By Michael Kahn

LONDON, Oct 1 (Reuters) - Hair samples taken from child mummies suggest the ancient Incas "fattened" up children chosen for ritual sacrifice months before actually killing them, British researchers said on Monday.

A chemical analysis of four mummies found high in the Andes mountains also indicates the Incans took the children on a lengthy pilgrimage prior to the killings, the team said. In the case of the 15-year-old "Llullaillaco Maiden" the road to death started at least 12 months before.

"We are looking at a process that began a considerable amount of time before their death," said Andrew Wilson, an archaeologist at the University of Bradford, who led the study. "The maiden was essentially being fattened up or prepared for her final fate at least 12 months before her killing."

The frozen mummies, some of them extremely well preserved, come from the world's highest archaeological site and offer insight into the victims' backgrounds and how they were elevated in social status and prepared for the rituals.

The vast Inca empire, with its advanced culture and powerful armies, spanned most of the Andes along South America's western coast at the time of the Spanish conquest in the early 16th century.

"This is the very first time we are hearing the account from the individuals themselves -- what they were eating and when they were separated from their normal existence and set on this path," Wilson said.

The researchers, who published their findings in the Proceedings of the National Academy of Sciences, analysed hair samples up to 25 cm (nearly 10 inches) long in the children aged 6 to 15.

This length represented about two-and-a-half years of hair growth, which gave researchers a picture of the doomed children's' lives over a fairly long period of time.

Changes in isotopes, or chemical signatures, in the earliest samples showed the children were fed a steady diet of vegetables typical of a peasant background, Wilson said.

But in the 12 months prior to sacrifice, the diet suddenly changed to food reserved for the elite such as maize and meat, likely representing the children's elevated status as offerings to the gods, the researchers said.

Other isotopic changes indicate that in their final three or four months, the children began a pilgrimage to the mountains, likely from the Incan capital Cuzco after receiving ritual haircuts, Wilson said.

The researchers do not know exactly how all the children died, though at least one was killed by a blow to the head.

"It looks to us as though the children were led up to the summit shrine in the culmination of a year-long rite, drugged and then left to succumb to exposure," said Timothy Taylor, a researcher at the University of Bradford.

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