A Biological Overview of Equus mannulus monoceros
A Physiological Study of Unicorn Ponies by Twilight Sparkle
As a unicorn, one might expect me to be biased towards my own race. As a researcher and a student of Princess Celestia, I can assert that I remain objective throughout this essay and others. The physiology of unicorns is largely unremarkable in comparison to other members of the Equus genus, and virtually unchanged in comparison to the evolutionary ancestor Equus ferus genesis, save for one feature: magic. Unicorns are remarkable among ponykind for their active magical ability, use of spellcasting and sorcery, and presence of a horn used for the manipulation of the magical aether as described in by Starswirl the Bearded in the Studium Magicae. Unicorns are defined by magic, which pervades across all aspects of their culture and physiology.
As stated above, the majority of Equus mannulus monoceros’ physiology is unremarkable. The skeletal system is neither as strong as that of an earth pony, nor as light as that of the pegasi. Similarly, the muscular system is less developed than the brawn earth ponies possess or the agile grace of pegasus physiology. Athletically, most unicorns can’t hope to compete with members of the other pony subspecies, as even with muscular training the cardio-respiratory systems of the unicorn are underdeveloped in comparison to the rest of ponykind. These drawbacks limit unicorns from many jobs requiring manual labor and as a result further emphasize their other talents.
Positioned atop the dorsal surface of the cranium is the defining structure of the unicorn. The Corneus Unus is a true horn, with a core of bone (Corneus Summa), surrounded by layers of keratin and protein. As the horn grows, the keratin and protein sheath develops along a spiral path along grooves in the Corneus Summa. This sheath carries pigment cells that assume the same color as the rest of the unicorn’s body, and differences in horn color are rare and may signify disease. The horn is very sturdy, and can heal from most non-magical injuries with no issues, allowing a unicorn to reliably use it for magical purposes.
Housed inside the Corneus Summa is the Corneus Nerve, a nerve that connects the horn directly to the Hippocampus and to the Magnus Gland, which translates synaptic messages into base spells for casting. Spellcasting takes an amount of exertion in direct correlation to the difficulty of the spell, with spells such as telekinesis becoming more difficult in relation to the mass, volume and range of the manipulated object. If a unicorn becomes practiced enough in magic, the exertion required for spellcasting lessens, and higher tier spells can be safely attempted. Magic proficiency varies between ponies, however, and much of it is based in the talents of the spellcaster.
The magical abilities of unicorns are far reaching, comprising the entire scientific branch of Thaumatology. Different schools of magic interpret unicorn spellcasting differently, but that is a matter for another essay. Unicorns are able to do much with their magic, with Pre-Classical examples of groups capable of moving the Sun and Moon. Such abilities have a high cost, however, as the caloric intake of a unicorn skyrockets with intense magic use. This required the Unicorn Kingdoms of the Pre-Classical Era to demand large amounts of food from the earth pony tribes under their control, a feudal system of serfdom that continued until the Great Migration in Equestria and until the Grand Revolution in Prance across the sea. The power held by unicorns influences their high-society culture even to this day.
Unicorns are known for the many noble houses in Canterlot and the many successful businessponies in Manehatten and beyond. With the abilities allotted to them by magic, many unicorns have pursued employment in manufacturing, engineering, and especially business jobs. This yields them higher incomes on average than pegasi or earth ponies, especially among the nobility. As Equestria industrializes, however, many of the old ways will soon be obsolete. Unicorns must adapt to the new world, or find that their power may perish with the last.
Yeah, there's really only one high point in unicorn biology.
The anatomy of spellcasting is quite interesting though. Unicorn physiology is an area I actually have spent some time considering. If you don't mind me unloading my headcanon here, I'd be happy to go into further detail. In either case, looking forward to more.
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Please do! I was really trying to fill space with the last essay and I like to think of this comment section as a constant flow of ideas.
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Great!
