Mantled howler monkeys (Alouatta palliata) in Costa Rica are turning yellow because of pesticides used in nearby plantations

Mantled howlers (Alouatta palliata) are New World monkeys found in Central and parts of South America, southwest of the Amazon basin. It's pretty common there, and it is considered to be the monkey you typically hear most from when you are out in nature in this part of the world. This strictly foliage-eating monkey is one of 15 species of howler monkeys, a genus that got its name from the load screams they used to interact with each other. Apparently they hold the record in Guinness Book of World Records for having the loudest call, where they have been measued to be heard clearly over 4.8 kilometers away.

A mantled howler. This is what it's supposed to look like; completely black with only a small patch of golden hairs to runs along its side. Photo by Steven G. Johnson, posted with CC BY-SA 3.0.

The mantled howlers are highly social animals

You might have already realized that the howler monkeys are highly social animals, or else they would likely not have evolved the ability to communicate over distances. The mantled howlers typically live in groups of 20-40 individuals (but more or less members is also possible), with most or all of the monkeys being unrelated to each other. The group is lead by an alpha male that selects the best females for mating, get the best food, and can chose the best resting place.

This species of howler monkey is also an important species in the rainforest ecosystem where they are a big part of seed dispersion and germination. They are currently considered to be Least Concern by the IUCN Red List (Cuarón et al., 2008), which means that they are unlikely to go extinct within the next 100 years. It does face some threat from deforestation, but it seem to be able to feed on most type of leaves, so it is still doing much better than most of the other monkey species in the region.

The mantled howler monkeys in Costa Rica are turning yellow

The howler monkeys are mostly black with only a small stripe of golden hairs that run along the flanks fo their bodies. That is unless they are in frequent contact with food that has been affected by pesticides, in which case they will gradually turn yellow.

Photo by Scott Robinson, posted with the CC BY 2-0.

During the last five years, people in Costa Rica began noticing that some howler monkeys had yellow fur on places where they were supposed to be black. The first individuals observed has creamy yellow colors on their tails and feet, but the yellow patches has seemingly grown in both size and the number of monkeys affected as the years go by.

This obviously caught the attention of primatologists and zoologists all over the world, and a research group consisting of Spanish and Costa Rican scientists looked further into this. The results have just been published in Mammalian Biology by Galván et al. (2018), and it's very interesting to see what their proposal for why this happens is.

As you have seen in the title, they believe this yellow coloration comes as a result from the monkeys eating from leaves that have been sprayed with, or are close to trees that have been sprayed with pesticides. Costa Rica is generally a country known to be great at conserving biodiversity, but they still use a lot of dangerous pesticides there as well. Both palm oil, pineapple, and banana plantations are common in Costa Rica, and all of these uses pesticides to prevent insects from damaging the crops.

Why does pesticides make the fur of the monkeys yellow?

It might sound crazy that a pesticide can actually physically change the color of the fur of an animal, so why do the scientists think this is the cause? According to their research paper the structure of the melanin in the yellow areas of the monkey fur is different from the black parts. Melanin is the compound responsible for deciding the color of the fur, and the mantled howlers should mainly have a version of it called eumelanin, which creates very dark grey/brown or black hairs.

The yellow regions of the monkey had pheomelanin instead of eumelanin, which has never been found in these monkeys before. The pheomelanin is rich in sulfur, an element that is not otherwise found in the pigment on the mantled howlers. But guess where you find plenty of sulfur? Yeah, in pesticides.

Pesticides with sulfur has apparently been more and more common in Costa Rica in the last decade, so while the researchers can't be completely sure that pesticides are the culprit, they have a good indication towards it. Discolored animals also seemed to be found in higher concentration in areas surrounding plantations that uses pesticides with sulfur, so the evidence is stacking up.

Finding the source, and why it matters

Despite the fact that pesticides are looking like a hot candidate, the evidence is not quite strong enough to say anything for sure, so hopefully the team will find time and funding to get more research done to be able to say for certain if the pesticides are responsible. If it turns out to be responsible, the Costa Rican government might have to change laws and regulations towards a type of pesticide that won't damage the nearly wildlife.

While many humans think that the yellow mantled monkeys are looking cuter than their natural color, they obviously have a big disadvantage when it comes to hiding from predators. Bright yellow tend not to blend very well in to the background in the jungle, and I would assume that these monkeys have a higher mortality rate from predators. We also don't really know if this sulfur does any damage to the monkeys or other wildlife, but there are no clear indications that it is harming them other than the change of fur color.

Sources

• Cuarón, A.D., Shedden, A., Rodríguez-Luna, E., de Grammont, P.C., Link, A., Palacios, E., Morales, A. & Cortés-Ortiz, L. 2008. Alouatta palliata. The IUCN Red List of Threatened Species 2008: e.T39960A10280447. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T39960A10280447.en. Downloaded on 10 December 2018.

• 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. https://doi.org/10.1016/j.mambio.2018.10.007

Thanks for reading

I hope you enjoyed reading this post, and learned something new about these monkeys. Thanks for stopping by!

I'm sorry for not being able to include a photo of any of the monkeys with yellow patches, but I could not find any that I could repost here without breaking any laws. But you can find a photo in the scientific article by Galván et al.