Color-changing monkeys

By Victoria Clare

Posted July 22, 2024

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D: Is there something different about you, Yaël? Is that a new outfit? No, wait, you got a haircut, right?

Y: Close—I got my hair dyed yesterday, Don.

D: Really? I could have sworn it was always blonde…

Y: Don’t worry, it’s just a tiny bit lighter now.

D: I knew it couldn’t have been anything dramatic—like what’s happening to Costa Rican black mantled howler monkeys.

Y: Are they getting their fur dyed?

D: No, but it is changing color. Researchers started noticing distinct yellow patches on these mostly black-furred monkeys, so they analyzed their fur. Usually, the hair contains a type of melanin—the pigment that controls hair and skin cells—called eumelanin, which is black, gray, or brown. But in the yellow fur, the scientists found a melanin type called pheomelanin, which contains sulfur, and is red, orange, or yellow. The monkeys already have some blondish-brown hair on their flanks, which might be why they’re more susceptible to this color change.

Y: What’s causing the pigment to change from one type to another?

D: Scientists think it’s a result of the monkeys ingesting sulfur. In Costa Rica, farms are often sprayed with sulfur-containing pesticides from small planes, and some of the pesticide lands on trees that the monkeys eat. Once inside the monkeys’ bodies, the sulfur mixes with the melanin structures in their hair and changes its composition, creating a different color. It’s a problem because a lighter coat makes the monkeys easier to spot for predators like jaguars. And the problem is growing—scientists have even seen some monkeys with completely yellow coats.

Y: It puts a bad hair day into a whole new perspective.

A grey howler monkey sits with arms raised, eating inside an enclosure

Researchers started noticing distinct yellow patches on these mostly black-furred monkeys, so they analyzed their fur. (Nicki / flickr)

Humans intentionally change our appearances all the time, sometimes in dramatic ways. Animals can make some dramatic changes too, like what’s happening to Costa Rican black mantled howler monkeys.

Researchers started noticing distinct yellow patches on these mostly black-furred monkeys, so they analyzed their fur. Usually, the hair contains a type of melanin—the pigment that controls hair and skin cells—called eumelanin, which is black, gray, or brown. But in the yellow fur, the scientists found a melanin type called pheomelanin, which contains sulfur, and is red, orange, or yellow. The monkeys already have some blondish-brown hair on their flanks, which might be why they’re more susceptible to this color change.

So what’s causing the pigment to change from one type to another?

Scientists think it’s a result of the monkeys ingesting sulfur. In Costa Rica, farms are often sprayed with sulfur-containing pesticides from small planes, and some of the pesticide lands on trees that the monkeys eat. Once inside the monkeys’ bodies, the sulfur mixes with the melanin structures in their hair and changes its composition, creating a different color. It’s a problem because a lighter coat makes the monkeys easier to spot for predators like jaguars. And the problem is growing—scientists have even seen some monkeys with completely yellow coats.

It puts a bad hair day into a whole new perspective.

Reviewer: Sarie Van Belle, the University of Texas in Austin.

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