Everyone loves a good monster story.

The problem is, monsters always insist on living in faraway lands, at the bottom of lakes, or in the depths of forests. While this adds to their mystique, it is little wonder that our knowledge of them comes exclusively from grainy photos and unreliable witness reports.

This is what first drew Benjamin Radford to the chupacabra, a supposedly vampire-like creature. Its roots are in Latin America, but stories about it have since spread to the rest of the world, including his native New Mexico.

"This was a much more local mystery to me," says Radford. "I didn't have to go to Inverness or Borneo, it was right here in my backyard." Helpfully, the chupacabra also seemed to be less shy than your average monster. That meant Radford had a good chance of figuring out whether or not it was real.

Tales of the chupacabra first emerged in Puerto Rico in the late 1990s. They described a bipedal creature four or five feet tall with large eyes, spikes down its back and long claws. This beast, people claimed, was responsible for killing and draining the blood of livestock, an act that earned it its name – which is Spanish for "goat-sucker".

What is remarkable is how fast the story travelled

In his extensive investigation, which took a total of five years and saw him travel as far as the jungles of Nicaragua, Radford even located the person who first reported this beast: chupacabra patient zero.

Her name is Madelyne Tolentino and she comes from Canóvanas, a town in the east of Puerto Rico. In 1995, she spotted a scary alien-like creature out of her window.

What is remarkable is how fast the story travelled. After more reported sightings, and links made subsequently in the media with livestock that had been found "drained of blood", the legend of the chupacabra spiralled out of control. First it spread around the island, then the rest of Latin America and into the southern US states. It also flourished online, where it was latched onto by UFO enthusiasts and conspiracy theorists.

Then, in the early 2000s, a different chupacabra arrived on the scene. This one shared some of the traits of earlier sightings but was a little less alien. This time it was described as a hairless, dog-like animal walking on four legs.

And unlike most monsters, this type is not based exclusively on sightings. Chupacabra bodies have reportedly been found.

When you have a body, everything changes

When Radford, a research fellow with the Committee for Skeptical Inquiry got wind of this story, he recognised a unique opportunity. Between the dead livestock and the actual specimens, he had the makings of an unprecedented scientific investigation into a creature that had already achieved infamy on a par with Bigfoot or the Loch Ness monster. This was a story he could really get his teeth into.

"When you have a body, everything changes," he explains. "You have DNA samples, you have bone samples, you have morphology." As with all his missions, Radford approached the chupacabra with an open mind, employing what he calls "investigative scepticism", conducting field work, collecting evidence and interviewing witnesses.

"I was of course initially sceptical of the creature's existence," he says. "At the same time I was mindful that new animals have yet to be discovered. I didn't want to just debunk or dismiss it. If the chupacabra is real, I wanted to find it."

The obvious place to start was with the chupacabra bodies. These have mostly turned up in Texas and other south-western US states, and Radford has recorded about a dozen in total. They are quite horrific-looking: hairless, with a gaunt appearance and burnt-looking skin.

However, DNA tests revealed a pretty mundane reality. The bodies have invariably turned out to be coyotes, dogs or raccoons – barring one that was actually a fish.

The reason these animals get identified as chupacabras is because they've lost their hair owing to sarcoptic mange

But despite clear DNA evidence, this version of events is a little fishy. The people who found and often shot these creatures were usually ranchers or rural folk who should recognise a coyote when they see one. So where does this confusion come from?

"The reason these animals get identified as chupacabras is because they've lost their hair owing to sarcoptic mange," explains Radford.

Sarcoptic mange is caused by itch-inducing mites called Sarcoptes scabei burrowing into the upper layer of the skin. It is very common. Alison Diesel of Texas A&M University, US, who specialises in inflammatory skin conditions in animals, agrees that this disease can be sufficiently gruesome to produce convincing monsters.

"The 'mangy dog' is typically very sparsely haired to near-bald, with red or hyper-pigmented black, thickened skin," she explains. Add to this the self-inflicted wounds from scratching and a hairless body, and you have yourself a "chupacabra".

Sarcoptes mites are thought to have infested humans for hundreds of thousands of years, but in more recent evolutionary history made the transition to dogs and other similar animals. This could explain why scabies in humans – the minor rash that results from the mites burrowing into our skin – is a relatively trivial affair, whereas in dogs it can cause death. These animals might not have had long enough to evolve an effective immune response to the mites.

