Hybridization, sexual imprinting, and mate choice

Hybridization of animal species is a special pattern of mating and hence a behavioral phenomenon, influenced by ecological, demographic, and individual factors. We examine demographic factors (sex ratio and mate availability) and characteristics of individuals (song and morphology) in an attempt to understand the occasional hybridization of three species of Darwin's finches (genus Geospiza) on the small Galapagos island of Daphne Major. We use field data from a 20-yr study to test five hypotheses. A relative scarcity of conspecific mates (the Hubbs principle) explains the hybridization of the rarer species (Geospiza fuliginosa and Geospiza scandens) but not the common species (Geospiza fortis). Female C. fortis pair with male Geospiza fuliginosa that sing songs similar to their own fathers' songs. Morphological trait values of their G. fuliginosa mates were also correlated with the trait values of their fathers. Geospiza fortis females pair with G. scandens that are morphologically similar to their mothers. These results imply that heterospecific mate choice is influenced by auditory and visual imprinting on parental traits in early life, consistent with the findings of laboratory studies of other groups of finches. Hybridization is most likely to occur when imprinting syndromes of closely related species are similar.