The colors of primates are among the most diverse phenotypes in mammals. These colors are mostly produced by the deposition of melanin pigments in hairs. Many species show considerable variability in pigmentation, but this is always temporarily fixed. Here we report the first rapid change in the pigmentation phenotype of a primate. In the last five years, the pelage of mantled howler monkeys Alouatta palliata inhabiting Costa Rica has started to change from fully black to yellowish, constituting a conspicuous color change. Raman spectroscopy analyses of hairs show that the change is due to a shift toward the production of the sulphurated form of melanin, termed pheomelanin. Most animals with anomalous coloration have been observed in forests surrounding intensive cultivations where sulfur-containing pesticides are frequently used. Exposure to environmental sulfur may increase the availability of sulfhydryls to cells, which may favor pheomelanin synthesis in melanocytes and explain the pigmentation shift. (C) 2018 Deutsche Gesellschaft fur Saugetierkunde. Published by Elsevier GmbH. All rights reserved.