For the better part of going on towards five decades worth, there's been a ongoing and gruesome phenomena seemingly clustered around the American mid-western regions (though Charles Fort collected similar cases from England in the 19th and early 20th Centuries) that involves what has been described as the mutilation of large herbivore wildlife such as bison, but more usually domesticated livestock like cattle, sheep and horses. Mutilation is not usually a word associated with natural processes like predation, rather deliberate acts for purposes other than taking down an animal as a natural food source. We'll return to that unnatural bit later on down the track.
These unfortunate victims usually have selected and nearly consistent bits and pieces of their body removed with near surgical precision. Most of the carcass is left behind, though the body is usually devoid of blood.
Firstly, from the outset, there's no denying the reality of the phenomena - ghosts and UFOs come and go like will-o'-the-wisps, but animal mutilations, like crop circles, stick around and are available for ongoing scrutiny and analysis, and both have some other common threads like clustering in geographical areas (crop circles tend to be associated with and clustered in SW England. Both are ongoing and both now number in the hundreds to thousands of individual cases. Further, there have been several (two) independent Federal inquiries into the phenomena, one by the FBI (1979), the other by the Bureau of Alcohol, Tobacco, Firearms and Explosives (AFT). Both studies produced conclusions that anomalies existed in some mutilation cases and that further studies were warranted. There have also been various State-level investigations as well as Senate hearings.
The most obvious answer to the mutilation mystery is that all can be attributed to natural predators doing their thing. In fact professional veterinarians usually associated with university schools of veterinary medicine or science proclaim from their ivory towers that these are indeed the results of natural predation by known carnivores, all by studying the photographic evidence - god forbid they'd actually do any on-site fieldwork. However, they do admit that not all of the hundreds to thousands of mutilation cases fall into such neat little piles. All agree by now that all cases cannot be easily explained by wild cats like mountain lions and bobcats or those from the canine side of things like wolves and coyotes, and similar predators such as foxes and wild dogs.
An obvious problem with the natural predation theory is the appearance of the phenomena in the mid-1960s (apart from still relatively recent England noted above). Prey and predators existed way before that as well as humans to report the cases.
In contrast, the man-on-the-land, the farmers, rangers, hunters, wildlife field biologists and vets who work for government departments and other outdoors types who are quite familiar with viewing and inspecting your run of the mill animal predation tend to take a more opposite view - there's nothing natural about these sorts of animal predations and the state the remains of the carcass are found in. This might appear to be a typical academic skeptic vs. eyewitness (I know what I saw) disagreements were it not for a few other unusual features.
Firstly, you'd expect to see signs of a predator-prey struggle - the ground should have been scuffed up a bit; the vegetation slightly disturbed. That's not what you find. You'd expect in many cases to observe the tracks of the predatory animal(s). You don't tend to find tracks. Often there are not even tracks of the victim in the immediate vicinity as if the animal were dumped from above at the spot. Thirdly, if natural, you wouldn't expect the ground/soil and surrounding vegetation in the immediate area of the kill to show any long-term lasting effects. Often the kill site can be identified way down the track by unusual changes in the condition of the soil and vegetation, usually for the worse.
Further, natural predators tend to avoid or shy away from the mutilated carcass even though it's an apparently easy meal. Ditto other livestock avoid the area as well. It's like a no-go zone. Some post-mortem biochemical studies suggest that the victim in question was tranquilized prior to the mutilation. One of the anomalies often reported via laboratory studies of the carcass are unusual alterations in the amounts of vitamins, minerals and other associated biochemical compounds. The findings of tranquilizers and/or anomalies in the standard biochemistry of the victim, points more to a human or an otherworldly culprit. What about the human animal?
Because the nature of the wounds often having the nature or hallmarks suggestive of surgery; well there's another predator that needs to be considered, the human predator. Well that doesn't really appear to wash either - again, no human footprints in the area prior to discovery; no tire tracks (and most of the kill sites are off the beaten track - a long walk to get to them). Further, to the best of my knowledge, no human(s) have ever been observed in the act, far less caught, tried and convicted. Further, no human(s) or cults (Satanic or otherwise) have claimed responsibility. In gun-toting America, especially rural America where rifles and shotguns probably outnumber their human owners, any other humans with animal mutilation on their depraved minds are really risking their hides - farmers seeing their livestock (and livelihoods) butchered are liable to shoot first and ask questions later!
Further still, the nature of the incisions hasn't been duplicated in the laboratory by lasers (an obvious human tool if humans were responsible) since there's no signs of heat or cooking on the carcass incisions. Laboratory tests show that the time and effort to do the high precision cutting job using sharp steel implements is quite considerable. Hides are tough, unlike human skin. Further, no evidence beyond reasonable doubt (say by the FBI or the AFT) of any human or cultist activity was ever uncovered by any official investigation
Of course there have been claims that the American government is actually responsible via clandestine or covert research into possible cattle-human transmitted diseases like 'mad cow disease' and scrapie, research via the National Institute of Health (NIH) and the Center for Disease Control (CDC) with covering support from the armed forces via 'black helicopters'. However, on balance, you'd think such work in the public interest wouldn't have to be done undercover! Surely the American powers-that-be can buy livestock at random for testing; ditto hunt other wildlife for routine and ongoing testing in the open. Some conspiracy scenarios make sense - this one doesn't.
You can't really pin this on some sort of unknown, undiscovered predator like the Chupacabra - an animal that would be in the providence of cryptozoologists since there have been no sightings of such potential critters in the area with the necessary 'tools' (teeth and jaws), and anyway no tracks, known animal or unknown animal.
Since the phenomena exists and requires explanation and resolution, perhaps that leaves the otherworldly. Are aliens in UFOs responsible? While there have, on occasion been reports of unusual aerial objects in the relevant vicinity prior to mutilations, it would seem on the surface to be relatively absurd. Why would aliens want bits and pieces and the blood of large herbivores?
Alien beings will, by definition, have alien minds, an alien psychology and alien motives - alien relative to us of course.
There is however an obvious parallel with alien UFO abductions. Humans who claim to have been abducted by the 'Greys' also claim to have had tissue samples taken from them - biopsies - and often have the unexplained scares to 'prove' it, though to the skeptics that hardly constitutes real evidence, far less proof. Still, it's another dot point suggesting that aliens, for whatever reason, just take tissue samples to monitor - what? Well, who knows? At least the humans aren't sacrificed on the alien's altar.
Needless to say humans (biologists, vets, nurses, medical doctors & researchers) do tissue sample work all the time and sometimes it's necessary to kill the subject under the knife first.
Ultimately, to date, the only reasonable conclusion that can be reached is that there is no 'smoking gun' proof yet to identify beyond question whodunit, if indeed there is just a single whodunit.
Science librarian; retired.