The forests of Costa Rica are home to the mantled howler monkey, a primate with a dark coat of fur and a few flecks of orange on their sides that is native to Central and South America. Over the past five years however, some individuals have been spotted sporting conspicuous blotches of yellow fur.
Researchers first noticed the unusual colouring through the use of camera traps, installed on bridges that the monkeys use to cross roads. At first it was a rare occurrence but, as time went on, the frequency of affected monkeys increased along with the spread of the discolouration. While the first photos revealed monkeys with light limbs or tails, as if they’d dipped their extremities in yellow paint, by 2017 the team were encountering monkeys that had almost fully changed colour.
Dr Ismael Galván, a researcher at the Spanish National Research Council (CSIC) was one member of the team that studied this phenomenon. His initial thought was that it could be a genetic issue. ‘When I saw these pictures I got really surprised,’ he says. ‘At first I thought that it was a rare mutation in some gene involved in the synthesis of melanin pigments, like the MC1R gene, as animals with these mutations occur from time to time in nature. These cases, however, are rarities, isolated.’
Probing further, Galván began to investigate the chemical nature of the pigment responsible for the anomalous colouration. By analysing the hairs of the monkeys, he discovered that the pigment being produced by the follicles had shifted from one called eumelanin that results in dark hair, to a lighter pigment called pheomelanin. It was this shift that alerted the team as to one potential cause of the colour change.
The pigment pheomelanin occurs when the eumelnain pigment you’d expect to see in a dark-haired animal is incorporated with sulphur. While the content of sulphur in hair follicles can be determined by genetics, environmental factors can also influence it. Most of the monkeys spotted with patches of yellow fur were observed in forests very close to intensive cultivations of pineapple, banana and African palm oil. One thing these plantations have in common is the fact that sulphur-containing pesticides are used on their crops.
‘We believe it is likely that pesticides are the cause of the change,’ explains Galván. ‘Recent studies show a strong use of pesticides in these cultivations, and a rise in the import and use of pesticides in Costa Rica in the last few years. Howler monkeys probably ingest significant amounts of pesticides with the leaves that they take in their diet.’
It isn’t yet known what impact the lighter fur might have on the monkeys, though one concern is that it will make them more easily detected by predators. The next step for the team is to test its hypothesis further to determine if pesticides really are causing the change, as they deem likely. As sulphur is a component of most pesticides it could mean a lighter life for the howler monkeys, though perhaps in colour alone.
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