Blood feeders are looking for blood that's close to the surface of the skin

But the chupacabras are only half the story. "There are two sets of bodies here," says Radford. There are also the reports of dead livestock. "Something is attacking these things, leaving puncture marks on their neck and supposedly draining their [bodies] of every drop [of blood]. So what's going on there?"

The answer, once again, is surprisingly straightforward. These animals are most likely the victims of ordinary predators, such as dogs or other canids. It is not uncommon for a dog to bite an animal in the neck and then leave it. Quite often the animal will then die from internal haemorrhaging, with no other injuries apart from puncture marks.

And while, thanks to Dracula, puncture marks in the neck tend to be associated with vampires, Bill Schutt of the American Museum of Natural History in New York, US, says unequivocally that this is not how real-world blood-sucking creatures actually behave.

"Blood feeders are looking for blood that's close to the surface of the skin, something not found in a jugular vein for example," he says.

In fact, if we compare the characteristics of real blood-feeding animals such as vampire bats with those of the chupacabra, there are hardly any similarities. Vampires, according to Schutt, are small and stealthy, with specialised teeth and digestive systems that allow them to extract nutrients from blood.

A creature the size of a dog "would starve to death pretty quickly on a blood meal," he says, owing to the lack of essential components such as fat.

When an animal dies, the heart and blood pressure stop

Besides the presence of these "tell-tale" bite marks, Radford thinks he knows why worried ranchers might attribute the death of their animals to a blood feeder. Having found a mysterious corpse, they would examine it and perhaps cut into it, expecting blood to spurt out – but they would be surprised.

"When an animal dies, the heart and blood pressure stop," he explains. "The blood seeps to the lowest part of the body, and it coagulates and thickens. It's called lividity, and it gives the illusion that they've been drained of blood."

So if all the mythology surrounding the chupacabra actually comes down to some fairly commonplace natural phenomena, why do the stories live on with such vehemence today?

Bizarrely, Radford says it might have something to do with an anti-US sentiment found across Latin America. This is particularly true in Puerto Rico, which is in the unusual position of being a non-state territory of the US.

"I spoke to several Puerto Ricans who felt that the US had exploited, short-changed, and ignored the island, in economic and many other ways," he says. Most recently, this resentment has played out in the island's ongoing debt crisis.

The original chupacabra had spikes on its back, big eyes… then over the years the idea of what it was became bigger and bigger

As for chupacabras, there are many Puerto Ricans who believe they are another indication of American exploitation and meddling; the result of top secret US scientific experiments taking place in El Yunque rainforest, not far from Tolentino's hometown.

This may be one factor, but the spread of such stories can largely be attributed to the internet.

"I would classify the chupacabra as the first internet monster," says Radford. "If the first sighting had been in 1985, a couple of people would have heard of it, but it wouldn't have gone viral and spread across the world."

Radford points out that the myth has changed rapidly. "The original chupacabra had spikes on its back, big eyes," he says. "Then over the years the idea of what it was became bigger and bigger until you have any mangy dog being called a chupacabra. Now, people go on Google and search 'mysterious animal attacking things'. It's self-perpetuating."

But what about those initial sightings? Having explained away the mysterious specimens and how they might operate, what does Radford think inspired Tolentino to come up with such a story in the first place?

The answer is unexpected. Radford noticed that Tolentino's 1995 description was similar to the alien from the 1995 movie Species, which had recently been released in Puerto Rico and which she had watched. The film was set in the present day, revolves around top-secret US scientific experiments, and was partly filmed in Puerto Rico.

The whole story is a perfect storm of scientific misunderstanding

"It's all there. She sees the movie, then later she sees something she mistakes for a monster," says Radford. While he is careful to clarify that he does not think any witnesses are lying, he does suggest that this sighting could simply be a result of an overactive imagination.

Even today, while it is difficult to assess the extent to which people still believe the chupacabra myth, it remains a widely-discussed monster, not only in Puerto Rico but also the rest of the world. In recent years, chupacabras have been reported as far afield as Russia and the Philippines.

"From my perspective there is absolutely no reason to believe that anything out of the ordinary is involved in the attacks on livestock," says Radford. Instead, the whole story is a perfect storm of scientific misunderstanding, misidentification of animals, media hype, cultural anxiety and mass hysteria, all potentially resulting from one woman's viewing of a film.

It goes to show, you can do all the rigorous analysis you want, all the investigation, all the science. At the end of the day, humans like stories and will continue to tell them, however peculiar or far-fetched.

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