Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part VII, Evolution Part 3

Introduction

Hello again! This is Part VII of the ongoing Intermediate series about equine anatomy and biomechanics. It's by no means complete because it's meant to be a springboard for further proactive study, but it does provide some immediate, useful insights nonetheless.

In previous parts we learned some terminology, systems, and some basics about equine evolution, a subject we're continuing in this Part VII. An equine artist should be well-versed in equine evolution as a matter of course. Why? Because it lends context to our creative decisions. When we know the why of his structure, we gain a new perspective, we see The Big Picture, and that means creating work advocating for this animal becomes more the focus of our goals. For example, it makes putting that extreme, "exotic" head on our Arabian sculpture that much more difficult. It makes recreating the Big Lick impossible. It means we won't be able to put tiny hooves, light bone, and post legs onto our Quarter Horse sculpture. When we understand the nature of his structure, our desire to arbitrarily tweak it for "beauty" or modern fads becomes that much harder to do. The equine is a creature of pure function, and it's this functionality that makes him so beautiful.

For this reason, it's important that our work be a beacon for all that's good for our beloved subject. It makes our art more authoritative and more respectful, and that always results in art we can be confident in. Validated visuals have a powerful impact on the breeding shed, and our art can be a positive force in advocating for this animal. For that, it's through the understanding of his evolutionary history that we begin to see the full scope of his ongoing story.

So with all that in mind, let's explore more evolution stuff!...

More Backstory

Despite his unique design, the horse still isn't the fastest land mammal on the planet. It’s the cheetah that has that honor. By using the peculiar flexibility of his feline spine, the cheetah can infuse two suspension periods into his gallop rather than one (like the horse) thereby increasing speed. But the main difference is that the cheetah cannot sustain his speed for long—he peters out rather quickly. In contrast, the equine can maintain his speed for a very long time. That's a remarkable feat for such a large, fermentation-dependent herbivore!

As a result, a curious thing to know is that the hooves throughout Equus aren't the same. Nope! Each one is built for their specific habitat. Those equids that evolved on hard, dry ground such as asses and zebras, developed hard, thick, upright hooves, placing the inner foot structures up higher in the hoof capsule. Their soles are also thicker, tougher, and more vaulted while the frogs and lateral cartilages are larger and located further back, protecting the inner foot better under these harder conditions. In fact, though clocked at speeds of 31 mph (50 kph), the hoof of Equus asinus is designed more for sure-footedness than speed, reflecting his habitat of rugged desert regions. In fact, Equus asinus asinus takes this foot design to an extreme, making him the preferred pack animal in rocky mountainous country. Hemionids, on the other hand, evolved on the open plain like horses, and so have hooves similar to horses, yet they're still rather narrow in comparison. Even so, developing on the grassland plains, Equus caballus evolved hooves that are rounder, wider, and more sloping, with less vaulted soles, smaller frogs, and lateral cartilages. Interestingly, the extinct Quagga and the Plains zebra also share similar hoof structures to the horse. This may also explain the differences in hoof structure between today’s breeds such as between the Drafter, who developed in spongey grassland and forest, and the Arabian, who arose in the hard deserts of the Middle East.

Now For Some Mythbusting…

Equids were limited to a small size during the first part of their history, kept at about 50-110 lbs (25-50 kgms). Then during the Miocene (about 18 mya) when the grasslands began to take over, his lineages grew to 165-1102 pounds (75-500 kgms). An interesting fact is that Equus is one of the stockier and heavier versions of prehistoric Equidae. The biggest proto-horse may have been the extinct Hippidion, a Clydesdale-size early horse who lived in South America during the Ice Age (about 2 million-10, 000 years ago). But in truth, most lineages were graceful deer-like creatures relatively small in size. But it was Equus that evolved into a modest-sized animal, no taller than about 15 hands. Only with the advent of modern breeding, about the last 500 years, did horses taller or heavier than 1,100 lbs (about 500 kg) appear. In fact, the old world “Great Horse”, or Destrier, was really just a wide and stout animal, similar to a large, robust Cob. An average suit of armor was only about 70 lbs (32 kg) and people were smaller back then, requiring the animal to carry only up to approximately 300 lbs (136 kg) along with armor he might have to bear himself. So the bulk of the animal was intended more for increasing the thrusting power of the knight’s charge rather than creating a towering giant. And this “Great Horse” only appeared gigantic because people at that time were significantly smaller than they are today, in both weight and height. So, contrary to popular opinion, he wasn’t built like modern draft horses, making the image of knights riding Shire-like horses more fantasy than fact. Such huge drafters are actually a result of the modern railway rather than medieval battle. Farmers had to deliver their products to the intermittent rural rail stations for shipment in the most efficient way possible, that being with the least amount of back-and-forth trips. This meant that two enormous horses were more efficient for this purpose than four smaller ones, who at the time were the standard draft type prior to the railway. Breeding large horses also inspired “one-ups-manship” among farmers, fueling the increase in the standard size of draft horses. 

Also, the concept of “breeds" and "pure bloodlines," as we know them today, are rather contemporary notions begun by the eugenics-fixated Victorians, and so by the respective registries. In the past, the horse was a utilitarian beast. So rather than closed “purebred” bloodlines determining a “breed," or fixed points of type, horses were originally bred and grouped based on the physical qualities best suited the animal for a specific job. In fact, the original application of “type” was to discern between a riding horse, a racehorse, a carriage horse, a war horse, a workhorse, etc. or even the general region or culture a type of horse could be found. And sometimes Farmer Bob simply bred a type of horse he needed. The point is, horses were previously bred according to their purpose and not according to fixed, standardized points of type with "closed" bloodlines. Also bear in mind that modern breeding in the developed world is fed mostly by luxury money, sport, and the show ring, and often not the everyday-living uses of yesteryear, something that's particularly obvious in many halter classes with popular breeds. So it’s smart for an artist to be objective about “breeds” and “breed type” because mythologies, misinformation, and rhetoric abound that advocate ideas that can be misguided and harmful. 

