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Gillian L. Moritz, Nathaniel J. Dominy; Selective advantages of mono- and dichromatic vision among nocturnal primates. Journal of Vision 2010;10(15):1. doi: 10.1167/10.15.1.
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© ARVO (1962-2015); The Authors (2016-present)
It has been suggested that the energetic cost of maintaining functional short (S-) opsin pigments might exceed the adaptive advantages of dichromatic vision under scotopic conditions. Such a view is supported by the convergent evolution and prevalence of monochromatic vision among nocturnal primates. For advocates of this hypothesis, the existence of functional S-opsin pigments in species such as tarsiers is taken as evidence of a recent adaptive shift from diurnality to nocturnality. The fact that the Bornean tarsier (Tarsius bancanus) features an M-opsin with a peak sensitivity of 543 nm, whereas the Philippine tarsier (Tarsius syrichta) exhibits an L-opsin with a peak sensitivity of 558 nm, has been interpreted as the relaxation of functional constraint at the M/L-opsin gene locus. Yet recently, a signature of purifying selection at the S- and M-opsin gene loci was reported for the aye-aye, a nocturnal primate. For such species, the advantages of detecting chromatic signals or cues are unknown; further, the tasks in which luminance cues alone might explain S-opsin inactivation are also unknown. To address these issues, we measured the spectral reflectance of foods consumed by Galago senegalensis, Tarsius bancanus, and T. syrichta. Next, we used irradiance spectra under twilight, full moon, and new moon conditions to calculate the radiance spectrum of each food object and to estimate the relative quantum catches for each species. The results of our models suggest that Galago, a monochromat, can subsist on luminance contrasts alone, whereas the tarsiers depend on distinctive, habitat-specific chromatic cues.
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