Size Matters: The Evolution of Unicorn Horns

With the recent discovery that unicorns do exist, we must take a serious look at these often misunderstood animals. Set aside any notions you may have of magical creatures frolicking beneath rainbows. Unicorns aren’t magic. There is no magic in them except perhaps that magic which drives all living things to survive and grow and ultimately pass their genes on to the next generation.

Within our modern taxonomy, unicorns belong to an order of mammals called perissodactyla (the odd-toed ungulates). They share common ancestry with moose, elk, antelope, etc; but their closest living relatives are Equus caballus, better known as horses. Therefore, zoologists have assigned them the scientific name Equus unicornicus.


Of course, the unicorn’s defining characteristic is its horn: a long, nearly straight protrusion from the forehead, pointed at the end and circumscribed by a continuous helical groove. By now, we’ve all seen these beautiful horns and perhaps marveled at their pearly luster, but how did unicorns evolve this distinctive feature?

Forget Natural Selection

The development of unicorn horns has little to do with natural selection in the typical sense. Forget about survival of the fittest. As we’ll soon see, growing large, cumbersome horns can make a species less fit for survival. Instead, unicorn horns evolved through a process which evolutionary biologists call sexual selection.

Female unicorns, which can be identified by their smaller, more diminutive horns, only become fertile for a few quick weeks per year. During mating season, boisterous males engage in almost ritualized contests against each other while stoic females watch on.

Males begin by snorting and stamping the ground with their hooves. This is followed by vigorous head movements meant to display the length and girth or each male’s horn. If one male or the other does not at this point back down, the situation soon escalates into what is best described as a jousting match. And when unicorns begin to joust, the unicorn with the biggest horn wins. Always.

Roughly 90% of male unicorns never get the opportunity to mate even once in their lifetimes. Their horns simply can’t compete. This may seem like a shocking statistic, but it is comparable to the statistics for other horned or antlered ungulates. For animals like the unicorn who must fight over mates, size truly does matter.

When Horns Get Too Big

As you can imagine, the intense competition among males creates strong evolutionary pressures that favor larger horns. Genes coding for the largest horn sizes are virtually guaranteed to be passed on to the next generation while genes coding for smaller horns are rarely if ever passed on.

But the top 10% of unicorn males, those that mate with 100% of the females, pay a high price for their reproductive success. Growing the largest horn requires nutrients. It requires energy. It requires increased caloric intake. And yet for males giddy in the throes of mating season, there’s little time for grazing.

Instead, a male unicorn will leech calcium and phosphorus from its own bones, diverting these minerals toward horn growth. If all goes well, the animal can replenish these nutrients after mating season is over. If not… well, let’s just say fending off rival males while suffering from calcium-depleted bones does not always end well.

In the remote plains and steppes where unicorns can be found, field researchers have observed animals in deplorable health in the aftermath of mating season. Males generally appear weak and malnourished, afflicted with broken ankles and legs in addition to bearing nasty scars about the face and all over the body. Many will not recover their strength in time for winter and will likely die–though if they were among that lucky 10%, their genes will already have been passed on to the next generation.

In their suffering, unicorns are not unique. They share their struggles with other horned and antlered ungulates, and indeed with many species throughout the animal kingdom. Some readers may still wish for the magical creatures of legend. But perhaps by studying unicorns from the perspective of evolutionary biology, we can better understand some of the magic that has made life possible on Earth.

Okay, unicorns aren’t real. The point of this article is that all creatures, both real and fictitious, must adapt to evolutionary pressures, doing whatever they need to do in order to pass their genetic information on to the next generation.

My research for this article began with a book called Animal Weapons: The Evolution of Battle by Douglas J. Emlen (click here to see an interview with the author). I did additional research concerning various horned and antlered ungulates, particularly the long-extinct Irish elk, trying to understand the behaviors and characteristics all these animals have in common.

When creating a new species, especially for the purposes of science fiction, a little research can go a long way toward establishing believability. I’ve also heard a rumor that doing your research can earn you bonus points in the Alien August competition here on Sci-Fi Ideas.

Article by James Pailly. Check out James’ blog for more great science articles.


  • Kirov

    Yep, evolution is the key to alien design. It’s probably what I should start from, though in actuality, I come up with something I think is cool, then try to explain it through evolution. If I can’t, there will sometimes be revision to the original idea. Sometimes.

    • J.I. Borrero

      Guilty of the same thing. Starting from evolving an organism tends to be a bit too abstract for me.

  • Leonardo Faria

    Most of this is highly speculative. The collection of data gathered by the ethologists is pathetically poor. The observational conditions of the unicorns are so adverse that we have to have the honesty to admit we know almost nothing about them. Your hypothesis that the horn has to do with sexual selection is as good as mine when I say it is a sample of vestigiality. It survived its ancestral function to transfix the stupid Hobbits that wanted to remove their scrotums believing they were a powerful aphrodisiac. Which also explains why female unicorns don’t have horns, without resorting to the sexual selection theory. As to the taxonomic reference to the perissodactyla order, that’s just absurd. It was the romantic iconography of the Victorian Age that imposed the image of the horse, and Hollywood went gaga over it. Whoever has seen a unicorn — and I have — knows it’s a big goat.

  • Excellent article! Particularly the point about evolution not always favoring what we might describe as the fittest.

  • If only male unicorns have horns, does that mean female unicorns aren’t magic? :/