The Modern Horse

At any rate, the horse evolved into an animal that ran first and asked questions later; his first instinct is "to get the heck out of Dodge" as quickly as possible. Elite racing Thoroughbreds can achieve bursts of speed for about 547 yards (500m) up to 38-46mph (61-74kph) while some racing Quarter Horses have been clocked at 50 mpg (80kph) during sprints. Yet these breeds have been artificially developed for higher speeds at shorter distances whereas Equus largely evolved for galloping longer distances at relatively high cruising speeds. Therefore, while most average horses cannot achieve these elite high speeds, they’re still quite fast and possess a high cruising speed for long distances. Their abilities are more than adequate to outrun and outlast just about any predator large enough to bring them down such as a wolf (who’s top speed is about 25-30mph [40-48kph]). Newborn foals are also able to keep up with the herd at a gallop within hours of birth because their joints are already precisely formed and they have an early ability to run. Another mechanism enabling immediate escape is the horse’s unique Stay Apparatus, which allows him to sleep while standing, but instantly take flight at the first sign of danger. Really, when it comes to a package combining speed, reaction time, strength, durability, and endurance, the horse is the finest running system today.

That means a predator’s best bet for bringing down an adult horse is an ambush, and chances of a successful one significantly increase at 50 yards (46m) or less. But horses developed a sensory system keenly designed for early detection of potential threats to provide a head start for escape. And once a horse starts running, it’s unlikely a predator can catch him. For instance, his eyes are located on the sides of his head and protrude outwards, providing him with a wide field of vision estimated at almost 350˚. He only has a narrow blind spot in front of his nose and right behind his tail, but a tiny shift of his head brings these areas into view. The horse has the largest eyes of any land mammal and a large portion of the cerebrum is dedicated to visual stimuli; it's believed that about one-third of sensory input to the brain is from the eyes, a testament to how much he depends on them peepers.

As for smell, the advantage is gaining information about something that’s hidden while also enabling communication or identification in a herd setting. Regarding the equine, many estimates have been made with inconclusive results. Suffice to say, his sense of smell is much more sensitive and perhaps operates on different levels than human abilities. While his head elongated for the battery of grinding teeth, it also resulted in a large surface area for the nasal cavity to detect smells. It’s actually been suggested that if his sensory mucous membrane in this long nasal cavity were spread out, it would cover his entire body surface. The horse also has a specialized smelling organ called The Organ of Jacobsen (or Vomeronasal organ), also present in some predators like lions (but not in humans), which some believe enables the horse to identify pheromones. This organ is also well-known in lizards and snakes who use their tongues to carry scents into their mouths. Therefore some assert that the behavior known as “flehmen," exhibited by both genders, may actually be an analysis of scent in which the lip-curl allows air to be sucked closely into the mouth and nostrils and across the Jacobson’s Organ. Indeed, many horses express the “flehmen” when they’ve encountered unfamiliar or pungent smells. And, since it’s often seen during sexual behavior, it’s also been hypothesized that it may help to detect pheromones. The “flehmen” may allow a precise analysis of territorial marking such as with dung piles left by breeding stallions as well. The “flehmen” response is also seen in other Perissodactyla, as well as in Artiodactyla, and even felines. 

The sense of taste, also called the gustatory sense, is closely linked to smell. While the horse’s sense of taste is largely unknown, it’s believed to be quite good and, paired with smell, his taste can detect certain toxic plants or tainted water. Indeed, some domestic horses pluck out those parts of the hay that are spoiled, eating only those parts still edible with great precision. Horses cannot vomit, so he needs to be cautious about what he ingests. Also, those who know horses know they do prefer certain foods or treats, and often have favorites they seem to truly enjoy.

Like humans, horses have binaural hearing, in which the ears (pinnae) detect sound concurrently. However, it’s clear that horses have a keener sense of hearing and a recent study indicated that horses hear sounds up to 2.73 miles away (4,400 meters). Humans can hear sounds in about the 20 Hz to 20 kHz range, being most sensitive to the 1kHz to 3 kHz limit. In comparison, studies have shown that horses hear in the 55 Hz to 33.5 kHz range, being most sensitive from 1 to 16 kHz. This means that horses can hear higher and lower frequencies than people and seem to be particularly good at locating low frequency sound. Recent research has even implied that, like elephants, horses may also communicate with frequencies outside of human hearing. Humans have only three ear muscles, all of which are vestigial, while horses have eight, moving each mobile ear (which can swivel up to 180˚) to quickly target and track sound. This is probably why their ears are so busy. 

Also, while humans have flat ears, horses have cup-like ears, which trap sound and flood it into the inner ear, allowing him to capture sounds humans may miss. Where the horse’s ears are pointing is a good indication of what the horse is focused on. And when a sound occurs, his ears automatically react to home in on it, in what’s referred to as the “Preyer Reflex," which improves the horse’s chances for survival. On the open plain, the only noises present, other than those created by the weather or the herd, could be stalking predators, and the horse only has to approximate the location of the sound in order to run in the opposite direction. If the sound is suspicious, he’ll orient his head and eyes to attempt to see the threat, while also freezing his body, probably scent the wind and also stop any chewing (to hear better). Hey may even briefly spook, only to spin around and assume the same behavior a bit farther away. If he decides the sound is coming from something genuinely threatening…VROOM, as they say. So a horse’s ability to target a snapping twig or panting predator would have meant the difference between escape and becoming somebody’s dinner. Stallions may also react more aggressively to sound since they’re inclined to be the protectors of their herd.

Horses can also loose their hearing with age, like us, with the high frequencies being the first to be lost then gradually working down the sound range. While hearing loss generally starts in the horse at about age five, it’s not usually obvious until the horse is around fifteen. However, since he has such a broader range of hearing, he can loose more of it without significant problems.

His sense of touch is apparent from his predisposition to seek physical contact from his herd mates, observed as rubbing, caresses, scratching, social grooming, etc. Horses also prefer to be similarly touched by people rather than being patted or slapped since these don’t mimic how horses actually touch each other. He’s also sensitive to the feather-touch of flies and quick to use his “fly shaker” muscles or tail to deftly remove them. Moreover, his eyes and muzzle are quite sensitive, having a higher concentration of receptors. Similarly, his facial whiskers, with follicles surrounded by nerve endings, are longer and more prolific on his muzzle and eyes, also constituting a sense of sorts. Indeed, there are more nerve endings in his muzzle than in our fingertips.

