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Horse girls considered

Thu 11 Apr 2019 by mskala Tags used: , , , ,

Uma Musume Pretty Derby (ウマ娘プリティーダービー, hereinafter UMPD; the first couple words translate as "horse girl") was a hit anime series in 2018. It quickly became a favourite topic of fan artists on Twitter and the federated network, largely because of the cute character designs. It's basically a sports anime, which is not exactly my favourite style, and it took me some time to get around to watching it, but I finally did in December and I became interested in a number of questions about the world-building.

I'm sure UMPD was never meant to be "hard SF" and the writers, throughout the series, just did what they thought would be cute without regard to whether it made any sense; but let's see how far we can go on the assumption that this show actually does depict a consistent world that makes some sort of sense. What kind of world is it?

This posting will be full of spoilers and assume some knowledge of the show. If you haven't watched it, maybe you should do so first, at least Episode 1 to get some idea of the world. To summarize, UMPD is a show about creatures called "horse girls," who look like human girls but have horse-like ears and tails, and are capable of running very fast. Their main activity is running in, and training for, horse girl races, which are a lot like horse races in our world. They're also something like Japanese "idols" (アイドル) and the winners of the races give song-and-dance performances to cheering crowds of glowstick-waving fans. Much of the show follows one horse girl named Special Week (named, like all the horse girls, after a real-life racehorse) as she pursues her ambition to become the best horse girl in Japan.

[An electronic billboard promotes Special Week's participation in the Dream Trophy race]

A note on terminology: I will write "true horse" where I mean the kind of horses that exist in our world (Equus ferus). Whether horse girls are properly human (Homo sapiens), or something really different from human, is an interesting unsettled question, which I'll address in detail below, so I will usually write "non-horse-girls" when I mean humans other than horse girls, and avoid using "humans" to mean a set that doesn't include horse girls except when I'm writing specifically in the context of an hypothesis that they really are a separate species.

Some specific hypotheses about what's going on in the UMPD world will be indented like this. These may conflict with each other; it's not my intention to claim that all are simultaneously true, only that each of them is worth considering.


Some questions

There are a lot of questions that immediately occur to the thoughtful viewer and are not answered within the show. Here are some we might want to think about. I don't have answers for all of them, and often what seems a good answer to one will make another more difficult.

Official comments from production personnel answer a few of these questions but don't really clear up much that viewers wouldn't have reasonably guessed. In particular, the studio says that there are no horse boys; that alliances between non-horse-girl humans and horse girls are known to occur (the original Japanese is marvelously vague as to what that might actually mean); that some horse girls compete in ways other than racing; that some horse girls are not good at running and train primarily for the song-and-dance idol thing (but we have seen that in the show already); that the matter of their fathers is a secret; and that there are no true horses in the UMPD world. (Japanese-language source.) All my hypotheses below are pretty much compatible with these points, although some would imply a certain amount of deception on the part of the studio.

A surprising kick

There's an important scene in Episode 1 when Special Week and Trainer-san first meet. She is watching a race and not paying attention to anything else. He kneels behind her and starts feeling up her legs. She is outraged, considering his actions to be sexual harassment, and kicks him in the face. Then they repeat this entire set piece - she's distracted, he kneels behind her and puts his hands up her skirt, she objects - again just a few minutes later.

[Just the kind of guy her mother warned her about]

That right there implies some degree of sexual compatibility between horse girls and human males. It may be taboo for Special Week to actually have sexual intercourse with Trainer-san; it may be even be physically impossible; but it is at least thinkable, enough so to be threatening. The existence of a massive commercial enterprise devoted to holding up horse girls as sex objects - "idols" - to human boys and men, also supports the idea that sex between human males and horse girls is thinkable and it is something that both horse girls and human males do think about. This may be a clue to how horse girls reproduce: human or at least human-like males are probably involved, else everybody would not be so interested in that.

On the other hand, girls in anime can also be molested by octopuses. That fact provides no useful insight at all into how humans usually reproduce. It's also easy to imagine that someone who has spent her whole life among non-horse-girl humans will have picked up their social expectations and taboos regardless of any direct personal relevance. So maybe Special Week's reaction to being unexpectedly felt up by a human male proves nothing specific about her reproductive biology.

There's more of interest in these scenes. Both times, Special Week kicks Trainer-san in the face and calls him a pervert. We don't get a clear view of her posture the first time around, but the second time, at the peak of the kick she appears to achieve a hip extension angle of about 80 degrees! In the blurred close-ups shown for the first face-kicking scene, her foot at least appears to be moving in the same kind of direction.

[Special Week extends her hip about 80 degrees]

She may be bending her spine backward and tilting her pelvis (her clothing and bag make it hard to judge this), so that not all of the 80-degree angle we see is really hip extension. It's also possible that the plane in which she's moving her leg may be tilted a little away from the camera, so that perspective foreshortening exaggerates the angle. But those effects could only account for a few degrees each. Most non-horse-girl humans can move their hips to a maximum extension angle of about 20 degrees. There's a lot of individual variation and many humans can go further than that, but the most flexible non-horse-girl humans can only reach about 40 degrees. Age and sex do not correlate strongly with this ability, and training can only improve it a little bit; it is mostly determined by the shape of the pelvis.

Special Week is leaning forward in the picture in a way that looks like she's doing it to raise her thigh, as a substitute for extending her hip further still, so she is probably at her limit of hip extension. And - as her later star status implies - she may be a more than usually capable horse girl. But even with all these effects in play, it seems clear that her range of motion in her hip is greater than that expected for a non-horse-girl by a basically unbelievable amount. How can she move her leg like this without injury? And given that she can, why aren't her other movements, especially her gait when walking and running, significantly different from human standard in order to make use of her ability?

[A true horse kicking, with the bone geometry labelled]

True horses cannot do it either. When they kick backward they do so mostly by flexing their "stifle" joints, equivalent to human knees; and that is also how non-horse-girl humans kick backward. The range of motion for true horses' hips is significantly less than that achieved by non-horse-girl humans, let alone Special Week. True horses can't even reach a hip extension angle of zero degrees measured in the way we measure it for humans. They can kick horizontally only because their spines are near horizontal when standing and we're inclined to look at the fibula (shin) rather than femur (thigh) when assessing the angle; it would be unheard-of for a true horse to safely bend the femur 80 degrees backward relative to the spine, as Special Week seems to. So in order to do this without injury, her hip joints must be not much like a human's, and even less like a horse's. What kind of animal is she?

Her ankle rotation in the picture also seems unusual, but that may be easier to justify with a comparison to the abilities of some ballet dancers. It's hard to measure in this shot, but it may not be far out of range for a non-horse-girl human, and it's easy to imagine that horse girls in general and Special Week in particular may have a little extra range. The hip seems like a bigger problem.

Trainer-san does something impossible here, too: he survives. Special Week kicks him directly in the face so hard that his entire body is lifted, flipped, and thrown a few metres away. Note that in the picture he is not just falling backward from a standing position; he was kneeling and the kick raised him into the air. She does that to him twice in Episode 1, and other horse girls kick him in the face under similar circumstances in Episodes 5 and 13; as he says, "I'm used to it." Groups of horse girls also gang up and kick him in a couple of other episodes, but not directly in the face, they're kicking forward instead of backward, and those scenes looks more playful than seriously violent.

