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enter image description here

This famous tapestry shows all the "modernized" traits of a unicorn--basically just a pale-colored horse with a single horn on its head. Of course, single-horned animals do and did exist:

enter image description here enter image description here enter image description here enter image description here

The only problem with such comparisons is that real-life one-horned animals have their horns on a practical area of the head--the upper snout. That way, the animals can have a longer reach against a charging predator, forcing it to brake its feet. The unicorn, by contrast, has its horn usually set on the forehead. That's no problem for the usual two-horned animal, as the V-gap resulted by the two horns makes it difficult for an opponent to strike directly at the skull. The two-horned animals also have their horns ridged to prevent slipping.

But the unicorn of classic mythology and fantasy doesn't have either advantage, so a unicorn can't use its single horn to fight other unicorns. And it doesn't look sturdy, either, so of/defense against predators can't be a factor, either. With those two eliminated, what else would the smooth single horn of a real-life unicorn be used for?

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    $\begingroup$ In the last but one picture I see also an horn on the forehead. $\endgroup$ – L.Dutch - Reinstate Monica♦ Sep 30 '18 at 4:14
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    $\begingroup$ Many alleged unicorn horns do have ridges on them, as they come from narwhals.en.m.wikipedia.org/wiki/Unicorn_horn And depictions of them often show a ridged horn, depending on the medium that was used tapestries such as the one you have chosen don't have a good resolution but carvings and paintings are different..upload.wikimedia.org/wikipedia/commons/thumb/1/19/… $\endgroup$ – Sarriesfan Sep 30 '18 at 6:18
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    $\begingroup$ Carrying donuts would otherwise be difficult for an animal with no opposable thumbs. $\endgroup$ – David Richerby Oct 1 '18 at 12:22
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    $\begingroup$ @DavidRicherby and bagels, for those who are trying to cut back on sugar and fat. $\endgroup$ – Paul Oct 1 '18 at 22:45
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    $\begingroup$ An uncursed or blessed unicorn horn can cure: blindness except from cream pies and venom, deafness, confusion, hallucination, stun, sickness, nausea (from eating tripe/eggs), lost attribute points. $\endgroup$ – Jason Bray Oct 2 '18 at 20:14

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The only problem with such comparisons is that real-life one-horned animals have their horns on a practical area of the head--the upper snout. That way, the animals can have a longer reach against a charging predator, forcing it to brake its feet. The unicorn, by contrast, has its horn usually set on the forehead.

The late Miocene toxodontid Trigodon had its horn mounted on its forehead. I'm not sure exactly why it had it there - sexual selection, probably - but it at least justifies the position of a unicorn horn.

enter image description here

The horn is rather stubbier than that of a typical unicorn, but if it was a display feature, then it could certainly grow longer given time. If that's enough to satisfy you, then there you go; however, justifying any kind of spiral horn, like those in most unicorn depictions, is slightly more difficult.

One leading theory for the spiralling of narhwal tusks is so that they remain straight, and therefore streamlined - in a spiral pattern, even if the tusk grows irregularly, the overall shape of the structure remains quite straight. Streamlining, obviously, is unecessary for a land animal; perhaps straightness is also something that attracts females?

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    $\begingroup$ And then, siberian unicorns could have pretty impressive horns $\endgroup$ – Eth Oct 1 '18 at 11:38
  • $\begingroup$ The narwhal's tusk has a special growth mechanism because, as a tusk, its base is off-center. If it didn't spiral tightly toward the center, it would likely twist off to the side, like the tusks of most animals. True one-horned animals don't have this problem, they simply grow equally fast from both sides - in fact spiraling would make it less likely for them to remain straight. $\endgroup$ – IndigoFenix Jan 20 '20 at 9:13
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It could be an organ for communication.

As we know, Dolphins and Whales communicate by sonar sound waves. The teeth of dolphins are arranged in a way that makes them act like sonar antennae (read more about it here). They basically hear with their lower jaw.

