tunnels to avoid predators.

This style of burrowing is called head-lift digging.

The snout and the head are essentially used as a shovel, with the neck muscles doing most of the work to lift the dirt out of the way.

And there's physical evidence of this adaptation in the skeletons of these animals.

For example, the back of the skulls and the neck vertebrae of head-lift diggers have enlarged areas for powerful muscles to attach, and the nasal bones at the tip of the snout tend to be thicker to support an abrasion-resistant roughened nose pad.

And when you've got thicker nasal bones and muscles that can support a heavier head, you're already part of the way to nose horns.

But none of the modern animals that are head-lift diggers have horns, which means horns probably didn't help with digging at all.

In fact, once the first horned gopher evolved its horns, it actually would have been worse at digging than its hornless ancestors.

The position of the horn and its shape make it inefficient when you're trying to tunnel forward.

And over time, the horns only got taller and moved away from the tip of the snout, making them basically impossible to dig with.

It turns out becoming a better digger was not what drove the evolution of the horned gopher's horns.

But having digging ancestors gave it the adaptations it would need to eventually support its horns.

Okay, so what about competing for mates?

While there aren't any modern animals with horns that dig by lifting dirt with their heads, There are definitely mammals alive today that use their horns to show off to or fight for potential mates.

One horned gopher species is even named for one of these beasts.

Its scientific name is Ceratogaulus rhinocerus.

But in a lot of those horned modern species like impala and kudu, the males have horns and the females don't.

And it looks like both male and female horned gophers had horns.

And just like in the digging hypothesis, their anatomy also gets in the way of them being able to use their horns to spar with each other.

Because they use their heads to dig, these rodents tended to have a shortened, stiffened neck.

Basically, the gophers wouldn't have been able to tilt their head down far enough to point their horns at their rival, especially in the species with the tallest horns.

And as far as using the horns for display instead of actual combat, well, the female would have needed to actually see the horns or the victorious male to be impressed by them.

But it looks like horned gophers actually had really poor eyesight, based on the size of the holes in the gopher's skull that the optic nerve would have passed through.

This is also the problem with the idea that the horns were used by gophers to recognize other members of their species.

If they couldn't see very well, a visual signal would have been pretty useless.

Which brings us to the final hypothesis-- defense against predators.

Remember how we said the gophers got bigger over time?

Well, it looks like these larger gophers probably spent more time foraging above ground than their smaller ancestors did.

And the Great Plains were only getting more open and grassy as the Miocene went on, providing even less cover for a rodent.

And these things put them at the mercy of potential predators.

Throughout the middle and late Miocene, while the horned gophers were getting ever larger horns, new predators arrived and diversified in North America.

The two groups that were most likely to have been a problem for the horned gophers were the skunks and badgers.

These new predators were semi-fossorial, so they would have had no problem going after burrowing rodents.

And their fossils are found in the same places as those of the horned gophers.

The gophers also might have had to worry about death from above.

Avian predators like hawks and other birds of prey have also been found at the same fossil sites as the horned gophers.

As the gophers got larger and pressure from predators increased, their horns got taller.

And these tall, robust horns could have been used both for protection and active defense by the gophers.

Lifting the head and tipping it backwards would have positioned the horns above the eyes and neck, helping to protect these vulnerable parts from predators trying to grab the gopher from above.

By doing this quickly using those powerful neck muscles, it could have been a forceful enough strike to deter a potential predator.

And even hungry weasels don't like to get hit hard in the face.

So it looks like the best hypothesis for why these rodents evolved horns was for defense.

The last of the horned gophers, Ceratogaulus hatcheri, survived in the grasslands of Nebraska and Kansas until the earliest part of the Pliocene epoch, around 5 million years ago.

And there hasn't been a horned rodent since.

From the adaptations of their digging ancestors, the horned gophers evolved a truly unique defense mechanism, one that no other rodent ever has.

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