Horse’s react very emotionally to sensory stimuli, typically with fear if the flight response is triggered. In fact, horses have a remarkably short reaction time to stimuli and usually by the time the horse has reacted, spooked and calmed down, people are still trying to figure out what happened! However, though some horses are very reactive, some are far less so, often referred to as “bombproof." While some of this might be genetic, much of it is how well the horse was schooled to help him respond to the expectations imposed on him. Also, how thoughtful a person reacts to a horse helps to positively shape his responses.

So when a horse behaves in ways that seem mysterious, unreasonable, or bizarre to us, he’s actually being biologically sensible. Really, he’s simply perceiving things that are beyond our ability to sense and which millions of years of evolution compel him to act upon. So because humans and horses evolved according to very different criteria, to us much of his behavior can be bewildering or alarming. Yet to him, how much of what we do is bewildering and alarming to him? Remember that nothing about our human impositions were things he was evolved to deal with—it's an alien landscape with alien expectations to him. For example, nothing in his biological past prepared him for a stable door, a bucket, or a plastic bag. So we must always remember that his natural behavior makes perfect sense to him and, therefore, should be respected. This means that we must “go to where he is” in order for him “to come to us." In other words, we must first adopt his point of view for him to work with us at full potential. Horses exist in a kind of constant existential fear unless they're compassionately taught how to deal with every strange aspect and imposition of the human world, from their point of view. We simply can’t take it for granted that he understands what a fence is or how to interact with a gate, trailer, wash-rack, screaming children, or even buzzing clippers. Especially a prey animal designed for instant, fearful flight. This is not only an important insight for horsepeople, but even for artists if we wish to express positive images. Truly, in order to be a responsible equine artist, we must first be a responsible horseperson.

Anyway, to create another survival advantage, Equus became a very social animal, forming groups for security and comfort. For example, a sentry is often posted as the rest of the herd dozes off. And because most predators tend to hunt in the morning, in twilight or at night, groups tend to be restless and more bunched up at these times in contrast to a more restful mid-day demeanor, when predators usually nap. So under natural conditions, a horse will always seek out other horses, yet he can bond well to other species too, including people. But this is why if he’s scared or anxious, he’ll usually gather close to his buddies, including humans, for comfort. So while some individuals are solitary, most are highly social and live in “harem groups” or “breeding bands,” or "bachelor herds" of ten individuals or less, but may occasionally number over twenty. The advantage here is mutual protection or aid with offspring. 

Young males are driven off, or other stallions steal young females during skirmishes, which mediates inbreeding. The band is the most stable social unit and made up of adult mares and their foals (up to 2-3 years), protected by a mature stallion. A hierarchical “pecking order” exists within the band, so there's a tendency for a horse to seek a leader or become one (which is one of the reasons why the relationship between horse and human can often go awry). However, there are exceptions. Nonetheless, this hierarchal rank is important in horse life and achieved through aggressive behavior such as threatening displays or gestures that may include shoving, kicking, biting and body language. Despite sex, age, size and tenure, however, generally the most aggressive or “bossy” animals achieve a higher position. However, knowledge about the terrain or where to find adequate forage and water can also contribute to the hierarchal position. By simply taking the initiative, boldly leading, can improve an individual's hierarchical status, too.

Rank can influence a horse’s access to reproduction rights and even to resources. Yet despite the seeming discord, horses are very gregarious and enjoy each other’s company, forming very strong bonds. Play and mutual grooming are popular pastimes. Mutual grooming is also an important bonding activity and some suggest that dorsal stripes, shoulder and hip crosses may have evolved as focus points for this social grooming. Curious thought, eh? 

Regardless, horses are emotional creatures who wear their emotions on their proverbial sleeve. They possess a language that’s diverse, nuanced, and complex involving vocalizations (such as whinnies, snorts, blowing, squealing, nickers, sighs, roaring, grunts etc.), scent, touch, hearing, and body language. This makes group dynamics a rather complex and nuanced interaction of social graces, strong friendships, partnerships, alliances, and power struggles that can confuse us if we aren't "listening." 

Nevertheless, usually an older, mature mare, who possesses the learned wisdom to help keep the herd healthy and safe, leads the band and determines daily travels and movement. The stallion typically protects the rear of the band while the lead mare directs the movements, with mares following her in hierarchal order. Stallions have been known to aggressively attack predators or threats and can show concern for the comfort and safety of the individuals in his harem. However, when a new stallion takes over a harem, it has been reported that he may kill the defeated stallion’s foals to remove them from his new band. Yet, bands having more than one mature stallion have been reported, working together to protect and maintain the harem. But it’s the dominant stallion who does most, if not all, of the breeding. But despite the role of the stallion, the band’s stability rests on the relationships between the mares, which are very close-knit and enduring.

Another other type of group is the bachelor band, comprised of harem-less stallions. Also, bands of juveniles of 2-3 years, of both sexes, can exist as well. A local population of bands is referred to as a herd, which may form during migrations. Herds may possess an inter-herd hierarchy that determines access to limited resources. Daily diurnal or nocturnal migrations for forage and fresh water are typical along with seasonal migrations. Truly, equines move a lot in their daily goings on. Though horses may eat other types of forage, they primarily consume grasses, low forage plants, and other high fiber, low-quality vegetation.

Mares are seasonally polyestrous and usually come into season in early spring or eleven days after birth. However, under natural conditions, mares are usually biannual in their pregnancies. Gestation averages about 332-342 days and normally produces one foal (twins are sometimes born, though rarely). Newborns are fully functional and can stand within an hour, usually within the first twenty minutes, and can gallop with the herd within a few hours. They continue nursing for about eight months, but begin to nibble grasses within a few weeks. Both mares and stallions become sexually mature at about two to three years. However, environmental conditions are a strong influence on a young mare’s ability to conceive, and she often won’t breed until she’s older or in a stable band. Likewise, a stallion has to earn a harem, which usually doesn’t happen until he’s about five to seven years old. In the wild, the lifespan is about twenty years while in captivity he can live as long as fifty years, though the average is about thirty-two.