The same principle could apply to Unicorns. They can create horse-like sounds, but do so only to communicate with other creatures. To communicate with another Unicorn, they create ultrasonic sound waves that are received and turned into nerve signals by their horns instead of their ears.

The center of the horn is packed full of nerve cells like a tooth. The ridged and tapering shape makes sound waves of different frequencies resonate in different parts of the horn. That enables them to distinguish different pitches of tones. By turning their heads and aligning their horns, they can determine the origin of the sound and thereby the position of the "speaking" unicorn.

Since young girls have high pitched voices, their singing might resonate in the horn as well. Thus the myth of virgins attracting Unicorns was born.

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    $\begingroup$ This would also make them more mystic, as they appear to understand eachother without people hearing anything. $\endgroup$ – Martijn Oct 1 '18 at 7:10
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    $\begingroup$ Having experienced raw and exposed dental nerves, I can attest to the effects of high pitched sounds being quite noticeable :P How we interpret that sensory stimuli is largely a matter of what our bodies expect to experience. +1 for referencing Dolphin teeth as precedent too! $\endgroup$ – Ruadhan Oct 1 '18 at 11:26
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    $\begingroup$ Don't Narwhal's also use their horn for this purpose? $\endgroup$ – Matt Oct 1 '18 at 13:54
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    $\begingroup$ What do you mean by "the V-gap resulted by the two horns makes it difficult for an opponent to strike directly at the skull"? Are you implying that the primary advantage of ordinary head-mounted horns is protection of the skull? Or are you trying to say that the force of impact from using a single horn as a weapon would be more harmful to the skull than in a two horned animal? Or are you saying something completely different? $\endgroup$ – user42528 Oct 3 '18 at 9:18
  • $\begingroup$ @Ben It's mentioned in an episode of "The Life of Mammals". $\endgroup$ – JohnWDailey Dec 13 '18 at 0:55
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Display.

A peacock's display feathers have no practical function and exist solely to demonstrate the animal's fitness as a potential mate. From a practical perspective, its display feathers are something of a liability.

The now-extinct Irish Elk had antlers so large in relation to body size that they seem unwieldy. It has been speculated that their large size served as indicators of the animal's health.

Likewise, a potentially fragile, functionally useless feature like a unicorn horn could serve as a signal to the opposite sex that this particular animal is of top genetic quality and an optimal mate.

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    $\begingroup$ A surprisingly unsubtle display, at that. $\endgroup$ – Shawn V. Wilson Oct 2 '18 at 0:48
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    $\begingroup$ When archaeologists can't figure out what something is for, they say it is a religious thingy. When biologists can't figure out what something is for, they say it is for attracting mates. $\endgroup$ – Keith McClary Oct 2 '18 at 4:42
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    $\begingroup$ Just to add to this, display features cost the bearer, a peacock's tail makes it easy to spot and easier to catch, therefore any male with a huge tail has clearly avoided predation. This is called an honest signal. A dishonest signal would have no impact on survival and so doesn't convey improved fitness but is "sexier". Dishonest signals can tend towards honest ones over time as males compete to be the "sexiest" by exaggerating said feature to obscenity! $\endgroup$ – Troyseph Oct 2 '18 at 7:59
  • $\begingroup$ I thought the "eyes" on the peacock tail also serve to scare things. As would the larger apparent size of the bird. $\endgroup$ – OrangeDog Oct 2 '18 at 9:11
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    $\begingroup$ Will upvote if you use the Irish Elk as another example ;) $\endgroup$ – josh Oct 2 '18 at 14:43
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It could be as simple as sexual selection

A lot of natural features that occur in animals have no other obvious reason for occuring and have become integral to a species. colour, frills etc.

The narwhal has one horn, we don't really know why it has it. But it's thought to be linked to dominance or mating displays of some sort.

