Tales of a mysterious monster that sucks the blood of livestock have exploded in Mexico, the U.S. Southwest, and even China since the mid-1990s, when the chupacabra, or chupacabras, was first reported in Puerto Rico (map).

Now, just in time for Halloween, scientists say they can explain the stories with the help of evolutionary theory.

In almost all these cases, the monsters have turned out to be coyotes suffering from very severe cases of mange, a painful, potentially fatal skin disease that can cause the animals' hair to fall out and skin to shrivel, among other symptoms. (Related: "'Balding' Bears: Mangy Mystery in Florida.")

For some scientists, this explanation for supposed chupacabras is sufficient. "I don't think we need to look any further or to think that there's yet some other explanation for these observations," said Barry OConnor, a University of Michigan entomologist who has studied Sarcoptes scabiei, the parasite that causes mange.

Likewise, wildlife-disease specialist Kevin Keel has seen images of an alleged chupacabra corpse and clearly recognized it as a coyote, but said he could imagine how others might not.

"It still looks like a coyote, just a really sorry excuse for a coyote," said Keel, of the Southeastern Cooperative Wildlife Disease Study at the University of Georgia.

"I wouldn't think it's a chupacabras if I saw it in the woods, but then I've been looking at coyotes and foxes with mange for a while. A layperson, however, might be confused as to its identity."

Chupacabra Evolution

Sarcoptes scabiei also causes the itchy rash known as scabies in humans. In humans and nonhuman animals alike, the mite burrows under the skin of its host and secretes eggs and waste material, which trigger an inflammatory response from the immune system.

In humans, scabies—the allergic reaction to the mites' waste—is usually just a minor annoyance. But mange can be life threatening for canines such as coyotes, which haven't evolved especially effective reactions to Sarcoptes infection.

The University of Michigan's OConnor speculates that the mite passed from humans to domestic dogs, and then on to coyotes, foxes, and wolves in the wild.

His research suggests that the reason for the dramatically different responses is that humans and other primates have lived with the Sarcoptes mite for much of their evolutionary history, while other animals have not.

"Primates are the original hosts" of the mite, OConnor said. "Our evolutionary history with the mites help us to keep [scabies] in check so that it doesn't get out of hand like it does when it gets into [other] animals."

In other words, humans have evolved to the point where our immune systems can neutralize the infection before the infection neutralizes us. (Related: "Future Humans: Four Ways We May, or May Not, Evolve.")

The mites too have been evolving, suggested the University of Georgia's Keel. The parasite has had time to optimize its attack on humans so as not to kill us, which would eliminate our usefulness to the mites, he said.

In nonhuman animals, Sarcoptes hasn't figured out that balance yet. In coyotes, for example, the reaction can be so severe that it causes hair to fall out and blood vessels to constrict, adding to a general fatigue and even exhaustion.

"Goatsucker" Explained?

Since chupacabras are likely mangy coyotes, this explains why the creatures are often reported attacking livestock.

"Animals with mange are often quite debilitated," OConnor said. "And if they're having a hard time catching their normal prey, they might choose livestock, because it's easier."

As for the blood-sucking part of the chupacabra legend, that may just be make believe or exaggeration.

"I think that's pure myth," OConnor said.

"Evolution" of a Legend

Loren Coleman, director of the International Cryptozoology Museum in Portland, Maine, agreed that many chupacabra sightings—especially the more recent ones—could be explained away as appearances by mangy coyotes, dogs, and coyote-dog hybrids, or coydogs.

"It's certainly a good explanation," Coleman said, "but it doesn't mean it explains the whole legend."

For example, the more than 200 original chupacabra reports from Puerto Rico in 1995 described a decidedly uncanine creature.

"In 1995 chupacabras was understood to be a bipedal creature that was three feet [about a meter] tall and covered in short gray hair, with spikes out of its back," Coleman said.

But, as if in a game of telephone, the description of the chupacabra began to change in the late 1990s due to mistakes and mistranslations in news reports, he said.

By 2000 the original chupacabra had been largely replaced by the new, canine one. What was seen as a bipedal creature now stalks livestock on all fours.

"It was actually a big mistake," Coleman said.

"Because of the whole confusion—with most of the media reporting chupacabras now as dogs or coyotes with mange—you really don't even hear any good reports from Puerto Rico or Brazil anymore like you did in the early days. Those reports have disappeared and the reports of canids with mange have increased."

First Chupacabras: Monkeys or Movie Madness?

So what explains the original chupacabra myth?

One possibility, Coleman said, is that people imagined things after watching or hearing about an alien-horror film that opened in Puerto Rico in the summer of 1995.

"If you look at the date when the movie Species opened in Puerto Rico, you will see that it overlaps with the first explosion of reports there," he said.

"Then compare the images of [actor] Natasha Henstridge's creature character, Sil [picture], and you will see the unmistakable spikes out the back that match those of the first images of the chupacabras in 1995."

Another theory is that the Puerto Rico creatures were an escaped troop of rhesus monkeys on the island, which often stand up on their hind legs.

"There was a population of rhesus monkeys being used in blood experiments in Puerto Rico at the time, and that troop could have got loose," Coleman said.

"It could be something that simple, or it could be something much more interesting, because we know that new animals are being discovered all the time."

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