Riddley's people follow a timeless human tradition of displaying their enemies' severed heads on a stick, not only as a warning to others but as a source of mystical knowledge.
Perhaps the best-known talking head was Orpheus: after he was pulled apart, his head washed up on the island of Lesvos, where in some versions of the story it continued to sing and delivered prophecies. However, Orpheus is not a typical case in that his head was not removed for this specific purpose, but became an oracle for reasons of its own (in fact, Apollo had to tell it to stop).
More commonly, heads are preserved because someone is after information from the departed spirits. In Voudon and other African traditions, a spirit can be consulted either through a prepared skull or an artificial construct into which the spirit is placed. Closer to home for Riddley, the Celts identified the head with the soul and prized the heads of their own ancestors as well as those of their foes; possessing a head implied possessing the strength and wisdom of the deceased. The deathless head of Bran the Blessed not only protected his country from harm, but entertained dinner guests for eighty years.
In Christian legend, John the Baptist is sometimes said to have continued his prophetic career after his decapitation; the Knights Templar were accused of worshiping a head called Baphomet, which some said was actually the Baptist. Visitors to the tomb of Karl Marx at Highgate may feel a similar principle is at work.
Artificial speaking heads are a later development, and not nearly as popular; of course they are not as easy to get, but there may also be a natural suspicion of the idea of artificial intelligence. Roger Bacon and the medieval magician Albertus Magnus were accused of building intelligent heads. Only in modern science fiction do we see the idea of connecting heads to machines, such as the iron hat that Eusa unwisely puts on. EB
The best-known oracular head in modern literature belongs to the pig in William Golding's Lord of the Flies (1954). Golding's novel, like Hoban's, features children in a possibly post-apocalyptic setting; its descent into savage ritual begins with the children sharpening a pole. BF
Heads also play a major role in Hoban's other novels: Pilgermann (the tax collector and several hundred Turks and Crusaders, decapitated); Kleinzeit and The Medusa Frequency (Orpheus, decapitated; Medusa and the Kraken as snaky heads); Fremder (Fremder's father, his head accidentally teleported across the galaxy). EB