In many contexts, animals assess each other using signals that vary continuously across individuals and, on average, reflect variation in the quality of the signaller1,2. It is often assumed that signal receivers perceive and respond continuously to continuous variation in the signal2. Alternatively, perception and response may be discontinuous3, owing to limitations in discrimination, categorization or both. Discrimination is the ability to tell two stimuli apart (for example, whether one can tell apart colours close to each other in hue). Categorization concerns whether stimuli are grouped based on similarities (for example, identifying colours with qualitative similarities in hue as similar even if they can be distinguished)4. Categorical perception is a mechanism by which perceptual systems categorize continuously varying stimuli, making specific predictions about discrimination relative to category boundaries. Here we show that female zebra finches (Taeniopygia guttata) categorically perceive a continuously variable assessment signal: the orange to red spectrum of male beak colour. Both predictions of categorical perception5 were supported: females (1) categorized colour stimuli that varied along a continuum and (2) showed increased discrimination between colours from opposite sides of a category boundary compared to equally different colours from within a category. To our knowledge, this is the first demonstration of categorical perception of signal-based colouration in a bird, with implications for understanding avian colour perception and signal evolution in general.
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We thank D. Osorio for comments on the manuscript; L. Jin, K. Kochvar, J. Bollinger and L. Deehan for assistance running trials; and R. Mooney and M. Booze for providing access to zebra finches from their colony. Funding for this study was provided by the Duke University Office of the Provost.
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Extended data figures and tables
Units are photons per cm2 per nm. Tungsten bulbs were used to illuminate each cage from a set distance. There was some variation in irradiance between cages. The blue line represents the mean absolute irradiance of our halogen bulbs, and the grey-shaded region indicates one standard deviation in either direction. The orange line illustrates standard Illuminant A, which we used throughout for visual modelling because it is a standard spectrum (and thus repeatable by other researchers) and closely matched the ambient lighting in our room.
Extended Data Fig. 2 Reflected radiance of experimental stimuli under experimental lighting conditions.
Reflected radiance is in arbitrary energy units; stimulus is Munsell paper covered with an epoxy cover. Below 400 nm, the values are so low that they reach the noise floor of the spectrometer, suggesting that the use of a trichromatic model (versus one that includes ultraviolet sensitivity) to examine chromatic distance between colours is appropriate.
a, b, Results show data across all birds (a) and for each bird individually (b). Overall, birds performed more poorly at all one-apart tasks compared to two- or three-apart tasks, perhaps suggesting that colours that are one step apart neared the physiological limit of colour discrimination. However, the pass rate for the 5|6 comparison (which crossed the category boundary; green box, grey-shaded area) was significantly higher than for comparisons that did not cross the category boundary (white-shaded areas; blue boxes) (paired t-test, t24 = 4.09, P < 0.01). Numbers in parentheses inside boxes are number of birds that participated in each type of comparison. Box plots depict the median (horizontal line inside the box), 25th and 75th percentiles (box), 25th and 75th percentiles ±1.5× interquartile range (whiskers) and outliers (circles). The horizontal grey line represents the expected pass frequency if birds flip discs by chance. Numbers in square brackets are Michelson contrast for a given colour pair.
a, b, Results show data across all birds (a) and for each bird individually (b). Pass rate was significantly higher for the three comparisons that crossed the boundary (3|6, 4|7 and 5|8; green boxes, grey-shaded area) than for those that did not (1|4 and 2|5; blue boxes, white-shaded area) (paired t-test, t25 = 6.07, P < 0.001). Numbers in parentheses inside boxes are number of birds that participated in each type of comparison. Box plots depict the median (horizontal line inside the box), 25th and 75th percentiles (box), 25th and 75th percentiles ±1.5× interquartile range (whiskers) and outliers (circles). The horizontal grey line represents the expected pass rate if birds flip discs by chance. Numbers in square brackets are Michelson contrast values for a given colour pair.
Extended Data Fig. 5 Results from greyscale (that is, hue information removed) one-apart discrimination experiments.
These greyscale experiments did not replicate the categories that we observed when hue information was included, indicating that categories are not structured based on brightness alone. Box plots depict the median (horizontal line inside the box), 25th and 75th percentiles (box), 25th and 75th percentiles ±1.5× interquartile range (whiskers) and outliers (circles). Numbers in parentheses below each comparison are Michelson contrast values. Linear mixed models showed that, in our greyscale experiments (that is, without chromaticity information), Michelson contrast between greyscale pairs significantly predicted pass rate. This finding is consistent with the possibility that category formation may be the result of both chromaticity and brightness information (see Table 1). Sample size was 18 birds for all comparisons.
The wavelength discrimination function of the pigeon C. livia (black dashed line) plotted against the spectral sensitivity peaks of the zebra finch (green, red, blue lines). Original data for the electroretinography-derived wavelength discrimination function are from Riggs et al.31 and have been inverted so that higher numbers indicate greater discrimination.
Extended Data Fig. 7 Schematic illustrating differences between continuous and categorical perception.
Under continuous perception (solid line), receivers perceive and respond in a continuous fashion to signal variation, meaning that any change in a signalling trait is associated with a concomitant change in receiver response. Under categorical perception (dashed line), such as described here for female zebra finches, receivers show enhanced discrimination of variants across a boundary (hash marks on x axis) compared to equally spaced variants within a category. Zebra finch line drawing by N. Silina licensed under a Attribution 4.0 International (CC BY 4.0) licence (http://supercoloring.com/pages/zebra-finch).
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Caves, E.M., Green, P.A., Zipple, M.N. et al. Categorical perception of colour signals in a songbird. Nature 560, 365–367 (2018). https://doi.org/10.1038/s41586-018-0377-7