It should be noted that body language is so developed in horses that they have an uncanny ability to pick up even the most subtle or most unconscious body signals, even including those from people! At the turn of the century in Berlin, a rather famous example was Hans von Osten, otherwise known as “Clever Hans," who was renown for apparently solving complex mathematical equations. However, it was discovered that Hans didn’t actually know mathematics, but he did know body language, the unspoken language of horses. He had learned to key onto a person’s involuntary physical responses and quickly became able to perceive even the most subtle tensing and relaxing of muscles in anticipation of a correct answer. So Hans would simply keep tapping his hoof until an unconscious cue from the human observer that indicated to stop at the right answer. He was particularly keyed onto Wilhelm von Osten, his owner, resulting in answers of surprising consistency, but did reasonably well even with other people. But if the person providing the unconscious cues didn’t know the answer, neither did Hans.

All this makes one wonder just what we’re unconsciously telling horses with our bodies, smells, touches, and sounds at any given moment! It’s known that a person’s emotional state can dramatically affect a horse, for better or worse, and why compassion, patience, calmness, consistency, and softness are so important to practice with horses. Since humans become part of the herd, in a sense, horses may look to people for appropriate reactions or responses to a situation, so it’s important for people to be aware of what they’re “telling” a horse at any given moment, particularly during a scary moment.

Above all, the horse is by no means stupid, despite what some may claim. When we believe he's dim-witted, it simply means we've failed to comprehend him. He’s actually quite intelligent and a quick learner who can exploit many types of learning strategies. He would have to be, wouldn't he? In order to prevail in our confusing world, he has to quickly make sense of what we want from him. In this aspect, the horse does a lot of "filling in" for us, whether we know it or not. In fact, a newborn foal is neurologically mature, unlike many other species, and has full capacity to learn within minutes of birth, and indeed, the first few days of a foal’s life are a critical learning period. The horse’s memory is infamous and he appears to never forget a lesson learned, good or bad. But happily for us, most horses have a forgiving nature! 

Domestication

Any number of other animals could have just as easily been domesticated for horse-like work. Think about it, what would oxen or elk look like today if they had been selectively bred for centuries for “rideable” characteristics? And humans have actually domesticated other rideable animals such as camels, reindeer, yaks, oxen, elephants, and various other critters. And, really, many of these other animals, particularly cattle, camels, and oxen (especially buffalo), are far better weight-bearers than horses, having far more rigid and straighter spinal columns.

During the beginnings of domestication, one wonders what these early horses looked like…were they close to the type of the Takh? Remember that centuries of selective breeding formed the modern horse into a more “rideable” animal, with high withers, longer legs, longer, arched neck, and proportional differences. But the Takh is quite different in both physique and temperament, so how prone were early humans to domesticating such an animal? Or perhaps these early horses weren’t Takh-like at all, and more Tarpan-like? Or something else? Then again, the Mongolian horsepeople of today still ride ponies that are rather Takh-like in build, too.  

Horses are a diverse bunch…a Falabella, an Akhal-Teke, an Arabian, and a Breton couldn’t be more dissimilar in conformation. This variation is a result of decades, sometimes centuries, of selective breeding for desirable attributes. But years of artificial selection have actually narrowed the genepool for many domestic lineages, some alarmingly so, a worrisome prospect for long-term sustainability of many populations. Yet domestic horses are a strange case. Recent studies have implied that the genetic diversity in the domesticated horse is uniquely extensive, suggesting that multiple episodes of domestication took place from genetically diverse populations. In other words, there’s a good chance that many genetically diverse horses, from many locations, from many genepools, at different times, were used to form the modern breeds today. Also, since relatively few mutations are present in the genetic material of domestic horses, it appears that most of his maternal genetic diversity was infused into the stock early at the time of domestication. Conservatively, it’s been hypothesized then that a minimum of seventy-seven wild mares are necessary to explain the present genetic diversity in the domestic horse, but actually that number was probably far higher. New research has also identified seventeen rather distinctive lineages, some of them showing a pronounced geographical link. This begs the question: Did domestication occur independently by different human societies in different locations, or was there a single moment of domestication that spread as a new technology? This issue is still being debated. Research is also being done on the paternal genes in the domestic horse. If early people bred horses like today, they used a few choice stallions to cover a number of mares, therefore the Y chromosome diversity of the domestic horse should be much less than the maternal DNA.

Generally, it’s believed horses were first used as food. This made sense in prehistoric times since horse’s meat and milk are uncommonly high in vitamins, minerals, and amino-acids important to the growth and health of the nervous and vascular systems in humans. But in terms of using the horse as a partner, the when, where, and by whom this first happened is still unknown. However, it’s believed to have first happened for milk, then meat, and then perhaps for hauling, with riding probably coming a bit later. In regards to that, it’s theorized that horses may have been favored for this because they’re not only faster, but also because they have cecal digestion which releases them from the hours of restful cud-chewing typical of ruminants. 

Nevertheless, the current belief is that horses were first domesticated during the Neolithic, Eneolithic, or Early Bronze Age, with the first domestic horses originating in the western part of the Eurasian steppes of modern-day Ukraine, southwest Russia, and west Kazakhstan, mixing with local wild herds as they spread across Asia and Europe. Current research also suggests that the wild ancestor of the domestic horse, Equus ferus, expanded out of East Asia about 160,000 years ago. Indeed, equine head carvings in antler and ivory, carbon-dated to 14,000-9,500 BC, have been found in caves in Germany, France, and Spain with what appear to be halters. In this instance, horses may have been used as pack animals, perhaps to haul mammoth meat and other materials. It’s ironic that an animal proven to be so instrumental in the shaping of human civilization could have such a fog surrounding his domestication.

Since domestication, feral herds have roamed, being descended from domestic stock. So these populations aren’t “wild," but simply feral domestic horses. Truly, it’s important to understand the significant difference between “wild” and “feral." Despite their untamed nature, feral horses are far more tractable owing to their heritage of domestication. Any of the feral horses can be captured and trained with relative ease. But in contrast, the Takh, a true wild horse, is notoriously dangerous due to his aggressive, untrusting, and virtually untrainable nature. The Takh is also far tougher physically that feral horses, being culled by nature and not by man. 