The kick in my screenshot from Episode 1 should kill him instantly. Surviving it is not really something he can expect to improve with practice or "get used to." Humans in our world kicked directly in the face by true horses do often die, and they're seldom if ever really kicked as hard as what's shown. But in the event, all Trainer-san suffers is a mild nosebleed - which by anime genre convention could have been caused just by the sight of Special Week's panties - and that is a clue to the natural hypothesis for what's really going on here.

In the Japanese idiom, this picture is an image. The animators are just horsing around; the scene didn't really happen as shown. Anime genre convention is that girls who feel sexually harassed may be shown inflicting impossible violence on the offenders (sometimes conjuring weapons like giant wooden mallets out of thin air for the purpose) without lasting consequences within the story, and this is meant to be perceived by viewers as comic exaggeration rather than literal reporting of events. Special Week's hip extension in this scene, Trainer-san's survival, and so on, are meant to be funny and cute and underscore the horseyness of horse girls - oh look, she kicks just like a horse! (or superficially appears to; never mind that true horses do not really kick like that) - and we are not meant to believe that the characters could do such things in a more serious context.

That's a dangerous path to start down because carrying the same logic to an extreme would suggest canonical evidence never matters at all: anything in the show that we don't want to believe, can be dismissed as "didn't really happen." But the genre convention of impossibly violent response to perceived sexual harassment is so strong and so widely accepted that it may be possible to draw an unambiguous line between this incident and other things in the show that we should still accept as literal.

Racing attire

In races classified as "G-II" and below, horse girls run wearing the same sort of clothing that human runners use, with the exception of their shoes, which have medium high heels and metal horseshoes attached to the soles at the front. They all wear the same clothing, even in the same colours, distinguished by numbered bibs.

[Silence Suzuka in uniform for a lower-level race, bib number 12]

In the Dream Trophy race of Episode 13, which seems to be a special event outside the normal series, they all wear a standardized uniform in a different colour for each horse girl. But in G-I races, which are the highest level of normal racing, horse girls run wearing elaborate costumes. Each horse girl good enough to compete in G-I races has her own unique costume for doing so, which becomes part of her trademark or brand identity, and many of the costumes seem inappropriate or even dangerous for high-speed running.

[Special Week receives her G-I racing costume]

In Episode 10, there's a G-I race in which one girl wears a bulky top hat with no obvious way for it to stay on her head (actually, someone who may or may not be the same horse girl wears such a hat in a couple of lower-level races too, which raises further questions), and another wears what looks like part of a suit of samurai armour. Most of the costumes include puffy skirts that would be expected to cause aerodynamic issues. Some include high boots that would impair the horse girls' ankle motions, and many, perhaps even a majority, of the shoes or boots have high heels. Because of the wide variation in costumes, it seems like some of the girls would have significant competitive advantages or disadvantages just because of costume choice, so if the horse girls or their trainers are allowed even a little freedom to select costumes, and they actually want to win, then it's surprising that they would not choose costumes better suited for running.

[Horse girls running a G-I race in individual costumes, many of which seem inappropriate]

We might guess that maybe the winners need to wear idol costumes for the "Winning Concert" after the race, and they don't have time to change, so they all wear their idol costumes in the race too, against the possibility that they might win. But first of all, that's silly. It's only plausible that the program of events would be arranged to give the winners reasonable time for any required costume change before they have to go on stage. Also, although they wear their G-I racing costumes at some other public events like press conferences, not only races, they do not wear racing costumes in the Winning Concert after all. Except in the Dream Trophy race, they change before going on stage, into standardized idol uniforms which have the same design for everybody and every event. Changing out of the complicated G-I racing costumes would be expected to take even more time than changing out of the lightweight "gym strip" racing uniforms used at lower levels - which could be worn under the bulky stage uniforms, making their removal unnecessary in a quick-change situation.

So, none of the obvious reasons for horse girls to wear the unsuitable costumes that they do wear in G-I races seems to make much sense - unless that unsuitability is actually deliberate!

G-I races are handicap races, like many true horse races in our world. The fastest girls are given the silliest costumes, which slow them down more, so that the chance of winning will be closer to uniformly distributed over the set of competitors. Having competitors matched evenly both makes the races more exciting to watch and improves the operation of the gambling business.

It seems possible that faster horse girls do wear more cumbersome costumes. In the Prix de l'Arc de Triomphe, for instance, Broye and El Condor Pasa wear a big flappy cape and long coat respectively, which would create a lot of aerodynamic drag; everybody else has less bulky clothing, they are the two horse girls most expected to win, and they are the ones who actually do finish first and second. But it's not clear-cut that the costumes correlate with someone's assessment of horse girls' speed in all G-I races, and testing this hypothesis would require a lot of data we don't have.

[El Condor Pasa and Broye about to race in the Prix de l'Arc de Triomphe; they have bulkier costumes than the other competitors]

The anime studio has claimed that there is no gambling on horse girl races and people just watch for fun, but as astute commentators on Internet fora have pointed out, given human nature that really means, at most, that there is no legal and officially recognized gambling on horse girl races. And I'm not sure we should even believe it that far. The studio might have to say it regardless of the truth if they're presenting this as a show for children to watch.

Furthermore, commentator characters within the show often refer to one horse girl in a race as "the favourite" - which in true horse racing has a specific technical meaning. The favourite is the competitor with the shortest odds in the betting market, especially if those odds are shorter than even (implying that bettors are behaving as if they rationally believe a greater than 50% probability she will win). Maybe this term is being used in a more vague sense for horse girls as just "the one who seems most likely to win," but I wonder. Even if we really believe horse girl races are only for fun, it would seem that the industry could provide the most fun by making it possible for every competitor to win, and that could be a reason for handicapping.

One significant problem with the handicap hypothesis is that although we don't know who assigns the costumes (Special Week at least did not choose her own; it was presented to her by Trainer-san as a surprise, with no indication of who designed it), it appears that the costumes are assigned per horse girl rather than per race. The same horse girl will wear the same costume in all her G-I races. That really limits the possibility for effective handicapping because it forces whoever's making the decision to commit permanently to a level of handicap based on the horse girl's performance in lower-level races, before her first G-I race. If her level of performance changes, or was guessed incorrectly, the handicap will be inaccurate and will not serve its purpose.

And horse girls seem to run just as fast in G-I costumes as they do in lower-level races where everyone wears identical plain "gym strip" uniforms; the costumes do not actually seem to have any effect on speed even though it's unbelievable that they wouldn't. And there never seems to be doubt that the winner of each G-I race will be one of the horse girls who performs well in lower-level races without costume handicapping; naturally-slower horse girls do not end up with anything like an equal chance of winning.

However, it could be that even though it does not work, this handicapping system is used anyway. For instance, it could be a matter of unassailable tradition. Sports often have rules that don't entirely make sense, but are still the rules players and teams follow. It could also be that the majority of horse girls who ever get to compete in G-I races are only able to do so once or twice in their lives, in which case locking in a single handicap level for a horse girl's entire G-I career isn't considered a big problem. The horse girls we follow in the anime are among the best in Japan and possibly the world. They participate in many G-I races over careers that are at least several years long, but maybe that's not a typical pattern. Or it could be that in fact the costumes do get changed or re-assigned sometimes and we just happen not to have seen clear examples within the narrow scope of the show.


