Data from: Visual ecology of true lemurs suggests a cathemeral origin for the primate cone opsin polymorphism

In contrast to the majority of primates, which exhibit dedicated diurnality or nocturnality, all species of Eulemur are cathemeral. Color vision, in particular, is strongly affected by the spectral composition and intensity of ambient light, and the impact of activity period on the evolution of primate color vision is actively debated. We studied three groups of wild brown lemurs (Eulemur fulvus) in Ankarafantsika National Park, Madagascar over a one-year span. We also used non-invasive fecal DNA collection and analysis to study the opsin genes underlying the color vision of 24 individuals. We quantify the color and brightness of dietary fruits and modeled the chromaticity and discriminability of food objects to different visual phenotypes under daylight, twilight, and moonlight conditions. We found that E. fulvus possesses routine dichromacy, unlike its congener E. flavifrons, for which polymorphic trichromacy has been reported. Our models suggest that dichromacy is well-suited to the foraging ecology of E. fulvus. The performance of modeled dichromats and trichromats is comparable under nocturnal illuminants and the luminance values of most diet items are detectable across light conditions. The trichromatic phenotype demonstrates a modest advantage under daylight conditions only. Our results, taken together with reports of polymorphic trichromacy in E. flavifrons, suggest functional ecological variation in the visual system of the genus Eulemur. Interspecific phenotypic variation in the color vision of a genus is both unexpected and instructive. Ecological differences between species of Eulemur could reveal thresholds for the origins of polymorphic trichromacy, which preceded the evolution of routine trichromatic vision in humans and other primates.

与多数专性昼行或夜行的灵长类不同，美狐猴属（Eulemur）的所有物种均具泛昼夜活动节律（cathemeral）。具体而言，色觉受环境光的光谱组成与强度影响显著，而活动周期对灵长类色觉演化的影响这一议题仍在被广泛讨论。本研究于马达加斯加的安卡拉凡兹卡国家公园（Ankarafantsika National Park）内，针对三群野生褐狐猴（Eulemur fulvus）开展了为期一年的观测研究。此外，本研究通过非侵入性粪便DNA采集与分析技术，对24只个体的色觉相关视蛋白基因（opsin genes）开展了研究。研究人员量化了褐狐猴食用果实的颜色与亮度，并针对日光、晨昏暮光与月光三种光照条件下，不同视觉表型（visual phenotypes）的个体对食物的色度（chromaticity）与辨别度（discriminability）开展了建模分析。本研究发现，褐狐猴（E. fulvus）具备常规二色视觉（dichromacy），而其同属物种黄额狐猴（Eulemur flavifrons，即E. flavifrons）此前已有多态性三色视觉（polymorphic trichromacy）的相关报道。建模结果表明，常规二色视觉非常适配褐狐猴的觅食生态位。在夜间光照条件下，模拟二色视觉者与三色视觉者的表现不相上下；且多数食物的亮度值在各类光照条件下均可被辨别。仅在日光光照条件下，三色视觉表型才具备小幅优势。结合黄额狐猴多态性三色视觉的相关报道，本研究结果表明美狐猴属的视觉系统存在功能性生态差异。同一属内物种间的色觉表型差异既出人意料，又极具研究价值。美狐猴属各物种间的生态差异，或可揭示多态性三色视觉的起源阈值——而这一特征的演化早于人类与其他灵长类的常规三色视觉。