霊長類における色覚の適応的意義を探る
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概要
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Primates are unique among placental mammals in having trichromatic color vision, while most mammals possess dichromatic color vision. It has been hypothesized that the adaptive significance of trichromacy in primates is to detect reddish ripe fruits against a dappled foliage background. However, a behavioral advantage of trichromacy for fruit foraging has not been demonstrated in wild populations. The present paper reviews color vision status and utility of color vision in various primate species and describes recent advances in examining the significance of trichromacy. New World monkeys, which express high intraspecific color vision diversity due to an allelic polymorphism of the X-linked opsin gene, provide the excellent model to explore the significance of trichromacy for frugivory. The comparison of fruit foraging efficiency between dichromatic and trichromatic individuals in free-ranging black-handed spider monkeys (Ateles geoffroyi) inhabiting a Costa Rican dry forest did not reveal any trichromat advantage. This result is explained via the luminance contrast between fruits and background leaves, which cues the detection and selection of edible ripe fruits when fruits are viewed from close distances. In addition, sniffing behavior toward fruits was negatively correlated with luminance and blue-yellow contrasts, suggesting that monkeys use olfactory cues when vision alone is insufficient to select edible fruits. These results suggest that an advantage of trichromacy is not salient under natural conditions where many sensory cues are available. To understand the significance of trichromacy, it is necessary to evaluate how trichromacy benefits fruit detection over long distances. It is also important to observe various social feeding behaviors to examine alternate hypotheses, such as mutual benefit of association.
- 2010-12-20