Ox and donkey are latter-day additions to nativity scene but tradition is here to stay, Benedict says in book on Jesus's infancy

As Christians around the world start thinking about setting up nativity scenes for Christmas, the pope has pointed out that the ox and the donkey – regular fixtures around the manger – are latter-day inventions nowhere to be seen in the gospels.

Benedict puts the record straight in his third book on the life of Christ, Jesus of Nazareth: The Infancy Narratives, which was released on Tuesday and looks destined for the bestseller lists with an initial print run of one million.

Having dealt with Christ's adult life and death in his first two books, the pope tackles the birth of the son of God and puts paid to some myths surrounding the newly born Jesus's spell in a stable with Mary and Joseph.

"In the gospels there is no mention of animals," the pope states. He says references to the ox and the donkey in other parts of the Bible may have inspired Christians to include them in their nativity scenes.

The Vatican itself has included animals in the nativity scenes it sets up each year in St Peter's Square, and Benedict concedes that the tradition is here to stay. "No nativity scene will give up its ox and donkey," he says.

Showing his scholarly approach to the Bible, Benedict also analyses the moment angels descended to tell shepherds the son of God was lying in a manger nearby. In a blow to fans of the carol Hark the Herald Angels Sing, Benedict writes: "According to the evangelist, the angels 'said' this. But Christianity has always understood that the speech of angels is actually song, in which all the glory of the great joy that they proclaim becomes tangibly present."

One aspect of the nativity story that Benedict states is absolutely true is the virgin birth of Jesus. Catholics, he argues, should see this, as well as Jesus's resurrection as "cornerstones of faith".

"If God does not also have power over matter, then he simply is not God," he writes. "But he does have this power, and through the conception and resurrection of Jesus Christ he has ushered in a new creation."