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    $\begingroup$ About narwhals horn: "The tusk is an innervated sensory organ with millions of nerve endings connecting seawater stimuli in the external ocean environment with the brain.[22][23][24][25] The rubbing of tusks together by male narwhals is thought to be a method of communicating information about characteristics of the water each has traveled through, rather than the previously assumed posturing display of aggressive “male-to-male rivalry“" (Wikipedia) $\endgroup$ – eis Sep 30 '18 at 19:56
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    $\begingroup$ @eis thanks, I take those sorts of things with a grain (or ocean) of salt. $\endgroup$ – Kilisi Sep 30 '18 at 21:40
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Magic

Unicorns can use their horns to purify foul water, strip away malign enchantments, and heal injuries. But magic resides in the brain, so the horn needs a direct connection to the brain in order to efficiently channel magical energy.

The horn's spiral pattern and sharp tip help the unicorn focus and direct its magical energies.

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  • $\begingroup$ This is a REALITY-CHECK question, meaning NO MAGIC! $\endgroup$ – JohnWDailey Oct 1 '18 at 13:28
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    $\begingroup$ Maybe it's for the Magic of Friendship only, because that one is totally real. ;-) $\endgroup$ – Alex Oct 1 '18 at 14:11
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    $\begingroup$ @JohnWDailey how do you check the reality of a mythical creature? $\endgroup$ – RonJohn Oct 1 '18 at 14:31
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    $\begingroup$ @JohnWDailey I'd argue you need a science-based tag if you want to get snippy about the inclusion of magic in an answer when the question is about a traditional magical creature. $\endgroup$ – Ash Oct 1 '18 at 14:35
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I like the other answers but I would like to point out one advantage of a horn on the forehead, used as a weapon.

Think about the line of forces when the horn is horizontal and impacts a target at speed - a bit like a medieval knight's lance. In a nose based horn, the shock and impact is transmitted along the entire length of the skull - its maximum dimension in most cases - and then transmitted through a pivot join (the neck).

Clearly the neck must be very strongly muscled and the skull strongly built to withstand impact, but the closer the horn is, to the neck, the stiffer the bracing and the more efficient it is as a physiological support. You can imagine a knight who holds the lance tight to his side, at his hip, near his mount on the horse, and compare to a knight who somehow holds a lance at his shoulder where the lance wouldn't make such a solid impact, and is a lot harder to brace and withstand the force of impact. Basically, you want the impact braced as close as possible and on a direct line with its support, and the forehead of a unicorn meets that goal better than its nose.

A second reason it's a solid position is that the skull shape may also be less fragile and better able to spread and dissipate the force, at the forehead. It's more likely that bracing in the bone structure, and massive or stronger bones, could evolve on the forehead area of an elongated skull, as it does in many animals such as buffalo, bulls, rams, etc, and this might be more efficient in an animal with a horse-style muzzle that isn't as heavily built as a rhino's muzzle.

So there may be sound design reasons based on physics and physiology, why a forehead horn is a good position for a weapon horn, even though in many/most animals it isn't done that way.

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  • $\begingroup$ Ivory and Dentine are both pretty soft materials as they go. I'm having visions now of a unicorn smacking into a target..and taking on an appearance similar to Pachycephalosaurus as the horn splats across its forehead :P But more realistically, if the horn hit its target even slightly off-kilter it'd probably snap straight off regardless of material. Half a ton of horse behind a point less than a centimeter across is a hell of a lot of pressure! $\endgroup$ – Ruadhan Oct 1 '18 at 8:56
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    $\begingroup$ Yes :) But I'm discounting that, because of the way the question is asked. The OP isn't asking "how could a horn shaped this way be a practical weapon, wouldn't it snap?". They are focusing entirely on whether a forehead mounted horn would have any practical benefit/purpose compared to the same horn mounted on the upper lip, nose, or front of an elongated skull. So I'm only considering that aspect, And for that, I can see some reasons why mechanically it would be better mounted, more rigid, and better able to withstand more impact (or the same impact with less bone/muscle/brain cushioning) $\endgroup$ – Stilez Oct 1 '18 at 9:20
  • $\begingroup$ Personally I like the idea that it allows use as a weapon with less adaptation for impact - as you say it's sharp and long, so it would work as a stiletto or penetrative weapon at lower speed, less impact, and maybe being forehead mounted lets it do so without needing major physiological adaptation to brace it, so it keeps the horse's natural graceful head lines, and only very minimal skull/neck/collar area adaptation would need to evolve. If it's nose mounted, the head is long, not naturally strong at the tip, and could deflect sideways or be damaged on impact, needing more adaptation/change. $\endgroup$ – Stilez Oct 1 '18 at 9:23
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    $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$ – Ash Oct 1 '18 at 14:30
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    $\begingroup$ @JohnWDailey Sorry, didn't mean to sound rude; I meant the assumption that they would have less reach doesn't to me seem correct; once the head / neck is lowered into position I can't see how there would be any difference in reach for a horse shaped animal $\endgroup$ – Whelkaholism Oct 3 '18 at 10:55
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I am going to reuse my answer to the following question (emphasizing my lasr paragraph):

