Pure-bred individuals of the two species are approximately the same size. Offspring of a female maniculatus crossed with a male polionotus produce offspring that are smaller than either parent. Conversely, when a male maniculatus is crossed with a female polionotus , offspring are oversized and typically die before birth, rare survivors being female. These effects are caused by a phenomenon called 'imprinting'. Patterns of imprinting may vary between species.
Hybridization can disrupt the pattern, resulting in a wide spectrum of abnormalities.
The bottom line, the study suggests, is that genes controlling imprinting rapidly evolve during speciation, short-circuiting the more gradual, underlying rate of genetic change.