Scientists are intrigued by a change seen in some howler monkeys in Costa Rica. Their fur, usually dark brown and almost black, has some yellow spots. Blond rings and spots are appearing in various areas especially on the tails. It is not thought to endanger the health of the animals although it could make them more visible to predators.
At least 23 howlers have this change. The change is seen in different parts of the country as well as on both female and male howlers. The first one spotted with this characteristic was noticed about 7 years ago in Sarapiquí. The phenomenon caught attention when more and more were seen with it.
This molecular change of yellow hair on dark haired animals could be a type of mutation that does not affect the whole population. The yellow hair is seen primarily on adult monkeys in Limón and Sarapiquí, though it has also been spotted in Santa Cruz, Nosara and Monteverde. One individual howler in Caño Negro is completely covered in fur of this unique color.
Samples of the hair were analyzed and it was found that the pigment responsible for the dark coloration incorporated sulfur groups in the chemical composition, creating the yellow pigment. Some speculate that the sulfur can be due to the presence of sulfur containing pesticides used nearby. Some causes have been ruled out, including a hereditary factor, inbreeding, and ultraviolet radiation.