How do I explain a unicorn discharging powerful electricity at a distance?

TL;DR: the unicorn shoots lightning through it.

While it is true that air does not conduct electricity as nicely as water, it is also true that there is no (engineering) problem that cannot be solved by judicious application of brute force.

Lightning is a thing after all, so we know a discharge will happen in air if there is sufficient difference in electric potential.

Copy the design of the eel. This is what the wiki says about it:

In the electric eel, some 5,000 to 6,000 stacked electroplaques can make a shock up to 860 volts...

Now notice what the wiki says about its typical dimensions:

The electric eel has an elongated, cylindrical body, typically growing to about 2 m (6 ft 7 in) in length, and 20 kg (44 lb)...

If your unicorn weights as much as a draft horse... let's say one metric ton. It will have enough size and mass to have a lot more electroplaques. A back-of-the-napkin calculation says that, keeping the same volts-to-mass ratio, the unicorn will be able to produce 43,000 volts. You can buff that up - nothing is keeping the unicorn from having even more electroplaques. Let's say the unicorn is able to produce 50,000 volts. That is about as much as a typical low end Tesla coil. Now check the pictures in the link. It also has this to say about the coil:

The high electric field causes the air around the high voltage terminal to ionize and conduct electricity, allowing electricity to leak into the air in colorful corona discharges, brush discharges and streamer arcs.

A pointy part in a body is much more likely to produce a discharge than a round part or a toroid, so the horn is perfect for shooting out lightning. As for how the beast does not fry itself, it may have glass or some other very good electrical insulator material in its hooves.

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    $\begingroup$ @JohnWDailey Of course it does, it would be used to discharge electricity, as stated by Renan. You can't seriously bring up unicorns and dismiss answers like this. It might seem like a reach to you, but I would argue that a unicorn is a reach in itself. Ask your question on biology.se to get the real answer to your question. $\endgroup$ – Douwe Oct 1 '18 at 14:01
  • $\begingroup$ @Douwe thanks. The irony is that there is an answer stating the horn does magic, which isn't even a tag in the question, and it has been better received than this one where I try to get some proper biology into it. $\endgroup$ – The Square-Cube Law Oct 1 '18 at 16:22
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Stabbing.

Most animals' horns are a huge liability for a charging strike. You either have to go head down and rely on your neck muscles to keep your head from being snapped off, or use the charge for terror and swing your nose more like a morning star to deal more damage. In either case, the common theme is a shitload of neck musculature.

Unicorn horns are aligned with their spines. They're much better suited than most animals for impaling the shit out of whatever squishy thing pissed them off.

(Side note, it's not that practical to impale things with your head. There are reasons this isn't a common adaptation. What do you do when you have hooves, an open field, and a dead dog stuck on your head?)

