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The adaptive origins of primates and anthropoid primates are topics of enduring interest to biological anthropologists. A convention in these discussions is to treat the light environment as binary—night is dark, day is light—and to impute corresponding selective pressure on the visual systems and behaviors of primates. In consequence, debate has tended to focus on whether a given trait can be interpreted as evidence of nocturnal or diurnal behavior in the primate fossil record. Such classification elides the variability in light, or the ways that primates internalize light in their environments. Here, we explore the liminality of light by focusing on what it is, its many sources, and its flux under natural conditions. We conclude by focusing on the intensity and spectral properties of twilight, and we review the mounting evidence of its importance as a cue that determines the onset or offset of primate activities as well as the entrainment of circadian rhythms.
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Supplemental Figure 1. Abbott H. Thayer, Blue Jays in Winter, study for book Concealing Coloration in the Animal Kingdom, ca. 1905-1909, oil on canvas, Smithsonian American Art Museum, 1950.2.12. This painting demonstrates how the color of light is as important as the subject itself. Thayer intuited correctly the blue color of woodland shade (Figure 3b) and argued that blue jays are camouflaged when viewed under this illuminant. The following passage exemplifies his views on the function of animal coloration: “The costume of the beautiful Blue Jay (Cyanocitta cristata) is a wonderful picture of a winter landscape—snow in shadow, snow in sunlight, sky, trees, and vinous-gray scrub—all are there, in true and exquisite comminglement. Here again we have a picture to show in aid of unconvincing words... Of course, the Blue Jay's costume is not confined to this one kind of background-matching. It pictures, perhaps equally well, a much nearer bit of snowy ground, thickly fretted with blue shadows, with some dark twigs or branches relieving against it. Wherever the bird alights in the winter woods, he looks like a vista through the tree in which he sits to one or another of these blue and snow-bright back-grounds. He bears a full obliterative shading (from dark blue and black to white), without which the delicate distance-picturing would be impossible. In summer the Blue Jay's perennially unchanged coloration is less closely fitted to its environment; but the bird is never conspicuous. The blue, seen in the leafy sylvan dimness, is usually soft and dull, and not sharply differentiated from the vinous ash-color of the breast and flanks; the white spots, as in all such cases, picture glints of sky, or lighter leaf-vistas; while the dark marks look like sticks and twigs and holes and shadows. Or again, when the clear, light blue of tail or wings gleams out with especial brightness, it may pass either for sky-shine on the leaves or for a bit of blue sky showing between them.” (Thayer 1909, pp. 115–116).