Spotting Fruit versus Picking Fruit as the Selective Advantage of Human Colour Vision

Bompas, Aline; Kendall, Grace; Sumner, Petroc · 2013 · Crossref

DOI: 10.1068/i0564

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Summary

This study investigates the evolutionary drivers of human trichromatic color vision, specifically addressing whether the selective advantage lay in spotting fruit from a distance or picking it at close range. While previous research suggested that the spatiochromatic properties of the human red–green (LM) color channel are optimized for close-up tasks, the authors argue that natural selection is driven by the advantage a phenotype holds over competing alternatives, not necessarily by the task it performs best. To resolve this discrepancy, the researchers tested whether the performance gap between normal trichromats and color-deficient individuals widens with viewing distance. The experiment employed a naturalistic visual search task involving human participants. Twelve color-deficient individuals (including dichromats and anomalous trichromats) were paired with normal trichromats matched for age, sex, and experience. Participants simultaneously searched for yellow pepper segments (simulating ripe fruit) hidden among green leaves in a real bush at distances of 1, 4, 8, and 12 meters. The stimuli were selected to mimic the spectral properties of fruit favored by primates in Ugandan rainforests. Participants were required to count the number of targets as quickly and accurately as possible. This paired design ensured identical lighting conditions and introduced a competitive element simulating primate foraging dynamics. The results demonstrated that normal trichromats outperformed color-deficient competitors in both response time and accuracy. Crucially, the magnitude of this advantage increased significantly with viewing distance. Statistical analysis revealed that the performance difference was greatest at the longest distance (12 meters), where the stimuli subtended less than 0.2 degrees of visual angle. At closer distances, the advantage was smaller. The effect was particularly pronounced in response times for full dichromats compared to anomalous trichromats. The authors conclude that spotting fruit from a distance was likely a more significant selective pressure in the evolution of primate trichromacy than picking fruit at arms' length. This finding reconciles the apparent contradiction between the channel's optimization for close-up tasks and the evolutionary benefit of long-distance detection. The study suggests that the LM channel’s spatial-frequency sensitivity may have emerged as a side effect of pressures favoring long-distance detection, rather than being primarily optimized for close-up selection. This implies that previous field studies focusing on close-up foraging may have underestimated the evolutionary advantage of trichromacy, as the greatest benefit occurs when visual cues like shape and blue–yellow color contrast diminish at distance.

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