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  • $\begingroup$ Spin, and let centripetal force remove it. With skill, let the dead dog go septic and fling it at an enemy den. $\endgroup$ – Harper - Reinstate Monica Oct 3 '18 at 3:20
  • $\begingroup$ If your legs bend the right way... just push it off. Or angle your head so the horn is pointing below level and shake until it slides off. $\endgroup$ – Matthew Jan 20 '20 at 17:58
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The horns purpose is to protect the beast's eyes. It can run as long as it can see, so it's most effective pre-horn predator learned to slash the eyes during the opening of every attack.

At first, the horn was short but sharp, making the predator pay for its attack with wounded claws. It's function was to discourage attacks, making all but the most desperate predators choose safer prey.

Later, as it grew longer in progressive successful generations, it became a true defense. Any animal wanting to reach a unicorn's eyes must expose its underbelly to a potential heart shot from the razor sharp weapon. In this role, the horn's placement mid-skull took optimal advantage of the horse's side mounted eyes, allowing the creature phenomenal aim without obscuring forward vision. In a front-eyed predator, the longer horn would have blocked some of its field of view; but in a side-eyed prey-animal, no such disadvantage came along with increasing length.

Would it have been more effective if it grew out of the nose rather than the skull plate? sure. But evolution doesn't require the best design, only the better design. A mid-skull mounted horn is better than no horn. That is enough.

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  • $\begingroup$ I would argue that your fourth picture shows you aren't presenting artwork consistent with your argument. That thing is an overweight unicorn with its smooth horn mounted in the classic unicorn location. I sure wouldn't want to try to claw its eyes with a frontal or overhead attack. I would get shiskabobbed while trying. Bulls and horses lower their heads during counterattacks, bringing a brow-mounted horn directly into the field of combat. I did read your whole question. And I addressed the superiority of snout-mounted horns in my fourth paragraph. Better is good enough for evolution. $\endgroup$ – Henry Taylor Oct 1 '18 at 13:52
  • $\begingroup$ You seemed to have missed the part about the horn being so long and so straight that it's delicate. $\endgroup$ – JohnWDailey Oct 2 '18 at 0:41
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    $\begingroup$ @JohnWDailey t'is no matter if it can grow back... $\endgroup$ – FreezePhoenix Oct 2 '18 at 12:10
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    $\begingroup$ @FreezePhoenix that might even be better, if they're herd animals like horses: stab, break off, and retreat would maximize damage while minimizing the danger of a double-hit or the issues with having a carcass stuck on the horn. $\endgroup$ – Morgen Oct 3 '18 at 16:15
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For hunting like narwhals.

As seen here it was recently (2017) seen that narwhals are capable of using their tusk as a hunting implement.

Drone footage has captured something no one's ever seen before - wild narwhals using their bizarre tusks to hunt Arctic cod by hitting and stunning them, making them easier to consume.

So if your okay with making your unicorns carnivorous (or at least omnivorous) you can have them hunt birds, or even small mammals with their horns.

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  • $\begingroup$ TIL: narwhal horn is actually a tooth. $\endgroup$ – Jordan.J.D Oct 3 '18 at 17:23
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To protect it from drop bears

But seriously, if you are introducing a new species (the unicorn), why not introduce another species that, during their joint evolution, necessitated the horn? Just like the ant-eaters long tongue is an answer to the tunnels in an ants nest, the unicorn's horn could be an answer to a property this other species has.

Maybe the horn evolved because unicorns hunted a woodpecker-like bird. As the bird evolved to dig ever deeper, the horn evolved to be ever longer. (Don't assume unicorns wouldn't hunt). Come to think of it, a variation on the drop bear might even be feasible, just don't call them drop bears I guess. The point is that when you introduce a fictional species, you might as well invent its ecosystem too.

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I'd like to challenge this assumption:

And it doesn't look sturdy, either, so of/defense against predators can't be a factor, either.

A fragile horn might be better for defense against predators than a more robust one, if the cost of growing one is marginal. Lizards which have a break-away tail and plants with detachable spikes have similar adaptations.

The unicorn is an equine, so unless there are some very good reasons for a different survival strategy, they're going to be herd animals. In a group defense situation, their default strategy may be similar to the muskox: create a circle with the young on the inside, horns pointing out.

