Why are these Costa Rican monkeys turning yellow?

Chameleons and squid can change their colors, but monkeys cannot—until humans get involved. Researchers have spotted typically black mantled howler monkeys (Alouatta palliata) in Costa Rica starting to sport distinct yellow patches on their tails and legs, the first evidence of a rapid change in the pigmentation of primate fur.

To figure out what was going on, scientists analyzed the fur of one of these color-changing monkeys. Mantled howler monkeys typically have a type of melanin—the pigment that colors hair and skin cells—called eumelanin that is black, gray, or dark brown. In the yellow hairs, the researchers noticed the melanin had changed to a sulfur-containing type called pheomelanin, seen in animals with yellow, red, or orange tones.

The researchers believe the animals are ingesting the sulfur when they eat leaves on the trees surrounding pineapple, banana, and African palm oil farms that have been sprayed with pesticides. This sulfur may be mixing with the hair’s pigment structure and changing its overall composition, scientists suggest. In recent years, farms in Costa Rica have used a greater number of these pesticides.