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Eleanor M. Caves, Patrick A. Green, Matthew N. Zipple, Dhanya Bharath, Susan Peters, Sönke Johnsen, and Stephen Nowicki (Feb 2021)
Categorical color perception differs between two finches that differ in coloration and may be adapted for signal function
The essential role of animal signals in interactions such as mate choice and aggression suggests that sensory systems might be adapted to perceive signal stimuli that are particularly salient for a given species. Many finches in the family Estrildidae display colorful signaling traits; for example, male zebra finches (Taeniopygia guttata) display carotenoid-based beak coloration, ranging from light orange to dark red, and females evaluate male beak coloration during mate choice. Prior work has shown that female zebra finches perceive this orange-red color range in a categorical fashion. That is, even though male beak color varies along a continuum from most orange to most red, females perceive this variation as simply either “orange” or “red.”
Is categorical perception a general feature of how birds in this group see colors, or has selection acted on the perceptual system of zebra finches in particular due to the important signaling function of orange-red coloration in this species? In this study, researchers from Duke University examined whether female Bengalese finches (Lonchura striata domestica), an estrildid finch having black, brown and white plumage and lacking any carotenoid coloration, also perceives the orange-red color range tested previously with zebra finches in a categorical fashion. The researchers found no evidence for categorical perception of orange-red coloration in Bengalese finches. Follow up experiments then showed that, in Bengalese finches, the ability to discriminate between two colors is dictated primarily by the difference in brightness between those colors, rather than whether colors fall in certain categories. In zebra finches, however, the importance of brightness differences in color discrimination depends upon whether two colors come from the same or different categories.
The results of this study are consistent with the hypothesis that selection on a signaling system may act on the perceptual systems of the signal receivers. It remains to be seen, however, whether the expression of categorical perception correlates consistently with the use of color signals across a broader range of species such as the clade of estrildid finches. The work also lends insight into the potential mechanisms underlying the phenomenon of categorical color perception in birds, and perhaps in other vertebrates as well.
Sensory systems are predicted to be adapted to the perception of important stimuli, such as signals used in communication. Prior work has shown that female zebra finches perceive the carotenoid-based orange-red coloration of male beaks—a mate choice signal—categorically. Specifically, females exhibited an increased ability to discriminate between colors from opposite sides of a perceptual category boundary than equally-different colors from the same side of the boundary. The Bengalese finch, an Estrildid finch related to the zebra finch, is black, brown and white, lacking carotenoid coloration. To explore the relationship between categorical color perception and signal use, we tested Bengalese finches using the same orange-red continuum as in zebra finches, and also tested how both species discriminated among colors differing systematically in hue and brightness. Unlike in zebra finches, we found no evidence of categorical perception of an orange-red continuum in Bengalese finches. Instead, we found that the combination of chromatic distance (hue difference) and Michelson contrast (difference in brightness) strongly correlated with color discrimination ability on all tested color pairs in Bengalese finches. The pattern was different in zebra finches: this strong correlation held only when discriminating between colors from different categories, but not when discriminating between colors from within the same category. These experiments suggest that categorical perception is not a universal feature of avian, or even Estrildid finch, vision. Our findings also provide further insights into the mechanism underlying categorical perception and are consistent with the hypothesis that categorical perception is adapted for signal perception.