National GeographicNow, just in time for Halloween, scientists say they can explain the stories with the help of evolutionary theory.
Flesh-and-blood chupacabras have allegedly been found as recently as June—making the monsters eminently more accessible for study than, say, the Loch Ness monster or Bigfoot. (See "Bigfoot Hoax: 'Body' Is Rubber Suit.")
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.
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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.
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