Mantled howler monkeys (Alouatta palliata) are typically pretty understated in terms of color, often carrying a dark coat of fur with a few flecks of orange on their sides. But in just the past five years, more monkeys have been appearing with larger splashes of yellow fur.
Since the early 2000s, scientists have been probing the genetics of primates in Costa Rica, including mantled howler monkeys.
An early study revealed that these animals’ genes don’t vary very much, and all 205 monkeys in that study had the same genetic sequence, called a genotype, that codes for a fully dark coat.
About five years ago, however, researchers began finding the peculiar yellow coloring. All told, the authors know of at least 21 individuals with it. At first, those splashes often just covered a small part of a monkey’s body, perhaps a hand or a section of the tail.
But over time, researchers have noticed that more of the affected monkeys’ bodies are yellow. Some now have little black fur at all.
The team’s hypothesis, reported online Oct. 31 in the journal Mammalian Biology, is that sulfur in pesticides used on banana, pineapple and oil palm plantations — frequently applied in copious amounts — could be triggering the effect. Indeed, most of the monkeys with yellow fur live near these operations, where they likely pick up sulfur-containing chemicals when they eat the leaves of nearby trees.
The researchers believe the yellow-colored monkeys are making less of the dark pigment called eumelanin and more of a compound called pheomelanin, which contains sulfur and produces lighter-colored skin cells and hair follicles.
They write that further research could bring the causes of this change into sharper focus, as well as the ramifications to the monkeys themselves. They share the forests of Costa Rica with six cat species that may hunt howler monkeys, and it remains to be seen whether the yellow patches might make them a more visible target.
Galván, I., Jorge, A., Sánchez-Murillo, F., & Gutiérrez-Espeleta, G. (2018). A recent shift in the pigmentation phenotype of a wild Neotropical primate. Mammalian Biology.
Zaldivar, M. E., Glander, K. E., Rocha, O., Aguilar, G., Vargas, E., Gutierrez‐Espeleta, G. A., & Sanchez, R. (2003). Genetic Variation of Mantled Howler Monkeys (Alouatta palliata) from Costa Rica. Biotropica, 35(3), 375-381.