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One group of monkeys is going through an unexpected聽transition 鈥� their coats are changing rapidly from charcoal black to creamy yellow.

Hair colour in primates is controlled by melanin. It comes in two forms: eumelanin, which is responsible for brown and black hues, and pheomelanin, which produces yellow and red tones.

The mix of melanin in an animal鈥檚 skin and hair is determined by DNA, so the natural hair colour of individuals belonging to the same species should remain relatively fixed.

That鈥檚 why Ismael Galv谩n at the Do帽ana Biological Station in Spain and his colleagues were so surprised when they spotted several mantled howler monkeys (Alouatta palliata) in Costa Rica that have patches of blonde hair on their limbs and tails. Normally, these monkeys have a fully black coat.

Pigment change

To find out what happened, the team collected samples of black hair and blonde hair from a mantled howler monkey. Galv谩n found its blonde hair contains pheomelanin instead of the eumelanin that the monkeys usually produce.

鈥淚 don鈥檛 know any precedent example of a conspicuous colour change in a given population of a wild animal,鈥� says Galv谩n.

Back in 2001, none of the mantled howler monkeys in Costa Rica had blonde hair. In 2013, a dozen mantled howler monkeys were spotted with yellow hair on the tip of their limbs and tails. The most recent observation suggests that there are at least 21 individuals with the yellow colouration. Moreover, the patches are increasing in size 鈥� and two of the monkeys have a fully yellow coat with no black hair.

Tainted diet?

鈥淭he real number of animals with anomalous coloration is probably higher than 21,鈥� Galv谩n says. 鈥淏ut in any case, the relative proportion of the yellow ones in the entire population is low.鈥�

It鈥檚 unclear what is driving the shift in melanin and thus the change in monkey鈥檚 coat colour 鈥� it is too fast to be explained by natural selection, and in any case yellow fur should make the monkeys more conspicuous to predators and so less likely to survive.

But because pheomelanins contain sulphur and eumelanin doesn鈥檛, the team suspects these monkeys may have ingested sulphur from the environment.

Most monkeys with blonde hair are found in forests surrounding plantations, where farmers use sulphurous pesticides extensively. Therefore, Galv谩n suggests that the leaves and fruits from the farms may be the source.

鈥淭his is the mechanism that we hypothesise, but it will have to be confirmed by future studies,鈥� Galv谩n says.