Australia currently has the largest population of feral horses, or “Brumbies” (the Austrailian term for their feral horses). Other feral horse populations include the Camargue horses of the marshy Rhone River delta of southern France, feral herds in Sweden’s tundra and forests, semi-feral ponies on the British Isles (the best known being the New Forest and Exmoor ponies), the feral ponies of Assateague island and, of course, the American Mustang (“mustang” is derived from the Spanish word for “wild," "mesteño"). But feral populations exist all over the world.

Sadly, the true wild horses haven’t fared as well as the feral herds. Of the three groups who existed, only one has prevailed to modern day. Equus caballus sylvaticus, the forest horse of central Europe, went extinct in the Middle Ages as his habitat was destroyed by agriculture. His blood may still exist in the Konik, a domestic pony bred in eastern Poland. Equus caballus gmelini, the Tarpan, lived on the steppes of southern Russia, but went extinct by human pressures and dilution with domestic stock. The last known genuine Tarpan died in southern Ukraine in 1879 and the last captive animal died in the Moscow Zoo a few years later. Today, the Takh is the only true wild horse left in the world. Believed never to have been domesticated, he formerly roamed the steppes of Kazakhstan, Mongolia, Sinkiang, perhaps southern Siberia, but went extinct in the wild from over-hunting and human pressures. However, a number of Takh lived in captivity since the late 19^th and 20^th centuries, becoming the genepool for the approximate 660 in captivity. While there have been two reintroductions, seemingly successful, the Takh isn’t out of the fire yet, being classified as Endangered by the IUCN on Appendix 1 of CITES. An important note is that equids are considered a “flagship species." They’re instrumental in the conservation of biodiversity in their native habitats by impacting the flora in ways necessary for the native fauna to thrive. Therefore, if equids slip into oblivion, the future of other species becomes questionable as well.

It’s been a long, chaotic history, but it was this type of environment that shaped him into the animal we know today, an archaic relic in the modern age. It’s strange that this unusual mammal would shape the advancement of the newest mammal yet, us. Indeed, life as we know it today wouldn’t exist without the essential contributions, and the many sacrifices, of this fascinating and ancient animal. 

Conclusion to Part VII, Evolution Part III

You may be asking yourself, "What the heck did all of that have to do with equine realism?!" And that's a fair question. It's not necessarily obvious. This is just the tip of the iceberg, too! But what all this information does is to provide perspective. To offer a backstory that helps to put him into better context for our creative decisions. It's so easy to regard this animal within the parameters of our short lifetimes, but his experience spans the millennia and he exists within a very different reality bubble than we do. Unless we know his backstory and his biology, we're going to miss much about this animal that could inform our work.

Above all, we can't forget that he's not "just a horse," but an autonomous individual existing in a parallel reality, and that deserves respect and thoughtful reflection. This gracious creature puts up with a lot from us, and much of it is bewildering to him. Indeed, he must wonder why he must trot and canter in so many circles! We owe a lot to his generous nature.

So while we're mulling over all this, prepare for Part VIII, the head. Yes, we're finally getting to the body parts! Until next time then...to know the horse is to love the horse!

"What an artist learns matters little. What he himself discovers has a real worth for him, and gives him the necessary incitement to work."

~ Emil Nolde

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part VI, Evolution Part 2

Introduction

Many apologies for the lateness of this installment. In Part I and Part 2 we learned a bit of backstory and what set the new stage for the equine's development. In this Part 2, we'll delve into more detail about what makes a horse a horse. 

Now it may seem a bit odd that we're discussing equine evolution in an anatomy series. But the fact is when we discover the why the horse is built like a horse, we can adopt modes of thinking that don't compromise our fidelity to our subject. Understanding his past makes it harder to be so arbitrary in our creative decisions, and we begin to adopt an advocate point of view for this animal we profess to love. Our work gains greater authority and responsibility, and that bodes well for our unfurling body of work.

So let's get to it!...

Now For Some Detail…

Little eohippus began life in the dark, dense forests of prehistory, nibbling on shoots and leaves, and scampering around like big bunnies. Over time, the forests gave way to grasses as the climate changed, but in order for the grazing lines to adapt to a life on the plains, a series of specific adaptations had to occur, starting about 20 million years ago. 

The early equids were browsers, eating the abundant fruits, shoots, and leaves which are greater sources of nutrients than grasses. But to exploit the grasslands, he had to develop a digestive system that could process grasses rather than forest vegetation. In many ways, Equus truly “is what he ate” since many of his later adaptations from speed, size, and intelligence, can be directly linked to the change in his diet. But digesting grasses is no easy task, being a comparatively poor food source relative to forest plants. It also has cellulouse, the complex sugar in fibrous plants. Ultimately, grasses cannot be broken down by a mammal without the aid of gut bacteria, which breaks down this cellulouse into volatile fatty acids the animal can process. Yet it’s a time-consuming method that requires a chamber for this food matter to be stored so the bacteria can work their magic. There are two different ways this relationship has been expressed in ungulates. One is ruminant digestion (or pre-gastric fermentation) and the other is cecal digestion (or post-gastric fermentation). Artiodactyls evolved ruminant digestion first, entailing four stomach chambers (though camels aren’t considered “classic ruminants” since they have three stomach chambers rather than four, though it functions similarly). The first two chambers, the rumen and reticulum are where bacterial fermentation occurs. Then the food material is regurgitated and chewed again, otherwise known as “chewing the cud." Upon being swallowed again, the food matter passes through a special opening into the last two chambers of the stomach, the omasum and abomasums, where further digestion takes place. Pound per pound, rumination is still the most efficient means for extracting nutrients from grasses largely because food matter ferments for the longest period of time. It takes about 70-90 hours for food to pass through a cow, for example. However, non-ruminant Artiodactyls do exist such as pigs, peccaries, and hippos. 

