The benefit of outbreeding might be explanation enough for the vigour of their offspring.

ECONOMIST: Adultery yields benefits to females as well as males

The MHC is also thought to act as a proxy for general outbreeding, with all the hybrid vigour that can bring.

ECONOMIST: Perfume science

Yet, despite their seeming success in the Darwinian game of outbreeding the competition, the Cape bee clones are apparently incapable of establishing self-sustaining hives of their own.

ECONOMIST: African honeybees