This is already going to deter the majority of predators, as the horn is long enough that they wouldn't be able to get close enough to harm one unicorns without running serious risk of being impaled.

For those that are foolish or desperate enough to make a run at it, the majority of the time they'd be skewered.

In this case, being able to give the horn a bit of a twist and have it break off may be the fastest and easiest way to put distance between the the dying predator and the unicorn it's impaled on. This is desirable for two big reasons:

  • Preventing the dying predator from fatally injuring the unicorn it's now attached to is a really good thing for the unicorn.
  • Having a corpse decaying on the end of a unicorn horn would be problematic, both from the risk of infection as well as general awkwardness.

This isn't as much of an issue for a species with a smaller horn and more robust frame, as they simply toss the predator off their pointy bits in the same movement with which they impaled the predator. An equine body type is pretty strong, but not well adapted to lifting with the neck, so the evolutionary cost of strengthening the head and neck could very well outweigh the cost of producing a long and thin horn.

A whitetail deer can grow up to 200 inches of robust antler over 120 days. We can approximate the shape of the antler by assuming that the thick and thin bits even out and it's basically a cylinder. The circumference of that cylinder we'll ballpark at about 4.5 inches. Based on those values, we get 322.29 cubic inches total and a growth rate of 1.6 cubic inches per day, which we'll round down to 1.5 cubic inches per day for convenience.

Similarly, we can approximate the unicorn horn using a cone of the same base diameter, and we get 19.34 cubic inches for a 3 foot horn. Even if the horn were as robust as the antler of a whitetail deer, the rest of the herd could cover for a unicorn that lost it's horn for the 12 days it would take to completely regrow it's horn, or the 8 days it would take to get to a usable 2 foot horn.

A delicate horn that's intended to break away should require even less material, and would have a correspondingly reduced regrowth period, so 8-12 days provides a generous upper bound.

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  • the horn is unpractical to fight predators on the ground and is more efficient against predators coming from the air, as it already points upwards when the neck is in rest position.
  • primary defense would be to outrun the predator, as for other equidaes. It is helpful mostly if predators are capable of sustaining unicorn's speed over long distances.
  • the horn is magic, so it is especially helpful if predator is having magic shielding itself.

Therefore, the purpose of unicorn's horn is most likely to defend against dragons.

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    $\begingroup$ Your answer could use more detail on how it would help with airborne predators/dragons, that functionality isn't intuitively obvious. $\endgroup$ – nullpointer Oct 1 '18 at 13:52
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    $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$ – Mołot Oct 1 '18 at 14:05
  • $\begingroup$ A dragon would toast a unicorn, at least the ones described in the OP's post. $\endgroup$ – FreezePhoenix Oct 2 '18 at 12:14
  • $\begingroup$ @nullpointer: clarified, hopefully. $\endgroup$ – PypeBros Oct 5 '18 at 8:02
  • $\begingroup$ @molot: imho, it is a response. OP Q:"what purpose would the horn be", A: "to defend against airborne predators that cannot be outrun easily" $\endgroup$ – PypeBros Oct 5 '18 at 8:04
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It could possibly be used as a tool, like primate hands.

They can use it to scrape away dirt or bark to access food.

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    $\begingroup$ If that were the case, wouldn't it be more useful on the snout like a rhino horn? The OP is specifically asking about forehead horns. $\endgroup$ – Arcanist Lupus Oct 1 '18 at 4:34
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Bear in mind that the image of the unicorn you quote is based on artefacts which were thought at the time to be unicorn horns, but which we now know to be narwhal tusks. So a "practical" unicorn could have a much shorter horn (see the Wikipedia article for some alternative illustrations) and possibly positioned better.

Medieval bestiaries had some very odd ideas about real animals, so if it turned out that unicorns were real you would expect that the bestiaries would have got some of it wrong.

