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The only rodents to ever have horns on their noses called North America home more than 5 million years ago. A new species of these horned gophers has now been discovered, and it suggests that they used their horns to fight off predators.

It was thought that there were five species of horned gopher, all of which belong to a group of squirrel-like rodents called mylagaulids. They lived between 16 and 5 million years ago. “The ones with horns are only found in North America,” says Jonathan Calede of The Ohio State University at Marion.

His co-author, Joshua Samuels of East Tennessee State University, spotted part of a horned gopher skull in a museum collection. The two realised it belonged to a sixth species, which they named Ceratogaulus cornutasagma.

The pair then fit the new species into the family tree of mylagaulids, to see when some of them evolved horns and how their bodies were changing. This allowed the researchers to test hypotheses about why the horns evolved.

Although the first horned mylagaulid was described in 1902, the function of the horns is still uncertain. They are probably not a way for males to show off to females, as both sexes have them. In 2005, Samantha Hopkins, now at the University of Oregon, proposed the gophers used their horns to defend against predators.

Calede and Samuels found a strong relationship between the gophers’ body size and the presence of horns.

“When the animals get bigger is when we see the evolution of horns,” says Calede. There then seems to have been an evolutionary pressure for the horns to increase in size. “As the animal gets bigger, the horns get bigger, but they get even bigger than you would expect based on the size of the animal,” says Calede.

This offers a clear story, says Calede. The first mylagaulids were burrowing animals, but large animals cannot spend too much time burrowing, because it takes too much effort.

So, the large mylagaulids must have spent a lot of time above ground, and that would have made them vulnerable. “When you’re a big, fat rodent living above ground, you have ‘lunch’ written all over you,” says Calede. A big, sharp horn would have helped them fight off predators.

The horns probably evolved from a thick nasal bone that the first mylagaulids developed to help them dig, says Calede.