Let's talk about the shoes specifically. The show contains plenty of shots of horse girls' feet, which look exactly like non-horse-girl human feet. They could presumably wear the same shoes that are worn by non-horse-girl humans, at least for street wear; there might be some kind of higher performance requirement (a need for stronger materials, etc.) to withstand the rigours of superhuman running speed, for shoes actually worn while running.

[Horse girls shopping for race shoes]

In Episode 10 there's a scene in which several horse girls shop for "race shoes" in what looks like an ordinary human shoe shop. It's not clear whether this whole shop specializes in shoes for horse girls; whether there are a few shelves of horse girl shoes in a shop primarily catering to non-horse-girls; or whether horse girls just wear the same shoes as non-horse-girls (perhaps with some slight modification that they do themselves) and so aren't limited to buying special horse girl shoes. One interesting point here is that they apparently have a choice about which shoes to buy. They have to wear standardized shoes in lower-level races; if they have a choice for G-I races then it's a challenge to the handicap hypothesis; and if these are actually shoes for street wear and not for racing, then it's odd that they would refer to them as "race shoes."

If it's a shop that only sells shoes especially for horse girls, that may imply quite a large population of horse girls to support the existence of such a shop. On the other hand, Tokyo is a big city, and we don't know that more than one such shop exists. Horse girls don't need to be a large percentage of the population for a city the size of Tokyo to reasonably contain just one or a few shops specializing in their shoes.

True horses wear shoes of a certain curved shape which is so distinctive that the word "horseshoe" is also used to describe many other objects with similar shapes, and the shoes are nailed to their feet. True horse anatomy makes this possible without serious injury. The shoes are usually made of steel for true horses in general, but often aluminum, or even some high-tech non-metallic material, for racehorses in particular, because of a desire to reduce weight. In the culture depicted in UMPD, the horseshoe is the iconic symbol of horse girls. It appears as a decorative element in their costumes, in the architecture of the school, in the video graphics associated with race advertising, and so on. They even eat horseshoe doughnuts in the end credits of the show.

[Horse girls eating horseshoe doughnuts]

It seems strange for the horseshoe symbol to have originated and to have become associated with horse girls, given that in our world this shape has a specific utilitarian function with respect to the anatomy of true horses. Horse girls do not have feet like true horses, and there seem to be no true horses in the UMPD world; where did they get the idea? Do other basically equine animals with horse-like hooves, such as zebras and donkeys, exist and wear metal horseshoes in the world of UMPD? They are not visible in the show, but that proves little given the scope and setting of the show. In our world you can spend a long time in Tokyo without ever seeing direct evidence of zebras or donkeys, even though they do exist in the world.

Horse girls have feet just like non-horse-girl human feet, and they cannot safely nail objects to their feet. But they do wear metal horseshoes that look like true horse horseshoes, fastened to the soles of their human-style shoes near the toes. There's a scene of Special Week's foster mother presenting her with an inscribed gift horseshoe to wear during a race. It is sort of plausible that horse girl horseshoes could simply be meant to improve traction, and that the significance of the horseshoe shape in the UMPD world's culture just comes from the fact that that is the shape of the things that horse girls happen to attach to their shoes. Some human runners in our world wear shoes with cleats or spikes, especially if running on turf; if runners were considered an identity group then just maybe a stylized cleat could be their symbol. The curved ridge created by a horseshoe attached to a human shoe does not seem like it would be ideal for improving traction, but it could be of some benefit, and there could be weight of tradition or other unknown factors behind why they use this method of traction improvement instead of others.

We don't get any unambiguous indication of what kind of fasteners are used to attach the horseshoes, but there are several scenes in which horse girls maintain their own or each others' shoes, and it involves tapping on the shoes with hammers. It seems clear that the horseshoes are fastened to the human-style shoes with fasteners at least a little like our world's horseshoe nails. Many of the shoes have thin soles. If the nails are long, then why do they not pass through the soles of the shoes into where the wearer will insert her feet, and make wearing the shoes impossible? If the nails are short, then especially under race conditions when those shoes are hitting the ground with extreme force in a way that might tend to drive the nails further in, why is there not a serious risk of a nail poking through the sole of the shoe and injuring the wearer's foot? Here's a thought.

Horse girls nail their horseshoes to the soles of their human-style shoes with nails that do pass all the way through the soles to the interior of the shoes. But horse girls are so strong that when they put on their shoes after attaching horseshoes this way, their feet bend the projecting nails, clinching the nails flat against the insides of the shoes and giving a more secure attachment to the horseshoes than would be possible with friction-held nails alone. These nails are really more like rivets, held in place by being bent, and they may be made of a more ductile material than our world's horseshoe nails in order to make it possible.

That demands an unusual strength from the human-style shoes to keep them from being ripped apart by the forces involved, but it's at least sort of plausible. These shoes could be made of stronger materials than the ordinary human shoes they resemble.

I have no explanation for the high heels except deliberate handicapping. Another question for which I have no answer is why Silence Suzuka in particular, even after barely recovering from an injury that could well have ended her career or even killed her, still wears lace-up shoes that frequently come undone just as she's walking onto the racetrack. That looks terribly unsafe. If the rules of the sport mandate a specific type of shoe, we would expect the rules to require a more, not less, secure way of fastening them.

[Silence Suzuka's shoe comes undone right before a race - again]

How fast?

Here's a collection of specific numerical references to horse girl speed in UMPD; I exclude stuff like stair-climbing which wouldn't be directly comparable to race speeds on a flat course. Most horse girl races seem to be over similar distances to true horse races, and the girls usually run at a little less than their top speed over most of the distance in order to conserve strength and sometimes (like velodrome bicycle racers, though to a less extreme extent) to use aerodynamic tactics, with a sprint near the end. Silence Suzuka - about whom we have the most hard data - seems to be unusual in often keeping a lead through the entire race, and a disagreement with her previous trainer about the advisability of doing that seems to have factored into her decision to move to Team Spica.

Episode 1, first race, Silence Suzuka: described as leading over the first 1000m with a time of 57.8 seconds (62.3km/h). Her trainer thinks that's an excessive pace and she will tire herself out before the finish, but in fact she seems to accelerate significantly near the end of the race.

Episode 1, Team Rigel tryout, Special Week: after a poor start, she runs the whole race (length unknown) in 1:48.09 for second place (second to El Condor Pasa, who was sufficiently far in front that Special Week did not immediately realize she came in second herself), but Trainer-san comments that she ran the last three furlongs (603.5m) in 33.8s (64.3km/h); that's sufficiently impressive to recruit her.

Episode 6: Trainer-san foreshadows the next episode by speaking hypothetically about how if "a horse girl who is running at 70km/h" were to fall, it could kill her; implying that at least approximately that speed is possible and maybe even expected during sprints.