Okay, so the key difference between the two of us is the structure of the horn. I have a small cortex, the thaumic lobe, emerging from the corpus callosum and poking out of the forehead. Also in there is the cerebellum minoris, the seat of magic "muscle memory" for routine spells like telekinesis. And yes, this warps the frontal lobe a little, in multiple senses of the word. There does seem to be a correlation between magical proficiency and madness.
Anyway, the thaumic lobe serves the role of your Magnus Gland, converting intent and the raw magical energy produced by a pony metabolism into coherent spells in the same way your mind can subconsciously do the calculus for predicting a ball's flight path in the time it takes you to catch it. Thaumic lobes very rarely achieve self-awareness, and as Rarity can attest, such episodes are short-lived and usually benefit the unicorn in the long run.
The rest of the horn consists of thin meningeal layers and an outer surface of keratin. The horn's spiral groove is caused by the shape and arrangement of the dermal papillae that produce the keratin fibers, and its coloration by the same chromelanin content as the rest of the unicorn's coat. (Yes, my headcanon contains pigment proteins.) The meninges are somewhat vestigial, because the horn's primary line of defense is magical reinforcement of the keratin, dramatically increasing its hardness, tensile strength, and shock absorption. This reinforcement is constant, unconscious, and efficient enough that it uses much less energy than a unicorn typically produces. If a unicorn doesn't have the magic needed to maintain this reinforcement, she probably has much bigger problems than a more easily concussed thaumic lobe, such as her imminent death or enslavement by a soul-consuming, ape-faced centaur.
A lesser version of this reinforcement applies to a unicorn's musculoskeletal system, especially the legs. (After all, unicorns had to keep up with the herd somehow!) This makes unicorns stronger and tougher than one would think from looking at them... but still not all that much when compared to the other tribes. It also makes it harder for unicorns to build up muscle, because their magic is always doing some of the work for them. The degree of reinforcement is directly related to the strength of the unicorn's magic.
On an interesting cultural note, this has made thin limbs very attractive to unicorns (well, to those attracted to unicorns, but that's a different story,) because all else being equal, underdeveloped muscles are an indicator of strong magic. Of course, once unicorns realized they found this attractive, all else ceased to be equal. It's entirely possible that trying to cultivate this look contributed to the more sedentary lifestyle of unicorn nobility. Amusingly, between her ludicrous magic and her scholastic obsession, Twilight ended up becoming one of the most beautiful mares in Canterlot without even realizing it. (Rarity was instantly envious. Applejack just tried to get as much food in the wasted little thing as possible before she keeled over.)
I think you see why I wanted to make sure you wouldn't mind me going into more detail.
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I had toyed with the idea of having the cerebrum extend out into the horn, much like your thaumic lobe, but I wanted to be sure that the removal of the horn by injury would not cause permanent brain damage (so as to allow for the show and other fanfics to remain consistent). The Magnus Gland is much like the Pituitary in terms of magical growth (the Cake Twins), but random spurts of magic can be common at young ages, even with Rarity, who I believe casted her first gem-finding-spell that helped her obtain her cutie mark.
As for madness, does magical proficiency equal psychosis or other mental defects? Maybe. Twilight isn't the most sane pony in Equestria, nor was Sombra (the other most powerful unicorn we've seen), and we have no idea about Starswirl.
Then comes the point about the musculature of unicorns. I personally believe that unicorns' base abilities lie in active magic casting, and that going beyond that would make them 'too overpowered' (though it would make sense then for them to be nobility). We rarely ever see them doing manual labor, so I assume that earth ponies are better suited for the job. Maybe there is some form of passive magic backing them up, but then why is it that Twilight was unable to push a snowplow in Winter Wrap Up?
As for the passive reinforcement charms? I agree that makes sense. The horn rarely ever gets damaged in the show and the inherent properties of the horn probably work to negate physical damage. Sombra's horn survived his death. Celestia's horn survived losing to Chrysalis. It makes sense that the horn is the strongest part of the unicorn.
This is all wonderful thinking, FoME! You really put a lot of thought into it!