In contrast, Perissodactyls evolved cecal digestion. This entails digestion in the cecum (equivalent to the human appendix), which grew to enormous size to provide the fermentation chamber. Food is chewed and travels to a comparatively small stomach, which then makes it into a slurry with digestive juices, and then passes the materials into the cecum for bacterial fermentation. Then from the cecum, the materials travel to the intestines for further digestion. Because cecal digestion is a more effective digestive strategy in animals under 5 kg, it’s believed Perissodactyla may have adopted it while still small during the Paleocene. However, cecal digestion extracts about 30% less energy from the same food material as does ruminant digestion, but the benefit is that it takes far less time to digest the food matter, only about 48 hours. This allows equines to flourish in niches where few other animals could survive or compete, particularly ruminant Artiodactyls. Biological data reveals that wild equines usually target the worst, lowest quality and highest fiber roughage they encounter. For example, while ruminates may eat just the leaves, equines will eat the stems left behind, as observed between Plains zebras and Gnus. In North America, feral horses and cattle compete for the same forage, but horses utilize territories not exploited by cattle, such as those far away from water sources (especially in winter) and at higher elevations. (An interesting factoid is that feral horses don’t compete with the Pronghorn, a native ungulate, since the Pronghorn eats mostly shrubs, herbs, and woody forage.) Indeed, many wild equines thrive in areas characterized by such poor quality vegetation that ruminants seem to avoid the area altogether. For example, wild asses and Takhi thrive in regions other large grazers deliberately avoid. Studies have also supported this data by showing that unless a certain level of fiber is provided by the habitat, a ruminant simply cannot support itself and will starve. The answer lies in the time factor. A ruminant can only process a limited amount of food in its system in a fixed period, which is a very long time. However, an equine’s response to poor vegetation is to simply eat more since its form of digestion can process nutrients much faster. So while per unit of energy a ruminant is more efficient per unit of time, a horse can extract far more energy from grasses than a ruminant. And we all know how horses love to eat! Nevertheless, his gut became large and heavy, so while the back of Equus appears hollow, due to his convex dorsal processes, his spine is really built in a soft arch to best support his heavy gut.

In addition, all this extra time a horse spends grazing amounts to about 15 hrs a day, in comparison to a cow who spends only about 8-10 hours a day grazing. All this “head down” time can translate into a greater vulnerability to predation, which has some interesting implications with how the equine evolved. For example, unlike ruminants, the horse’s digestive system allows him to “eat on the go," one moment munching at lunch and the next avoiding becoming one. This digestive design is also ideal for long journeys to search for new grazing areas or migration into new regions. So while cecal digestion is considered rather primitive, it could actually be considered a strange evolutionary benefit for Equus.

But with his cecal digestion, he also had to have teeth that could chew grass in sandy, gritty soil without quickly milling them down to nubs. Unlike fruits, shoots, and leaves, which either depend on being eaten to propagate or can suffer the loss of some material without sacrificing itself, the blades of grass are the plant itself. And plants enduring heavy herbivore depletion usually evolve various defenses. In grasses, this defense entailed infusing sturdy silica particles, or phytoliths, into the cell walls. Essentially, the nutrients in grass are locked in this silica skeleton, similar to glass powder, which lends shape and stiffness to the blade of grass, even when it’s dead. But to pulverize this silica skeleton to release the nutrients, lots of chewing is involved, but which rapidly wears down teeth because of the abrasive silica particles. In response, the horse lost his short, gently cusped bunodont teeth in lieu of a design that could withstand and exploit this abrasion. The gentle dentin cusps became elongated into long prongs sheathed in hard enamel, which then turned into slicing ridges. Hard cementum, once vestigial on the old bunodont teeth, encapsulated the entire tooth and filled in any spaces, particularly between the dentin prongs. Eventually, he developed continually erupting teeth (hypselodont), long crowned teeth (hysodont teeth) with long ridges made of blended dentin cusps (lophodont teeth) coated in tough enamel and encapsulated with hard cementum. So by altering the structure of the tooth and adding layers of materials of differing hardness (and therefore different rates of wear), nature created a grinding surface that responded to the abrasiveness of grass like a self-sharpening blade. This structure could withstand the silica particles and more efficiently turn grass into the necessary consistency of milled, fine cornmeal, increasing digestive efficiency (parts of the silica skeleton that aren’t pulverized won’t digest because gut bacteria cannot penetrate the silica shell). 

The fine texture of chewed grasses also avoids deadly blockages in the horse’s digestive system, which is already vulnerable to obstruction with its various bottlenecks. Any grass bits longer than 3/8-1/2 inches (particularly 1/2-1 inches) can be lethal to a horse by causing obstructions and colic, which is why old horses, who often have dental problems, typically “quid” (chewing and sucking out the flavor of hay, then spitting out the fibrous wad rather than swallowing it). They realize the potential danger. The horse doesn’t chew his cud nor can he vomit, so whatever he swallows goes through his system on a one-way trip. So his teeth make his food both safe to eat and nutrient-rich for his body…a big responsibility for such unassuming anatomy!

His teeth also changed orientation in his skull, forming into front incisors for nipping off grass in the grazing position and developing rows of large grinders in the back. This resulted in a long diastema, which elongated his head below his eyes. For example, about 40 million years ago, with Mesohippus, this battery of grinding teeth was increased to six (the number in living horses today) on either side of the jaw, top and bottom, because his premolars adapted into grinders. And over time, the head of Equus would become longer and larger, pulling the teeth out from under his eye orbit and deepening his jaws to make room for these new, bigger, high-crowned teeth and the larger chewing muscles required to activate them. Another interesting benefit of a long head with a high-placed eye is that he could now spend the necessary long hours to graze, yet still have eyes reasonably high enough to scan the grassline for predators. Furthermore, the mechanics of his mandible joint changed to discourage for and aft motion, but to favor side-to-side rotary chewing, like a shearing motion still distinctive today.

Another adaptation for grazing were his lips, which in the modern horse have an almost prehensile quality, unusually sensitive, and adept enough for selecting choice bits of food. This design was produced by a shortening of his nasal bones that permitted a fleshier and more flexible muzzle advantageous for selective grazing. Actually, some extinct lineages had skull structures that implied very prehensile lips, such as those of the tapir. The shortened nasal bones also benefited his nostrils by allowing them to enlarge and become fleshier, enabling greater intakes of air. His muzzle also grew in size to facilitate the heavy breathing for running over distance and to accommodate the chewing of lots of grass.

