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I came upon an interesting quote after going down the rabbit hole on a variety of subjects. While deep in the data, I came upon this quote

oat and farm animal owners likes to have mules with their herds because donkeys or mules will actually attack predatory animals such as foxes and coyotes.

Which raised quite a few questions in my mind. I was able to find additional sources as it relates to the predisposition of the given species.

Seemingly, donkeys and mules will act aggressively in confrontation with predators and attempt to attack to the point of killing many of them. Is there something about the physiology of a donkey/mule that makes them much less timid? Perhaps body structure? Seemingly a horse has equal or even better tools to try and defend itself.

Here are the sources (the first 2 are not the most authoritative by any means):

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  • $\begingroup$ I was able to find additional sources Can you please indicate these sources? $\endgroup$
    – Remi.b
    Apr 19 '19 at 11:33
  • $\begingroup$ It has been completed $\endgroup$ Apr 19 '19 at 15:44
  • $\begingroup$ Different organisms evolve different behaviors, evolution is a whatever works kind of designer. it can influenced by all kinds of things, social behavior, what kinds of predators there are, offspring care needs, herd size, and a hundred other things. $\endgroup$
    – John
    Apr 21 '19 at 14:08
  • $\begingroup$ We know what wild donkeys are like because they still exist in Africa, population 570, and they are more like small zebra's than horses. Wild horses are from Eurasian grasslands and have all been captured for domestication and eaten, so current wild horses are feral. The are completely different animals, they diverged 5-10 million years ago. They are much more genetically distant than a wolf, labrador and an alsatian, and their habitat and behaviours are completely different, including the dung strategy of donkeys and the semi-desert versus steppe habitat. $\endgroup$ Apr 22 '19 at 9:35
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    $\begingroup$ I can’t speak for other mules but mine does not like dogs so I assume she would not like foxes or coyotes. I’ve seen her literally stomp a dog with her front hoofs. If the dog runs, I’ve seen her chase the dog and start stomping if she can get close enough to the fleeing animal. Our horse, well, he doesn’t give a hoot if a dog is nearby or not. $\endgroup$
    – Jack
    Mar 10 at 18:46
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So horses, donkeys, and zebras are all of the genus Equus. Mules are not a species, since they are the product of the cross-breed between horses and donkeys, and are sterile (therefore do not comply with the biological interpretation of species).

Given that 2 out of the 3 species from the species of the genus Equus are aggressive against predators (zebras see reference and donkeys), it might be possible that the "aggressive" behavior is the ancestral trait of the genus, and horses evolved afterward the behavior of fleeing before fighting. Furthermore, it looks like the "aggressive" trait is dominant over the "fleeing" trait since mules (cross-breeds between horses and donkeys) inherit the aggressive trait. There are no sources that necessarily say that a dominant trait is associated with ancestral (common ancestor) provenance, but for the case of single genes, this is the most likely scenario.

So if we think that horses evolved the trait of fleeing before fighting (being the "aggressive" trait the ancestral one), then one should ask why did they evolve such behavior (why is a cringy word for evolutionary biologists, but serves the purpose here). And this is probably due to the exposure horses had to different types of predators. I wouldn't be surprised if horses had more contact with human beings than donkeys or zebras, thus they evolved this behavior as a way of coping with extensive hunting.

Alternatively, some traits are not "modular" traits that can evolve independently from other traits. More often than not, complex behaviors, like the "aggressive" trait, are determined by a large number of genes, some of which have phenotypical effects elsewhere. This means that if a species has a "selective pressure" for one trait, other traits can also be modified over time as a causal effect of this selective pressure. Horses were domesticated long before donkeys were, and zebras haven't been domesticated at all. Domestication is usually conferred by a process of selectively breeding individuals that express the least amount of aggressiveness, or show signs/traits of "neoteny". If horses were selected to produce neoteny in the species, the non-aggressive behavior might also have been selected as a causal effect of this selective-breeding. This would explain why horses, which have been domesticated way before donkeys have, do not show aggressive behavior as donkeys or zebras do.

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