In most bird species, young individuals take their parents as a model for what their later sexual partner should look like. This is the process of sexual imprinting, made famous by the Nobel prizewinner Konrad Lorenz. Nonetheless, most birds prefer mates in which specific traits are exaggerated compared with those of their parents. Because learning about a specific stimulus usually leads to a preference for this familiar stimulus over an unfamiliar one, it has generally been assumed that imprinting itself could not give rise to preferences for novel, exaggerated traits.
In their new work, the researchers tested the sexual preferences of zebra finch males that were raised by white parents differing in beak color. For one group of males, the researchers painted the beak color of the mother orange and that of the father red; for another group, the mother got a red beak and the father an orange one. When the males reached adulthood, they were tested with females with a spectrum of beak colors ranging from extreme orange to extreme red. Males in both groups preferred females with beaks that were more extremely colored than their mothers' beaks. This "peak shift" effect is known from other learning processes in which animals learn to distinguish different stimuli, but the presence of this effect in the imprinting process has not been demonstrated previously. The outcome of the study shows that the skewed mating preferences that are crucial for driving the evolution of sexual dimorphism and exaggerated traits in birds may result directly from sexual imprinting.