Adaptations for grazing also occurred in his cervical spine, largely being for length to get those teeth down to the grass. Very early on, the horse’s cervical column pirated the first thoracic vertebra, making it more like a neck bone, rendering it functionally part of the neck, increasing length and flexibility. Also, his Sternomandibularis muscle, the “hugging muscle," developed for grazing so he could jerk his head back in that typical grazing, nipping motion with the incisors (in contrast to the possible snout-pushing motion of Hyracotherium).

A plains lifestyle also meant he could no longer rely on dodging about forest cover to escape predation. Rather, he had to increasingly rely on his fleetness of foot, in a straight line, for a sustained time over hard semi-level ground, to survive. Fortunately, Equidae was built for a running escape from the onset because of their narrow ribcages and scapulas on the sides of the torso, orientating them on the same plane whether running, rearing, or standing (in comparison, humans have a wide ribcage with scapulas situated on the back). 

All the same, an important adaptation for running first occurred in his spine, which is still characteristic today. In Hyracotherium, the door for this necessary spinal change had already been opened, already differing this animal from related lines. How? Well, because his lumbar vertebrae were smaller and more condensed than other relations, even losing some vertebrae to establish the characteristic six lumbar in modern horses. The transverse processes of his lumbar were also more vertical and closer together, reducing their rotary capabilities. This established the horse, very early on, as primarily a natural “transverse” galloper in which a stride has a leading side (in contrast to a “rotary” galloper who runs with a cross-firing lead, like a lion). To improve the stability of the spine for a fleeing tactic on the plains, evolution began to straighten and stiffen it while further compressing the lumbar span to stabilize the coupling between the body and hindquarter. Over time, the lumbar vertebrae became even more rigidly constructed, with longer vertical transverse processes situated closer together and with a peculiar articulation between his last lumbar and sacrum. So rather than flexible lateral motion or rotation, this type of spine became increasingly limited to a coiling motion, the spinal structure characteristic of Equus today. 

All these changes, started in Hyracotherium, were first seen in Parahippus, who possessed the first horse-like spine. This type of spine is very efficient for this new mode of escape, acting like a coil and spring mechanism specifically designed for acceleration and an enduring, rapid “cruising speed." And, indeed, for a large prey animal with a fermentative gut, who runs in a straight line, it’s far more efficient to have a stiff "pole" launched by that spring rather than a wiggly, wobbly pole. So by diverging from a dodging, flexible spine to the more rigid and straight posture built for speed in a straight path, Parahippus could literally outlast predators in the chase at high speeds, a survival tactic characteristic of Equus today. Nonetheless, the horse wasn’t the first to achieve this design since it was the camel family that has earned that distinction. The camel was the first to achieve the straightened, stiffened spine, and high withers for anchoring and a shortened lumbar span necessary for a running escape on the plains (this family was also the first to achieve the ruminant digestion and teeth necessary for feeding on grasses). Thusly, because the horse got a relatively slow start with a cursorial lifestyle, his back is still more flexible and arched than a camel’s or most Artiodactyl descendants. 

His pelvis also changed by lengthening both ichium bones to increase leverage for the developing hamstring muscles that propelled him forwards, the Semitendinosus and Semimembranosus muscles (the "semis"). Not coincidentally, his first two tailbones were pirated by the sacrum, pulled forwards and made larger to functionally serve as the root for these enlarged hamstrings. And today, these muscles in the modern horse attach at the first two tailbones and the end of the sacrum and run down to the femur and tibia, becoming powerful motor muscles for propulsion. And somewhere during evolution, the Semitendinosus also developed a thick tendon in the middle of its muscle belly to amplify its forces and to become part of the Reciprocal Apparatus. So the dock on the modern horse really begins at the third tailbone and that bump sometimes seen on the tail head is really the Semitendinosus muscle. Additionally, the sacral spines on Parahippus were the first to slope backward, opposed by those of the lumbar that slope forwards. All of these changes together imply that this early horse may even have had a precursor system of the Reciprocal Apparatus.

Also in Parahippus, the horse began to increase the length of his withers to anchor the longer neck and larger head, perhaps also developing a crest. Withers didn’t develop in the browsing lines for there was no need, so their necks were round or tubular, like a dog. And started with Hyracotherium, and completed with his plains descendants, grazers also lost certain spines on their cervical vertebrae, implying that their necks began to rely on yellow ligaments for passive support rather than active muscle contraction. 

Additional change occurred in his limbs, which needed modification for speed on the open plain. A common misconception maintains that the legs of early browsers became progressively longer first, making the animal increasingly larger as an inevitable outcome of equine evolution. But height and size only came into play after his digestive track, teeth, and, especially, his spine adapted to a cursorial lifestyle. The proportional differences between Hyracotherium (55 mya) and Mesohippus (40 mya), for example, were still relatively similar. In the lineages leading to Equus, the lateral digits became vestigial, leaving the 3^rd digit to bear all the weight. The first digit to be lost was the first (or thumb), then the fifth (or pinky). Later the second (pointer finger) and the fourth (wedding finger) became vestigial toes or dewclaws, and now remain as the splint bones in the modern horse. 

But it wasn’t until Parahippus (22 mya) that the legs themselves began to elongate by “telescoping” or lengthening the bones in his appendicular column, particularly those of his “hand” and “foot." This changed the proportions of his legs from those of his browsing relatives by making his upper limbs (scapula, humerus, and femur) comparatively shorter than his lower limbs. This is a typical characteristic in speed animals who require leg structures based on long levers with high fulcrums, or a short upper limb ratio to a longer lower limb ratio. So leverage became oriented about one-thirds up on the appendicular column (from the scapula to the elbow, or femoral joint to stifle) to act upon the remaining two-thirds (from the elbow to the toe, or from the stifle to toe). This means that with minimal effort by the upper limbs a great deal of leverage could happen at the toe, increasing stride length and therefore increasing speed, allowing him to “eat up the ground” with relative ease. In response, his limb structure simplified, removing the muscles below the carpus and tarsus, making the distal limbs passive levers activated by tendinous servos whose muscle bellies are located high on the limb (above the knee and hock). This reduced the weight of the distal limb, thereby further maximizing speed efficiency. For this reason the lower limbs of modern horses aren’t muscled. The "bone" of his legs also increased to allow a large, heavy herbivore have enough support, especially while escaping predators.