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Unicorn horns take years to grow, but once they are fully grown and the animal is mature, the unicorn diet changes to include meat. Having no hands, unicorns use their horns like a modern day shish kabob skewers. Unicorns that fail to grow a horn sufficiently long enough to cook meat will often times catch fire. So cooking meat on their horns is part of the natural evolutionary selective process. Only the most fit get to eat tasty kabobs and the rest turn into a pile of charred flesh.

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  • $\begingroup$ I have suggested an edit to remove part of your post. The part I have suggested to remove does not seem to fit in with the rest of the answer, and definitely does not answer the question. Please make sure that you keep answers on-topic. If you think that part of the post does fit in, please edit the answer to explain in greater detail how it relates to the rest of the post. $\endgroup$ – John Locke Oct 1 '18 at 21:08
  • $\begingroup$ Thanks @John Locke, I'll see if I can make my thought process more clear. Thanks for the edit! $\endgroup$ – GrandWizard Oct 1 '18 at 21:47
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    $\begingroup$ Is this practice of roasting kebabs on their horns tied in with mate selection in any way? I don't think a unicorn could reach the kebab he's just cooked without hands...but he could offer that bit of deliciousness to his paramour! $\endgroup$ – elemtilas Oct 2 '18 at 2:10
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Perhaps the unicorn's horn is placed higher on the head to allow it to align with the vertebrae and thereby reinforce a blow. It may be difficult to wield such a long weapon defensively, so maybe the unicorn is aggressive and attacks other unicorns or some other animal it perceives as a threat or as inferior to itself. Then having the long horn aligned with the spine would be beneficial in a charge.

Additionally, if the horn is long it will also be heavy, and portions of horn farther out will apply more torque to the animal's neck. Having the horn on the nose may strain the unicorn's neck, and so placing it closer to the central support of the neck may make it easier to tote as well.

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Unicorn fencing!

The unicorn uses its horn like humans use foils, épées and sabres - to majestically parry its opponent's horn and go in for a fatal stab.

To be sufficiently sturdy and flexible, the horn would possibly have to be made of some unusual material that a horn typically wouldn't be made out of.

For example, perhaps it somehow consumes rock and its body absorbs the metals/minerals into its horn? (After all, humans consume and absorb iron through different foods.)

Unicorn fencing has already been documented by a canny few on the internet.

(Taken from here on deviantart, image copyright of AdamSteve1984.)

(Taken from here on cafepress, design by Niadra.)

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  • $\begingroup$ Would the downvoter like to explain? $\endgroup$ – Pharap Oct 4 '18 at 18:53
  • $\begingroup$ Simple--it doesn't answer the question. $\endgroup$ – JohnWDailey Dec 13 '18 at 0:58
  • $\begingroup$ @JohnWDailey Why not? Most animals have some form of sparring. Deers use antlers for rutting, why wouldn't unicorns use their horns? $\endgroup$ – Pharap Dec 14 '18 at 1:27
  • $\begingroup$ "Deers is an accepted plural, but it is rarely used.". It's certainly less common, but it is still in use in certain parts of the world. I read the question, but I reject the premise that it can't be used for combat. The question only considers two techniques - charging (as a rhinoceros would) and gripping (as a deer would). This answer considers an alternative technique. If unicorns were to use their horns for parrying and thrusting then they would be perfectly adequate for fencing with. $\endgroup$ – Pharap Dec 14 '18 at 16:59
  • $\begingroup$ @JohnWDailey Exploring an avenue that the question has not explored is not 'disrespecting' the question. The question said that the way rhinos and deers use their horns for combat would not work with unicorn horns, so this answer proposes an alternative way for the horns to be used for combat. $\endgroup$ – Pharap Dec 15 '18 at 2:11
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As some of the repliers have pointed out here, there are animals that are thought to have had a single, large horn in the middle of the forehead, such as the toxodont Trigodon or the rhinoceros Elasmotherium. There is another toxodont, Hoffstetterius, that has an even more pronounced base for a horn core. Kubanochoerus is an extinct pig that has a very pronounced projection of bone in the center of the skull. No one knows what it was used for. There is also an extinct bovid, Tsaidmotherium, that has an asymmetrical set of horns with one being placed close to the center of the skull near the forehead. Ironically it seems to be a member of the same group that includes one of the possible real-life inspirations for the unicorn, the chiru (Pantholops hodgsonii)

Many sets of plate armor for horses also have spikes set in the middle of the forehead, though whether that is for function or aesthetic purposes I do not know.

