ten Cate, C. and Verzijden, M. (2006). Sexual imprinting can induce sexual preferences for

exaggerated parental traits. Current Biology, 16, 1128-1132.

This study assessed whether sexual imprinting in zebra finch could give rise to peak shift in male mate preference. Previous studies have shown that zebra finch learn sexually dimorphic signals through imprinting their parent’s traits. Males tend to choose mates based on beak color, and prefer beak colors that match their mother while avoiding females that match their father. The researchers wished to test whether this learning process could be affected by peak shift. Peak shift is a well-documented, inherent part of discriminative learning. When animals learn to discriminate between two similar stimuli, they tend to over-generalize the desired stimuli to distinguish it further from the undesired stimuli. In this study, the researchers expected the male zebra finches to over-generalize the color of their mother’s beak and prefer females with even more exaggerated “mother” coloration.

To test this hypothesis, the researchers used nail polish to experimentally manipulate the beak color of white parents raising broods of males. In one group the mother’s beak was painted orange and the father’s beak red and in another group the colors were reversed. After the males had reached sexual maturity they were presented with eight females whose beak color had also been manipulated with polish. Some of the females were the same color as the father and mother and other females had orange and red colors that were more exaggerated than the parents’ colors. The males’ preferences were measured by how many mating song the males sang at the females they saw. As expected, males preferred females that had the same color as the mother, and they further preferred the more exaggerated coloration (purer orange or red). The males also tended to sing less to females that matched or exaggerated the fathers’ color. These results suggest that males base their mate choice on learned sexually dimorphic signals and that this learning results in peak shift of color preference.

This study gives an alternative explanation for the origins of sexually dimorphic and exaggerated sexual signals. In class we discussed the idea that sexual signals may be arise because of inherent receiver biases or feature detectors that are naturally tuned to particular colors, movements, sounds, etc. We also discussed the possibility that some signals may be honest signs of an animal’s condition, and can thus be used by others to assess the viability of mates. We did not discuss how the learning process itself may shape the evolution of signals. This study has shown that peak shift does occur for the zebra finch (and presumably in other animals that learn from parent models how to distinguish between the sexes). Peak shift can explain how sexual signals can evolve into more exaggerated traits because each successive generation prefers a more exaggerated form of the parent model. This explanation is not appropriate for all animals and does not exclude the possibility of the other theories also contributing to the evolution of sexual signals.