Episode 7, Tennoushou (Autumn) G-I, Silence Suzuka: runs the first 1000m in 57.4 seconds (62.7km/h), then accelerates, and suffers a near-career-ending fracture of her lower leg, though not actually a fall.

Episode 11, practice for the Japan Cup, Broye: her time for a practice run is shown on a stopwatch as 56.26s for "lap 2" and 2:03.18 total; Trainer-san comments that he doesn't believe it, and it's better than Rudolf's record-setting time. If we assume the practice run is the same distance as the actual race, and that in turn is the same distance as our world's Japan Cup for true horses, then it's 2400m and her time works out to 70.1km/h.

Episode 12, Japan Cup: a commentator says that the horse girls as a group have covered the first 1000m in 60 seconds (60km/h) and that that is an average pace.

For comparison:

2009, Usain Bolt: 100m sprint in 9.58s (37.6km/h), our world's current male human record over this distance

1988, Florence Griffith-Joyner: 100m sprint in 10.49s (34.3km/h), our world's current female human record over this distance. (Record is officially accepted but there is some evidence that it may have been "wind assisted" and not recorded as such because of a defective measuring device.)

1999, ISTAF, Hicham El Guerrouj: 2000m in 4:44.79 (25.3km/h), our world's current male human record for this distance

2017, Mítin Internacional de Catalunya, Genzebe Dibaba: 2000m in 5:23.75 (22.2km/h), our world's current female human record for this distance

2008, Winning Brew: two furlongs (402.3m) at 70.8km/h, current Guinness world record for fastest speed of a Thoroughbred racehorse; notably, this was a female horse

1999, Tennoushou (Autumn) G-I, Special Week (Thoroughbred stallion of this name): 2000m in 1:58.0 (61.0km/h), fastest whole-race speed of his career

From these numbers it looks like we can conclude that horse girls run about as fast as true horses in similar-length races. This is between two and three times the speed of non-horse-girl human runners on the same distances, and a shade less than twice as fast as non-horse-girl human sprinters in 100m races. The 100m race is usually thought of as the fastest race for humans. (At shorter distances they don't have time to accelerate to full speed; at longer distances they start to tire.)

I really can't think of any sensible explanation for how they're able to run so fast. Horse girls have what look like human skeletons. Living muscles should not be able to move limbs that size and with that geometry at the speeds they do. Living bone should not be able to withstand the stresses placed upon it by moving that way. True horses manage to run at these speeds in normal mammal bodies only by having completely different body geometry from humans. Horse girls are shaped exactly like humans except for their ears and tails, and their speed seems to contradict the limits of biophysics. Maybe we can speculate about their bones being made out of something like silk (which is biologically produced in real life and might be strong enough) or Kevlar (not naturally occurring in our world, but chemically similar to silk, and it could plausibly be produced by exotic biology), and some other weird biology for their muscles.

At least their diet seems quite reasonable under the circumstances. They emphasize foods that might be eaten by both horses and humans. Special Week in particular has a thing about carrots, but other horse girls like them too. She eats carrots in large quantities, she uses denial of carrots to motivate herself, she sleeps with a carrot body pillow, writes letters on carrot stationery, and so on. Is it too much of a stretch to point out that even her girlfriend looks like a carrot, with an orange and green colour scheme?

[Silence Suzuka, and Special Week's body pillow, both resemble carrots]

But anybody might have a strong individual food preference. Horse girls also eat many other things that true horses would not - such as red meat. When a group of horse girls pig out at an expensive restaurant in Episode 7, it's on Japanese cuisine - hotpot, sushi, and so on - exactly such as might be eaten by non-horse-girl humans in a similar situation.

[Horse girls eating expensive Japanese cuisine]

It is notable that we do not ever see horse girls eat things humans couldn't, like the fresh grass and hay that are the usual diet of true horses. It sure looks like they have human-like digestive systems and cannot digest cellulose in the way true horses do. Except for the emphasis on "horsey" foods, but only as far as those overlap with human foods, and which could well be more cultural than biological, horse girls seem to eat just like humans do.

[Oguri Cap and Special Week with large lunches]

But they eat more. We can't infer too much without knowing both the restaurant price level and how much he gets paid, but Trainer-san complains that it cost him two months' salary to satisfy seven horse girls. In other scenes, horse girls are seen routinely gobbling down large multi-course meals for breakfast, lunch, and dinner. This makes a lot of sense: running as much as they do at the speeds they do, horse girls must be burning far more calories than is usual for humans, and it's plausible that they have typical human digestion and metabolism, just with the numbers increased. They would then naturally eat the same food as non-horse-girl humans, just in larger quantities.

Are they human after all?

They sure look human.

Horse girls seem to be very much like humans in very many ways. They talk like humans; they move like humans, except faster when running. They eat the same things humans eat. Notably, on almost all points where humans and true horses differ - such as diet - horse girls are like humans and not like true horses. And there is a entire commercial industry built upon holding horse girls in front of human males as sex objects. That bears on the interesting question of how they reproduce: it seems natural to guess that they are human, and they reproduce like, and with, other humans. But what we've seen of horse girl families creates some challenges to that view, for which we'll have to account.

Horse girls are human. They have some sort of genetic factor that gives them special ears, tails, and very high athletic ability; but other than that factor and its consequences they are biologically identical to any other members of Homo sapiens. Their general reproductive biology and sexuality are identical to those of non-horse-girl human females except maybe for some cultural differences in expected sexuality, and they reproduce as other human females do, by mating with typical human males. Only females can be horse girls. Two non-horse-girl parents cannot have a horse girl daughter, so horse girls always have horse girl biological mothers and non-horse-girl biological fathers. It is unknown whether a horse girl can have sons or non-horse-girl daughters (and thus whether a horse girl can have same-mother half or full siblings who are not horse girls too), but the answer is probably "no." A horse girl could presumably have non-horse-girl half-siblings sharing the same father.

I at first thought the horse girl determining factor (HGDF) that makes someone develop as a horse girl would naturally be some sort of sex-linked cluster of genes on the X chromosome, but on careful consideration I don't think that works and I'm not sure HGDF can be part of the main human genome at all.

The issues for genetics come from the fact that it doesn't appear possible for two non-horse-girl parents to have a horse girl daughter; horse girls' biological mothers, at least, must be horse girls too. In Episode 2, when told that Special Week has never met another horse girl before, Silence Suzuka asks what about her mother, and is told it's complicated. In Episode 12, when Special Week's foster mother comes to Tokyo and refers to Special Week as "my daughter," people seem surprised - although that one's not clear-cut, since in each instance they also have other reasons for reacting as they do. But it certainly appears that everybody expects a horse girl's mother to be a horse girl herself.

It also seems likely, though this is less well-supported, that horse girl mothers can only have horse girl offspring. Note that the only biological relatives of horse girls that we have seen in the show are other horse girls, though granted we have seen very few examples of horse girls having relatives at all. There's Special Week and her birth mother; the three Mejiro sisters (who refer to a grandmother in one of their conversations); Narita Brian and Biwa Hiyahide, who are described as sisters; and there is a very brief shot in Episode 12 of two child horse girls and one adult horse girl, implied to be a family, watching Special Week on television. Apart from the absence of males, families including both horse girls and non-horse-girls seem to be exceptional and the only known example is by adoption.