Chris Lavers goes into some details at to what a single horn in the middle of the forehead might be useful for in his book The Natural History of Unicorns. Similar to some other repliers here, he mentions that a single, straight horn in the middle of the forehead would be much better for spearing and thrusting than a typical bovid horn. However, he also provides some direct observations that support this. In some parts of Africa, the horns of cattle are artificially manipulated during growth for aesthetic purposes. Some calves have their horns manipulated so they point straight up through the middle of the forehead, rather than projecting out to the side. As adults, these bulls are oftentimes dominant in competing with other males over access to females because they can be much more aggressive in fights due to their forward-pointing horn structure, and they do seem to fight with a thrusting motion rather than locking horns.

I remember there was some suggestion that horns in the middle of the forehead were hard to develop because of how horn cores usually develop at the boundary between two bones or something. I think it was in the same book, but I am not sure. Notably in horned toxodonts and Elasmotherium the horn is thought to have been keratinous.

As people have pointed out, a horn in the center of the forehead would be much better suited for charging behavior because it is aligned with the spine. I'd like to add that a horn or plate in-like with the spine was one of the lines of evidence that led people to think that pachycephalosaurs butted heads, and while direct headbutting is controversial it is pretty clear they were using their heads to hit something. Plus a lot of animals often have large sinuses in the forehead region that can act as good shock absorbers when the horn makes contact.

A unicorn would almost certainly have a ridged horn. Those ridges in living horned animals aren't really used for anything but are the result of natural periodic growth due to the fact that the horn grows from the base continually throughout life (in contrast to deer antlers or pronghorn prongs which are shed annually). Mark Witton goes into some detail how this works here. The ridges don't usually prevent slipping. An horned ungulate usually doesn't want to get its horns stuck in a struggling predator, the purpose of the horn is to keep the predator away from its softer body by prodding it with the pointy end. They're not designed to kill in the same way that spears are.

The really gracile, ridgeless, spiral-shaped, white horn you see in a lot of unicorn depictions is clearly modelled after a narwhal tusk, which is a modified tooth. Narwhal tusk was often sold as unicorn horn in medevial Europe, though the actual legend likely draw more from Asiatic rhinoceroses and the chiru (which has close-set horns that can look like a single horn from a distance). A "real" unicorn almost certainly wouldn't look like a narwhal tusk, unless the horn was grown and shed annually like a deer antler (which have no covering). Otherwise it would probably be brownish or black (due to a keratin covering), ridged, non-spiraled, and possibly much more robust.

There is also the possibility that a unicorn doesn't just have horns, but tusks as well, and the horns and tusks are used for different tasks. There is a tendency in many ungulates to lose tusks when horns are developed (this is seen in ruminants, brontotheres, and rhinoceroses, among others) but there are exceptions. The Indian Rhinoceros mostly uses its tusks in fights between members of the same species and tends to use its horn for defense against predators, or so I've heard. Kubanochoerus and horned toxodonts also have both tusks and horns. A unicorn could use tusks or enlarged canines in social disputes, but then use the horn in a thrusting or jabbing motion against predators. The comparison would be like a lance or a spear compared to the shorter, more sword-like reach of a cow or goat horn. The horn may not be sticking straight up as in popular depictions, but angled forward so that the frontal, parietal, and nasal bones can form a better base of support. A really long horn would force predators to stay further back to avoid getting stabbed, so long as it's not so long that it interferes with feeding. Getting struck by the side of the horn probably wouldn't be fun either.

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