AFAIK cats aren't domesticated. It's more that we have a deal with them. They moved in on their own when they realised humans provide shelter if you hunt their pests.
They aren't as domesticated as dogs, that's for certain, but you can't just take a bobcat or other small wild cat kitten and expect it to be a good pet that won't try to attack you when it gets older. Even the more exotic breeds of housecat like the Savannah (crossbred with wild servals) have behavior issues. Housecats have been domesticated to the degree that they continue to act kittenish in adulthood.
We've never prized docility in horses the way we do in other "pets"; in most cases, we've either bred them to be easily-spooked speed monsters (race horses; courier horses), or angry muscular bulls (war horses.)
And there has never really been any kind of long-term eugenics project for cats; they mostly domesticated themselves, which had different results. There are no "work cats" that we've selectively bred with other "work cats" to improve their demeanour; there are purebred cats, but they're basically assholes and we basically don't care. (Why this is, I'll never know.)
Docility is much prized in horses. I've owned ex-racehorses, a Percheron war horse, an ex-police horse, and a big quiet warmblood. They're all "docile"; they'll accept training and won't bite or attack without serious provocation.
Horses have been bred to be larger and more docile. The natural size for horses is a large pony.
It's unclear how this started. Przewalski's horse, the last remaining feral horse breed, is not the genetic ancestor of the modern horse, but a diverging line.
(Horses are somewhat different in the Americas than in Europe and Asia. They're not native. Feral horses in the Americas are descended from ones brought over from Europe by early Spanish conquerers. Amusingly, the pedigrees are known; the Spanish expeditions were Government operations and there's surviving paperwork. Most were good Andalusians.)
> They're all "docile"; they'll accept training and won't bite or attack without serious provocation.
I guess I'm using "docility" a bit differently. There are two somewhat different commonly-used meanings, and it'd be helpful if they were different words:
• the animals everyone tends to commonly use the word "domesticated" for, which usually have specifically eusocial behaviours, like coming and laying down beside a human. Dogs are the central example; but numerous animals, from ferrets to hamsters to pigeons, are actually like this.
• the animals that aren't afraid of humans, and will tolerate their presence, and maybe learn skills from them. Horses, cows, sheep, chickens, etc. Farmers and breeders call these species "domesticated" (compared to their wild cousins), but they're not called that by lay-people. "Able to be livestock" might be what the average person would call these. (Oddly, some common "pets", like guinea pigs, are actually more in this category.)
One of the major differences, in my mind, is that the animals everyone calls domesticated, like being around humans enough that—if raised in a human environment—they'll often defend their human "family member" against their own kind. But this is not a behaviour you see with the technically-domesticated species; a wolf, or a fox, or a cow, might defend its territory if it's feeling territorial, but it won't specifically defend you, even if you raised it. It knows humans are a sometimes-helpful thing, but its instincts haven't been hacked enough to consider them "kin."
I'm not actually sure where horses fall on this measure, having not had much personal experience with them. Your input?
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Also, fitting into neither category, there are a few extremely-intelligent species that, by this measure, we might call "domesticated" without having had much human-mediated human interaction at all. Corvids and chimpanzees both understand human social behaviour well enough to "befriend" individual humans, but this doesn't translate to them having a default-positive association with humans in general.
Also, under this distinction, I'd say that most wild animals that have assumed a "city habitat" like raccoons, skunks, squirrels, increasingly foxes in Britain, etc. are "technically domesticated." They're pretty much as docile if raised as pets as a horse or a cow would be. Definitely less unpredictable than a "pet" monkey. These species are doing the same thing cats did to get where they are; they just haven't spent as many generations evolving under the constraints cats have yet.
There are lots of books about horse behavior. It's been studied pretty throughly. Most horses are willing to socialize with humans. They're not submissive in the way that dogs are, though. Once you understand some horse body language (see "Talking with Horses", by Henry Blake) they're much more willing to socialize. Horses are flight animals, herbivores, and herd animals, and their behavior comes from that. Dogs are pack animals and carnivores. Different mindset.
(My current horse is possessive of me. I recently turned him out in an arena with another horse he likes, and the two played around a bit. Then the other horse came up to visit me. My horse ran over, ears pinned back and teeth bared, to chase the other horse away. But he wasn't "defending" me; the other horse wasn't a threat and my horse knew that. He was just showing the other horse that I was his human.)
I was of the understanding dogs, our friends, are omnivores.
From wikipedia:
Unlike obligate carnivores, dogs can adapt to a wide-ranging diet, and are not dependent on meat-specific protein nor a very high level of protein in order to fulfil their basic dietary requirements. Dogs will healthily digest a variety of foods, including vegetables and grains, and can consume a large proportion of these in their diet, however all-meat diets are not recommended for dogs due to their lack of calcium and iron.[14] Comparing dogs and wolves, dogs have adaptations in genes involved in starch digestion that contribute to an increased ability to thrive on a starch-rich diet.[18]
Horses are very much domesticated. It was probably very difficult, requiring many generations of breeding.
By comparison, zebras are very hard to work with.
http://theconversation.com/why-zebra-refused-to-be-saddled-w... "In many ways, zebra appear very like horses (or ponies, given their size). Yet underlying differences in behaviour have meant that while horses and donkeys have been successfully domesticated, the zebra remains predominantly wild... Horses were initially kept as a food animal."
Of course there are "work cats". Most large barns will have a few, for rodent control. You want them big and tough enough to take on a rat. Working cats should be fed about half what they need, so they stick around but have to hunt. Once they've cleared the barn area of rodents, they'll expand their hunting territory to find food.
(The problem is keeping cat fans from feeding the working cats. The cat people got out of control at the Stanford barn years ago, and the cats got fat and lazy. The overweight cats had to be traded out for more useful semi-feral cats.)
Those aren't really "working" animals in the sense of having been bred and trained to tasks that are substantially different than wild behavior. They're just following their instincts, which happen to be beneficial to humans in this case.
>We've never prized docility in horses the way we do in other "pets"; in most cases, we've either bred them to be easily-spooked speed monsters (race horses; courier horses), or angry muscular bulls (war horses.)
Draft horses and other working horses beg to differ.
They're generally calm and docile, and don't spook easily. Especially draft horses working in cities, they're gentle giants, because they have to be when working on traffic and noise. Police horses too.
As for cats, they don't give you the unconditional love that you get from a dog. They need to respect you first.
The police horse thing is about 20% breeding, 80% training. I owned a former San Jose police horse for years. Big AQHA quarter horse. He was fine with traffic and crowds. But the first time he saw cattle, he lost it, snorting and prancing away. I was laughing; quarter horses are supposed to have "cow sense" bred in. This urban workhorse had never seen cattle.
"Not one of our domestic animals can be named which has not in some country drooping ears."
It feels disingenuous to exclude breeds based on mutation / selective breeding, as the quote refers to the effects of breeding on domesticated animals, which are by definition going to be mutations of other species.