[A family group of horse girls]

Gwern has written some interesting stuff about the plausibility of the Bene Gesserit breeding program in Dune, and the inadequacy of Mendelian genetics at explaining inheritance of complex traits like intelligence, which leads to the inadequacy of breeding based on Mendelian genetics for producing complex traits. But the trait of being a horse girl looks like it obeys the hard nonlinear inheritance patterns that intelligence does not. We have seen no evidence of there being a continuum of different degrees of horse girl status. Nobody has the tail and not the ears, nor vice versa. You either get the whole package or you don't. Even if there is some hidden continuous or nearly continuous variable and a threshold for it beyond which someone suddenly gets the entire horse girl phenotype (which Gwern describes as a very successful model for some complex traits with visibly simple apparent manifestation), it doesn't seem to match what happens in UMPD. In particular, the inferred rule that horse girls must have horse girl mothers and it's surprising when one seems not to, doesn't go with the hidden-variable concept at all.

In the footnotes to the Dune article, Gwern makes some comments about the inheritance of race in My Little Pony: Friendship is Magic, which might be of interest for horse girls if only for the equine aspect; but as he admits, there are so many free variables in the models under consideration there, that it's hard to justify any of them as necessarily implied by the available evidence.

If HGDF were only a matter of having a special sex chromosome with otherwise our world's human genetics, then because each horse girl will inherit a non-horse-girl X or Y chromosome from her non-horse-girl father, she must be able to develop as a horse girl while bearing only one of the special sex chromosomes, that coming from her mother. It can't be necessary for both sex chromosomes to be special. Then in turn, half her offspring wouldn't inherit a special chromosome at all and would develop as non-horse-girls.

We could suppose that there exist "carrier" non-horse-girl men who possess at least part of HGDF without being horse girls, and maybe they are the only ones able to father horse girls, but then it seems hard to exclude the possibility that male carriers could have horse girl offspring with non-horse-girl mothers, contradicting the rules that seem to apply. Some of these issues can be reduced by assuming there are mechanisms other than straight sex-chromosome genetics that cause horse girl embryos carried by non-horse-girl mothers, or non-horse-girl embryos carried by horse girls, to spontaneously abort. But that statement alone doesn't really answer the question (it only comes down to saying "it is answered, somehow") and it might mean that horse girls end up with very low fertility, hard to justify on evolutionary grounds.

If we keep the hypothesis that horse girls are part of the human species and by definition of that, they are fertile with other humans, then the best hypothesis seems to be that HGDF is something outside the main genome, something that reproduces asexually while the main genome reproduces sexually. In particular: it could be in the genome of, or it could basically be, an intracellular parasite or symbiont. There's precedent for that in mitochondria.


Mitochondria are basically former parasitic bacteria which have evolved to become permanent and necessary cooperative parts ("organelles") of their host cells, and in sexually-reproducing eukaryotic organisms like humans the mitochondrial genome is inherited only from the mother. Maybe horse girls are simply humans with special mitochondria, or other things that are like mitochondria, and embryos develop as horse girls if and only if they inherit horse girl mitochondria-type things. That would neatly provide both the requirements that horse girl daughters always have horse girl mothers, and that horse girl mothers always have horse girl daughters. Embryos with XY genetics and HGDF might either abort, or develop as female regardless of the Y chromosome. XY horse girls would have reduced fertility because YY is not viable, and they might not be fertile at all, as the rare "XY female" humans in our world; but this doesn't make the whole thing completely implausible. Existence of XY horse girls might have something to do with why many horse girls, including Special Week and Silence Suzuka, are named after stallions. (The two racehorses with these names were also half-brothers, both sired by Sunday Silence.) It also might be an alternate explanation for the fact discussed under a different reproductive hypothesis, below, that some horse girls seem to be quite masculine in certain ways.

The link to mitochondria is interesting because of the role of mitochondria in metabolism, and the biophysics of horse girls: when running they're evidently able to burn far more food energy than is possible for non-horse-girl humans, and that suggests they have special metabolism and maybe special mitochondria. Mitochondria and hypothetical mitochondria-like things have long been a subject of fascination for science fiction authors. (A Wind in the Door; Star Wars; etc.) Also consider Wolbachia, a genus of bacteria that infects many insect species, has complicated effects on their reproduction, and in some cases seems to be in the process of evolving into a cooperative role such as mitochondria have. Effects of Wolbachia on host insects may include forcing offspring to be of specific sexes. (That's a vague statement because the exact effects vary a whole lot across different species.) If HGDF is basically an intracellular parasite or symbiont combining features of both mitochondria and Wolbachia, it could explain all the observed or guessed rules of horse girl inheritance while keeping horse girls within Homo sapiens and not having too many implausible additional consequences. So that seems the best hypothesis for how inheritance of the horse girl phenotype could occur.

Horse girls have special intracellular bodies: different mitochondria, mitochondrion-like organelles, or intracellular symbionts, in addition to or instead of non-horse-girl human mitochondria. The horse girl determining factor is something in the genome of, or simply equals the presence of, these bodies. They are passed asexually from mother to daughter and they result in all-female offspring, either by aborting embryos with XY sex chromosomes or by causing all embryos to develop as female regardless of the non-horse-girl sex chromosomes. Horse girls otherwise have the same genetics as non-horse-girl humans.

This hypothesis seems quite strong. It seems to explain most of the biological and cultural aspects of horse girl reproductive biology and sexuality, as far as we can see those things in the show. It is not open-and-shut, though. I see at least two other major hypotheses for what might be going on, and some possible ramifications of this one.

Horse girl of steel, boy of Kleenex

There are some baffling cultural aspects of the horse girl industrial complex, and attempting to explain them points to a rather dark possibility.

Within the first few minutes of the show, Special Week narrowly avoids closing her tail in the doors of the train - which would be the horse girl version of the classic anime trope of an inattentive girl getting her clothing or handbag caught. Then she finds she's gotten off at the wrong station; gets stuck in the ticket gate because of using it incorrectly and has to be rescued by a station attendant, whom she promptly denounces as a "pervert"; she buys a bunch of snacks but then drops them on the ground when she's distracted by the sight of the racetrack; twice inside five minutes Trainer-san sneaks up behind her and she doesn't notice he's there until he puts both hands up her skirt, despite her repeatedly-expressed concern about big-city perverts doing exactly that sort of thing; she gets so interested in watching the race and the concert that she misses the curfew for being allowed into her dormitory; trips over her own feet; is unprepared for the race starting signal; and so on. In Episode 2, she wins a race and then, although this has been her dream career path forever, it appears that she has never before that moment realized it will mean she's expected to sing and dance. A lot of this awkwardness can no doubt be explained by saying she's a country hick who has never seen a big city before. I did just one or two of those things myself on my first visit to Tokyo. But is that all? These events and some of her behaviour in later episodes build up a picture that Special Week may not really be very smart, nor very good at situation awareness. She's an airhead.

But she is a superhumanly strong airhead. She is fixated on Silence Suzuka, another strong horse girl who can probably take whatever Special Week can dish out. That may be fortunate, because if Special Week had a boyfriend and he was an ordinary human, would she one day forget herself and snap him like a twig? Can horse girls be trusted to be gentle with boys?

