The European motif of the devilish werewolf devouring human flesh harks back to a common development during the Middle Ages in the context of Christianity, although stories of humans turning into wolves take their roots in earlier pre-Christian beliefs.

Their underlying common origin can be traced back to Proto-Indo-European mythology, where lycanthropy is reconstructed as an aspect of the initiation of the kóryos warrior class, which may have included a cult focused on dogs and wolves identified with an age grade of young, unmarried warriors. The standard comparative overview of this aspect of Indo-European mythology is McCone (1987).
A few references to men changing into wolves are found in Ancient Greek literature and Folklore. Herodotus, in his Histories, wrote that according to what the Scythians and the Greeks settled in Scythia told him, the Neuri, which was a tribe to the north-east of Scythia, were all transformed into wolves once every year for several days and then changed back to their human shape. He added that he is not convinced by the story but the locals swear to its truth. This tale was also mentioned by Pomponius Mela.

In the second century BC, the Greek geographer Pausanias related the story of King Lycaon of Arcadia, who was transformed into a wolf because he had sacrificed a child in the altar of Zeus Lycaeus. In the version of the legend told by Ovid in his Metamorphoses, when Zeus visits Lycaon disguised as a common man, Lycaon wants to test if he is really a god. To that end, he kills a Molossian hostage and serve his entrails to Zeus. Disgusted, the god turns Lycaon into a wolf. However, in other accounts of the legend, like that of Apollodorus' Bibliotheca, Zeus blasts him and his sons with thunderbolts as punishment.

Pausanias also relates the story of an Arcadian man called Damarchus of Parrhasia, who was turned into a wolf after tasting the entrails of a human child sacrificed to Zeus Lycaeus. He was restored to human form 10 years later and went on to become an Olympic champion. This tale is also recounted by Pliny the Elder, who calls the man Demaenetus quoting Agriopas. According to Pausanias, this was not a one-off event, but that men have been transformed into wolves during the sacrifices to Zeus Lycaeus since the time of Lycaon. If they abstain from tasting human flesh while being wolves, they would be restored to human form nine years later, but if they do not abstain, they would remain wolves forever.

Lykos (Λύκος) of Athens was a wolf-shaped herο, whose shrine stood by the jurycourt, and the first jurors were named after hi…

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Some modern researchers have tried to explain the reports of werewolf behaviour with recognised medical conditions. In 1963, Dr Lee Illis of Guy's Hospital in London wrote a paper entitled On Porphyria and the Aetiology of Werewolves, in which he argues that historical accounts on werewolves could have been referring to victims of congenital porphyria, stating how the symptoms of photosensitivity, reddish teeth, and psychosis could have been grounds for accusing a person of being a werewolf.

This is argued against by Woodward, who points out how mythological werewolves were almost invariably portrayed as resembling true wolves, and that their human forms were rarely physically conspicuous as porphyria victims. Others have pointed out the possibility of historical werewolves having been people with hypertrichosis, a hereditary condition manifesting itself in excessive hair growth. Woodward dismissed the possibility, as the rarity of the disease ruled it out from happening on a large scale, as werewolf cases were in medieval Europe.

Woodward suggested rabies as the origin of werewolf beliefs, claiming remarkable similarities between the symptoms of that disease and some of the legends. Woodward focused on the idea that being bitten by a werewolf could result in the victim turning into one, which suggested the idea of a transmittable disease like rabies. However, the idea that lycanthropy could be transmitted in this way is not part of the original myths and legends, and only appears in relatively recent beliefs. Lycanthropy can also be met with as the main content of a delusion, for example, the case of a woman has been reported who during episodes of acute psychosis complained of becoming four different species of animals.

The beliefs classed together under lycanthropy are far from uniform, and the term is somewhat capriciously applied. The transformation may be temporary or permanent; the were-animal may be the man himself metamorphosed; may be his double whose activity leaves the real man to all appearance unchanged; may be his soul, which goes forth seeking whomever it may devour, leaving its body in a state of trance; or it may be no more than the messenger of the human being, a real animal or a familiar spirit, whose intimate connection with its owner is shown by the fact that any injury to it is believed, by a phenomenon known as repercussion, to cause a corresponding injury to the human being.

Werewolves were said in European folklore to bear tell-tale physical traits even in their human form. These included the meeting of both eyebrows at the bridge of the nose, curved fingernails, low-set ears and a swinging stride. One method of identifying a werewolf in its human form was to cut the flesh of the accused, under the pretense that fur would be seen within the wound. A Russian superstition recalls a werewolf can be recognized by bristles under the tongue.

The appearance of a werewolf in its animal form varies from culture to culture. It is most commonly portrayed as being indistinguishable from ordinary wolves, except for the fact that it has no tail (a trait thought characteristic of witches in animal form), is often larger, and retains human eyes and a voice. According to some Swedish accounts, the werewolf could be distinguished from a regular wolf by the fact that it would run on three legs, stretching the fourth one backwards to look like a tail.

After returning to their human forms, werewolves are usually documented as becoming weak, debilitated and undergoing painful nervous depression. One universally reviled trait in medieval Europe was the werewolf's habit of devouring recently buried corpses, a trait that is documented extensively, particularly in the Annales Medico-psychologiques in the 19th century.
Various methods for becoming a werewolf have been reported, one of the simplest being the removal of clothing and putting on a belt made of wolfskin, probably as a substitute for the assumption of an entire animal skin (which also is frequently described). In other cases, the body is rubbed with a magic salve. Drinking rainwater out of the footprint of the animal in question or from certain enchanted streams were also considered effectual modes of accomplishing metamorphosis.

The 16th-century Swedish writer Olaus Magnus says that the Livonian werewolves were initiated by draining a cup of speci…

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