The plane of limb motion narrowed to eliminate inward or outward instability, too. For example, people can turn their palms upwards or downwards, known as “suppination of the manus," because the human radius isn’t fused to the ulna. Similarly, the browsing lines, the radius and ulna weren't fused but their movement was inhibited by a special design at the top of the radius. But in the grazing lines, the ulna became fused to the radius, totally preventing this “suppination of the manus” to keep the toe permanently pointed forwards. Likewise, the joints of his legs became structurally oriented on the same plane so that each time the horse articulated his legs, his toes would automatically be pointed forwards for flight. In the hindlimb, this resulted in an angled hock joint that keeps the toe relatively pointed forwards despite stifle flexion. The evolution of the equine leg also resulted in a cascade of check ligaments. 

Another interesting change happened to the equine clavicle, or collarbone. In humans, the clavicle is connected by bony attachment, keeping the ribcage centered between the arms while also creating a bony attachment for the arms directly onto the torso (a relic from our primate ancestor who needed this design for climbing and swinging from branches). But like many large, hoofed animals with a suspension-based gait, the horse lost his clavicles during evolution, opting instead of the Shoulder Sling of muscle attachment. In the horse, this produced fluid, dynamic shoulder motion conducive for an agile, athletic long stride paired with shock absorption. Therefore, this large, speed animal could jump, pivot, stretch, and sprint at high speed without continually fracturing his collarbones. But, the loss of the collarbones forced his giant neck muscle, the Brachiocephalicus, to search for another attachment, ultimately finding the humerus, thereby linking his head directly to his arm and, therefore, to his entire foreleg. 

Changes also occurred in his feet. The footing on the plains was quite different from that found on the forest floor, being abrasive and rough with hard surfaces and coarse plants. This heightened the need for tough horn and cornfied structures rather than soft footpads to carry a large herbivorel at speed over distance. His ancestors had a plantigrade stance, but as certain lines evolved, such as Hyracotherium, digitigrade stance developed. And in the lineage that produced the modern horse, this stance became unguligrade to increase speed and torque in a plains habitat. So during evolution, the feet of Hyracotherium's grazing descendants simplified, streamlined, and grew in size to serve the cursorial lifestyle, resulting in greater dependence on the middle toe, which lead to gradual atrophy or even loss of the other digits in some descendants. In the modern horse, the unguligrade stance found full development, causing the atrophied digits to disappear altogether and the digital toe pads (or central pads) became incorporated into his sole, forming the frog and digital cushion. His old distal metacarpal pads are believed to have become the ergots and his ancient proximal metacarpal and wrist pads may have become the chestnuts. Plus, the sensitive laminae of his foot are actually modified periosteum of the coffin bone, richly laden with Sharpey’s fibers, lashing the hoof capsule onto the once ancient middle toe. In all, the modern horse literally walks on tiptoe on his middle digit and on stilts telescoped from his “hands” and “feet." Remember the "LandStriders" from The Dark Crystal? Yeah, something like that. Interestingly enough though, Equus still has the capacity to recreate these vestigial side toes II and IV. Normally, these digits remain as the “splint bones," but sometimes a foal is born with these toes fully developed, “hoofies” and all. In fact, during fetal development, equine embryos still have three toes at six weeks, which diminish to the familiar design after about five months in the womb. Caesar was even said to ride a horse with “feet that were almost human, the hoofs being cleft like toes,” which soothsayers foretold as proof of Caesar’s destiny to rule the known world.

Predation was also a determining factor in equine evolution. Originally the Perissodactyla were the most plentiful and diverse hooved animals. Then the evolution of Artiodactyla blossomed, replacing the Perissodactyla as the dominant hooved animal. Indeed, even today the Artiodactyla are more plentiful and successful than the Perissodactyla, many of which are currently near extinction. Nevertheless, by the end of the Ogliocene, as early horses started to enter the plains, they initially competed quite well with the Artiodactyls and diversified quickly into large numbers during the Miocene. 

But what's particularly interesting is how Artiodactyla may have primed the predator species on the plains, creating a habitat full of smart, fast, and efficient hunters. Predators had become more intelligent to outwit Artiodactyla prey long before the horse lineages showed up. It was actually a constant “cold war” of intelligence and wits between these animals that the early horses stumbled upon as they ventured onto the plains. So during these tentative first steps, early horses had to deal with packs of roving predators already well-equipped to make them an easy lunch. Hardly an optimistic beginning, yet this may have hastened and diversified equine evolution as those unsuitable were quickly removed from the gene pool. Also, the frontal lobes of Hyracotherium weren’t impressive, leading some to consider him rather dull-witted, but this “cold war” environment could have accelerated the development of equine brainpower since those simply too dim were quickly devoured. And as for the predators themselves, they were a diverse gaggle of looming danger. Since dinosaurs had been gone for some 10 million years before the Eocene, Hyracotherium didn’t have to worry about them gobbling him up. But on the other hand, he did have to worry about eight foot tall “terror birds” and the creodonts, an extinct group of carnivores who used an ambush and chase tactic to take down early camels, which they used on early horses. These creodonts would later evolve into modern cats and dogs during the Miocene. In fact, the Oligocene saw the beginnings of the Saber-Toothed Cat, an imposing predator, indeed. So the early horses certainly had their evolutionary work cut out for them to change from a browser lifestyle to a cursorial one…and they had to do it quickly.  

Conclusion to Part VI, Evolution Part II

Fascinating stuff, huh? "You are what you eat" is literally true for the equine! It also suggests that all of these changes are necessary for his lifestyle, and therefore for his well-being. It also implies that there's very little fudge-factor in his biological underpinnings since he's such a delicately balanced biomechanical system based entirely on functional attributes. This has critical implications regarding many of the breeding trends we see today in some popular breeds, and thus in our creative decisions in the studio.

In the third part of "Evolution," we'll learn even more about his biology for more informed creative choices. The horse has undergone many fundamental changes through the eons, each one of them distinct to the equine. He's a unique animal—archaic, ancient, and full of surprises!

So until next time...may our thinking evolve, too!

"We must keep the goal for integrating new information ever in front of us."

~ Carole Mayne