We should not read too much into the face-kicking scenes of Episode 1 because as I've said, they are probably not meant to be taken literally. But suppose they are literal. Special Week has demonstrated that she will immediately deploy deadly force against someone who touches her thighs without permission; she has no hesitation or moral compunction in doing so, and she gives no warning. Other horse girls display similar behaviour. The questions of whether she has a right to act this way, i.e. whether some kind of Castle Doctrine applies to her body; and whether it is plausible that Japanese society would encourage or tolerate this level of self-defence; are interesting but not the point. Note that a teenager in our world would not be allowed to ride the Tokyo subway with a 9mm handgun on each hip, shooting perceived chikan at her own discretion.

The non-horse-girl male who might consider approaching Special Week faces a real risk that just one mistake in judgment of the extent to which his advances are welcome, could mean instant death. Will that make her very popular with the boys, in a world that also contains many other girls who are less dangerous and volatile, let alone in Japan where even the regular humans can't maintain population replacement fertility and an important contributing factor is that young men are terrified of such risks? Just how cute will she have to be in order to have a chance of finding a partner?

On the other hand, consider this strain of fandom, which is easy to find on Twitter.

[Fans begging for abuse, mostly from Hatsune Miku]

I wasn't able to find anyone saying literally "I want Special Week to kick me to death" in English on real-life Twitter, not least because Twitter's "content filter" cannot be completely turned off and makes such searches difficult. But given the perversity of human nature, it would be naive to think she has no such fans in her own world. "Some of them want to be abused." But how many? We have the following hypothesis, which may not be very pleasant to contemplate but the truth doesn't care about our feelings.

Horse girls are human, not a separate species but just a special phenotype with different ears, tails, and very high athletic ability. Their sexuality and reproductive biology are basically identical to those of other human females; they reproduce by mating with ordinary human males. But they don't know their own strength. Because of horse girls' superhuman physical abilities and no more than average human levels of caution and personal responsibility, sex with a horse girl is as dangerous for the non-horse-girl partner as sex with a true horse would be. Boys are frequently injured, sometimes fatally, in encounters with horse girls. Every horse girl who succeeds in getting pregnant figuratively does so on a pile of broken bodies. Special Week's father is not in evidence probably because her birth mother killed him in bed.

Thus, although horse girls might be able to find a few willing partners who were masochists or suicidal, they would be unable to find enough partners to maintain a population replacement birth rate, and they would quickly become extinct, if not for the existence of a massive public relations complex devoted to convincing men and boys that horse girls are "cute" instead of terrifying; are sexually desirable; and are worth the risk. That is the real purpose of the horse girl racing business and especially the idol thing. The Horse Girl Yakuza Illuminati may be using racing profits in combination with threats to buy the silence of bereaved families.

On the advisability of human/true horse sex, see the Enumclaw horse sex case. There is also the old story about the death of Catherine the Great, which is not actually true, but would've been physically possible. This sort of thing can't be expected to go well for the human partner.

Further ramifications are possible. In earlier historical periods before the modern racing industry developed, maybe horse girls were revered as goddesses, and placated with virgin sacrifices. Consider the Witch of the Westmoreland: usually interpreted as a shapeshifting centaur, possibly the goddess Epona, but the ballad also could have been garbled over the centuries from what was originally a description of a horse girl. "And fast and fleet went she..." Also consider the dire warning in the last line of the ballad: only a knight so strong that "there's none can harm" him, dare "lie with" such a creature.

Maybe the UMPD anime series itself is part of the propaganda effort, an attempt to lay groundwork for an eventual invasion of our world from the other place, where horse girls lurk, awaiting the time when the stars will become right.

(This is why I'm not allowed to write anime plots.)

Triploidy and parthenogenesis

If we think that the mother-daughter inheritance of the horse girl phenotype, apparently obligate in both directions, is too hard to reconcile with human genetics; or if we think that involvement of human males in horse girl reproduction would necessarily imply a "boy of Kleenex" scenario and that's too cruel; then maybe we should explore the idea that horse girls do not interbreed with humans, and thus are not part of Homo sapiens after all. Maybe they're really a separate species. Then we have to ask how they're able to reproduce, given that they sure look like mammals at least, mammals always reproduce sexually, and there don't seem to be any horse boys. Note we also have the studio's claim that there are no true horses in the UMPD world, so if you were imagining a "Catherine the Great" scenario with the involvement of true horse stallions, please put it out of your mind.

Consider the following.

Horse girls are a separate species, perhaps genetically related to, but not part of, Homo sapiens. They may be basically triploid humans, with 69 instead of 46 chromosomes and enough genetic differences to avoid the health problems experienced by literal triploid humans. There are no males of this species, and they are not fertile with male humans although non-fertile sexual behaviour may be possible or even necessary as some kind of hormonal trigger. Their actual reproduction is asexual, by parthenogenesis: a horse girl can become pregnant and bear a child near-identical to herself without a genetic contribution from anyone else. Thus, horse girls only have horse girl mothers and horse girl daughters, and no male biological relatives.

Although it is not known in mammals, there are all-female species of lizards with almost exactly this biology, including the triploid genome and non-fertile sexual behaviour with each other or with males of related species in which males exist. If lizards can really do it, then it is reasonable for humanoids in science fiction to do it.

Many humans have at least a few triploid cells, and some human tissues (e.g. in the liver) are normally polyploid. Entirely triploid human embryos are not rare in our world but they almost always result in miscarriage; it's rare for a baby with this condition to be born alive, and the longest-lived known example was a boy who lived seven months after being born. So we'd have to assume that HGDF is not just another half-copy of a standard human genome but contains other things that allow horse girls to actually survive and function with a triploid genome.

We don't get to see much of Special Week's birth mother at all. She only appears alive on screen for a few seconds in Episode 2. Even her memorial photograph, later in the series, is only ever seen almost completely obscured by glare. They at least have the same basic hair colour, but the mother doesn't have the daughter's white forelock. That difference may be an argument against parthenogenesis if the forelock is a genetic thing, since we'd expect mother and daughter to be as similar as a pair of identical twins.

It's also hard for parthenogenesis to explain Narita Brian and Biwa Hiyahide, two sisters who look totally different from each other; or the three Mejiro sisters, who also have significant differences in appearance. These examples don't absolutely disprove parthenogenesis, because all kinds of weird things could be happening with epigenetic factors (gene activation and so on), especially if these creatures have a triploid genome. But they carry a fair bit of weight. Parthenogenesis doesn't look like a really strong explanation for what's going on in UMPD.

The Ackbar hypothesis

The two main hypotheses for horse girl reproduction presented so far each require novel biology. If horse girls are within Homo sapiens then we need a fair bit of stretching of how the genetics or epigenetics might work in order to account for the observed rules of inheritance; the nearest precedent for the necessary biology is among insects and also requires unique bacterial involvement. On the other hand, if horse girls are not within Homo sapiens and reproduce by parthenogenesis, then we must basically conclude that they are mammals with the reproductive biology of exotic lizards.

Either way there is some precedent in the real world, but it doesn't seem solidly plausible. It would be better if we could come up with a simpler explanation that requires little or no novel biology. And there is one: they could just be another species of mammal, and just like other mammals, without any unusual genetics, intracellular parasites, or similar. Everything we've seen is basically consistent with standard mammal reproductive biology except that horse girls apparently have hidden estrus, which is unusual for mammals - as far as any evidence in the show goes, horse girls don't "go into season" as mares do. But humans have hidden estrus too, so it's clearly possible in mammals. Horse girls seem to be in all matters of reproductive biology just like humans, except for one thing: the absence of horse boys.

Well, maybe horse boys really do exist after all, and we just haven't recognized them because they don't look like what we expect. Humans are good at ignoring facts inconsistent with expectations. Consider the following.

Horse girls are a separate species, at least mammals and perhaps related to humans, but not necessarily very closely related to humans. There are two separate sexes of horse girls, existing in approximately equal numbers, and they reproduce as other mammals do, in male-female pairs. Some individuals may at least sometimes engage in non-reproductive sexual behaviour, as seen in some other mammal species (humans, bonobos, etc.), including romantic involvement with same-sex horse girls or with humans of either sex. They are not cross-fertile with humans.

However, horse girls have little or no sexual dimorphism. They can tell the difference among themselves - perhaps largely by smell, as is the case for true horses - but the two sexes of horse girls both look like female humans with special ears and tails, and both sexes of horse girls are treated as "girls" and referred to as "she" in the context of human society. The fact that many horse girls are male may be a kayfabe secret not widely known to humans outside the racing industry. Horse girls retire from public life when they start families, partly in order to maintain this secrecy, and that is why we don't see horse girls older than at most their early twenties.

Note that humans don't have much sexual dimorphism either. We can usually identify each other by sex, but not always, and it's plausible that aliens of a species not closely related to us might not be able to. True horses have what may be even less sexual dimorphism than humans. The males tend to be bigger and faster, but females sometimes compete against males in horse racing, and sometimes win, to a significantly greater extent than is true for non-horse-girl human runners. Non-expert humans may be unable to tell the difference between male and female true horses if they can't see the genitalia.

So if horse girls really are of two different sexes, but they are at most as sexually dimorphic as true horses, and if like humans they usually wear clothing that conceals their crotches, then it might be plausible that the general human public might perceive them as all one sex; and then it's plausible that the general human public might have developed cultural expectations under which they perceive the apparent single sex of horse girls as "female."

Some horse girls are bigger and faster than others. Some have big breasts and some are flat-chested. Some have a whole lot of what could be called macho swagger in their behaviour, and others don't. Give Broye a pair of trousers and a sword, and she could easily be a character of either gender in Revolutionary Girl Utena, Rose of Versailles, or some other anime that inherits from traditional shoujo manga. Vodka speaks Japanese in a way that only someone who wished to be perceived as masculine would; she arguably is already presenting herself as a horse boy, regardless of whether they exist. El Condor Pasa may be another candidate for a male horse girl - she's as emotionally volatile as a boy, and it was her idea to trick Special Week into telling Broye (in French, which Special Week herself doesn't understand), "Don't get cocky!" Is she just an American Latina with different cultural background from the less aggressive Japanese-born horse girls, or is there another difference between her and some of the others? Are Special Week and Silence Suzuka actually a straight couple? (In that case, which is which?)

Also, note that male humanoids who look female, or vice versa, with many different kinds of "gender identity" and many different sexual preferences of their own, are a staple of anime. The term "trap" exists in English-speaking fandom for a male character who looks female, hence the title of this hypothesis; at least some fans do not consider that pejorative and object to others' claim that it is a "transphobic" slur; and an ongoing bitter argument is claimed to exist within fandom over the question of whether it is "gay" for a male fan to be attracted to "traps."

Having about half of the horse girl population be male in a sense that is relevant to their reproduction, but all of them treated as "girls" by human society, doesn't seem like it would be out of place within the spectrum of anime. Compare too with the dwarfs of Pratchett's Discworld, who are humanoids with two sexes that even they can't easily distinguish, all dwarfs likely to be perceived as male by non-dwarfs, a taboo (breaking down toward the end of the series) against revealing or making a public issue of one's own or anyone else's sex, and courtship rituals that revolve around politely finding out whether the other dwarf actually is male or female. I would expect horse girls to be more able to recognize their own sexes than that, but we could keep the aspect of non-horse-girls failing to do so.

Let's talk pronouns. Japanese first-person pronouns are often gender-specific, so anime characters tend to announce their claimed gender in a way that is much less true for speakers of other languages. It is made more complicated by the fact that there are more than two choices, and the rules for using them are not absolute. I haven't transcribed and examined every single word of dialogue in the show, but I've found examples of horse girls using five different pronouns, with different gender status, including some that are not standard for girls and women.

[Episode 10 eye-catch of Vodka in racing costume]

Let's think about Vodka. She refers to herself by a pronoun usually reserved for males. She's tall and relatively flat-chested. She practices more martial arts and less "idol" stuff than other horse girls. She wears shorts in her G-I racing costume and when not wearing a prescribed uniform of some kind, instead of the skirts favoured by most other horse girls. (Exception: she wears a dress at the Dream Trophy gala in Episode 13.) Ponytail hair styles are usually considered primarily feminine in Japan, as in the West, but Vodka wears a long thin ponytail gathered low on the back of her head - which is very masculine in Japan because it is associated with samurai. Other anime series, including Azumanga Daioh and Lucky Star, contain jokes about girls looking like boys when they put their hair into that style.

[Lucky Star, Episode 3:  Konata teases Kagami about her ponytail making her look masculine]

We are told that horse boys do not exist, but canonical evidence puts Vodka maybe 80% of the way to being a horse boy; and her appearance and behaviour are not treated as remarkable by her peers, so she is probably not unique in these attributes. If there are two really different kinds of horse girls, she may be an example of the less feminine kind, and the horse girls themselves may not care about the fact that both kinds are called "girls" by outsiders.

[Special Week and Broye stare each other down before a race; Special Week looks girly and Broye doesn't]

Broye seems only able to speak French, which like English doesn't use gender-specific first-person pronouns, so we don't get a self-identification clue there. The Japanese subtitles on her dialogue render her first-person pronoun as "私", which is probably watashi (this kanji could also be read as watakushi or even atashi, but either of those would probably be spelled out in kana if they were intended). But the Japanese pronoun would reflect the translators' choices more than Broye's own, anyway. Her racing costume, as mentioned, could easily be a boy's, and so could a lot of her behaviour. Like Vodka, she's taller than most of the other horse girls. Her breasts are noticeable, but not as large as some other horse girls'.

[Viewer comments over television coverage of Broye, including "ブロワイエイケメン過ぎる" - colours edited for legibility]

There's an interesting shot in Episode 11 that looks like it's from some kind of online video stream, with viewer comments scrolling across. It goes by fast, but one of the viewer comments is "ブロワイエイケメン過ぎる・・・! 惚れた・・・" (Broye ikemen sugiru...! Horeta...). The subtitles translate that as just "Broye's hot!" but a more literal translation would be "Broye is too handsome! I fell in love..." The word ikemen would in our world only be used to describe an attractive young man; its application to a woman stands out like Vodka's use of ore. So it seems clear that some of Broye's fans perceive her as some sort of masculine character.

There seem to be two distinct flavours of horse girl to horse girl attraction. Special Week consistently says "She's pretty" (kirei) about horse girls she admires, especially but not only Silence Suzuka; but some horse girls are more likely to say "She's cool" (kakkoii). For instance, that is what Teio says about Rudolf. Among straight non-horse-girl humans, it'd be typical for a girl to be pretty and a boy to be cool. Among horse girls it could come down to individual variations in personality and aesthetics; or some kind of "butch"/"fem" cultural distinction within a single sex; but it would also be consistent with a critical biological difference relevant to their reproduction, that is, two different sexes.

The biggest problem with all this is that it's hard to believe the humans would be okay with it. Humans care a lot about sex and gender, even, perhaps especially, when it's none of their business. A nearly-human species with two sexes that are to be considered female girls and male girls, does not fit into the consensus human ontology of persons, especially not if these creatures are to any degree sexually compatible with humans. A Japanese idol unit in which half the members were known to be male in any important sense might have trouble drawing a crowd.

It doesn't matter whether we fans as a subculture decide to agree that "traps aren't gay," whatever that even means; the percentage of human males in the general public who are happy to knowingly look upon males of any species as sex objects just isn't the majority of the population. Japan is not kind to real-life homosexuals, especially not male ones, despite the pervasiveness of the idea of homosexuality in Japanese pop culture. And if horse girl racing including the idol thing is a global phenomenon, it necessarily includes other places where human society is even less accepting of this sort of thing than Japan. If human society in the UMPD world, all over the UMPD world, is such that everybody has no problem at all with male and female horse girls all being considered "girls" and giving idol concerts for an almost entirely male human fan base who know that that's what they're watching, then that human society must be really different from our world's, in ways that ought to have other visible consequences.

So in order for it all to work, maybe we have to conclude that the existence of two different kinds of horse girls is a kayfabe secret: it's something the horse girls know, some humans involved in the industry know, but the general human public doesn't know. It would be a difficult secret to keep. In the UMPD world with no publicly-acknowledged and solidly plausible explanation for how horse girls reproduce, there would be many fans asking exactly the same questions I'm asking - and coming to similar conclusions. Are such thought crimes brutally suppressed by the Horse Girl Yakuza Illuminati?

Like fairies

One peculiar thing the studio personnel said in that interview was that horse girls are "like fairies." Maybe that just means they're sort of legendary and magical. But what if it means something specific about how they reproduce? Fairies are thought to swap their own babies with human babies to have them grow up as strange humans - there's a whole set of legends about "changelings." Exactly swapping horse girl babies with human babies would solve no explanatory problems because it would leave us with horse girl babies growing up in human families, which does not appear to occur in the ordinary course of events, and it begs the question of horse girls being able to make babies of their own to swap. But what if just the aspect of stealing human babies is retained?

Horse girls are not fertile; they cannot reproduce biologically at all. They kidnap human babies and do something to them with sufficiently advanced technology to turn them into horse girl babies.

There are a number of problems with that hypothesis. It's nightmarish, probably even worse than "boy of Kleenex," and it contradicts the apparent canonical event of Special Week's birth mother having given birth to her in a hospital in the standard human way. Since, from the opening slide show in Episode 1, horse girls appear to have been around for at least a few centuries, and the setting of UMPD seems to be a similar historical period to our own, there are a lot of constraints on the technology that could be involved. It's also hard to reconcile with the apparent amity between horse girls and humans. Unless, perhaps, humans are so completely enthralled by horse girl propaganda and mind control that they don't even realize how they are being preyed upon. Nightmarish, as I said.

Spe-chan has two mommies

Special Week's foster mother maintains a shrine for the spirit of the dead birth mother, something which even Special Week herself does not do and which would ordinarily only be done for a family member. Maybe she does this just for Special Week's benefit, because of the family relationship created by the adoption; but it could well speak to a connection between these two women that existed well before, and that goes deeper than, just the fact that one adopted the other's daughter. They must have been at least close friends for the birth mother to choose this non-horse-girl, with apparently no husband and no other children of her own, as the person she wanted to raise Special Week, and for the foster mother to agree to it. There surely existed other horse girls, or even non-horse-girl women with more stable families and experience of parenthood, who could have been asked instead.

[Special Week as a baby with her mothers]

The most natural explanation for how this family came about would be that when Special Week says she has two mothers, she doesn't just mean it literally. She has two mothers like "Heather" does.

Special Week's mothers were a same-sex romantic couple, and even more so than is usual for horse girls, she has no father with continuing involvement in the family. Unless the parthenogenesis or "like fairies" hypotheses are true (which would make conception a non-issue), she was conceived by any of the means that might be used by a female same-sex couple in our own world, most likely artificial insemination from a sperm bank donor because of the parallel to breeding of true horses. This family structure is unusual, but probably not unique, in the world of UMPD.


I've reviewed a number of questions about the world-building of Uma Musume Pretty Derby, and proposed answers to some of them. It's very difficult to build up a consistent picture. For the big question of how horse girls reproduce, there are three main hypotheses. They could really be human and interbreed with other humans, which clears up the questions about their social and presumed sexual interactions with other humans, but makes the apparent rules of inheritance a little hard to justify. They could reproduce by parthenogenesis, which neatly solves the inheritance of horse girl status and absence of fathers or other male relatives, but fails to explain the variation in appearance that we see among closely related horse girls. Or they could be a separate sexually-reproducing species with two sexes that both happen to look like "girls" to the human eye; that resolves almost all biological problems, but it is hard to fit into a global human culture that would resemble ours. All in all, some things seem to have reasonable explanations, but many unresolved questions remain.

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For insight about the post-race song-and-dance idol performances, we can turn to a seemingly very similar part of a very popular real-world American sport, which has something similar: at certain intra-game victory points, the players sometimes perform a dance routine for the audience, in a way that is seemingly unrelated to the game itself.

I am referring, of course, to American Football, and its tradition of celebratory dancing after touchdowns. This is not exactly the same as in the horse girl races, but the similarity is striking.

Some of the details seem to differ because of the inherent differences between the sports: the only natural place to interject a dance in a race is at the end. Others are presumably due to gender roles (with the competitors being exclusively female in the horse-girl races, and typically exclusively male in American Football, it's fairly clear why the post-race dancing is of a more "girly" idol-singer style.)

I think this is an important real-world precedent to consider when evaluating the "idol" parts of UMPD.
Silence Suzuka - 2019-05-10 10:47
Good thinking! There is also the whole tradition of "cheerleaders" in American football. Maybe we can understand this as simply as by saying that horse girls are their own cheerleaders.
Matt - 2019-05-10 10:57
I read this entire article and I think it was a very interesting read. I had similar questions watching this series. Personally I could believe the 'sperm' donor theory the most as it explains why we do not normally see fathers in the situation and would allow the horses to breed for the best genetics, just like real horses do.
vodka uwu - 2020-01-25 12:33

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