When the weather is fine and sunny, monkeys lounging in the sun often have another monkey meticulously picking at their fur as if catching “fleas”, with nimble and realistic movements akin to how humans might catch fleas.
It’s common to see monkeys grooming each other for “fleas”. Even solitary monkeys or those performing in alleys as entertainers will often, when they have a moment free, meticulously part their fur and patiently groom themselves, using their delicate fingers to pick at what appears to be “fleas”. People have habitually referred to this as “monkeys catching fleas” without any doubt. However, in reality, they are not catching fleas because monkeys rarely have fleas on them. So, what exactly are they catching?
Actually, after monkeys sweat, the moisture from their sweat evaporates, leaving behind salt crystals combined with dirt from their skin and hair roots. These crystals are what they are picking at—they are salt grains. The sweat contains about 0.9% salt. When we sweat on a hot day, the sweat on our face often tastes salty when it reaches our mouths. In the zoo, monkeys’ diets aren’t deliberately salted, so over time they may feel a lack of salt and thus pick at these salt grains attached to their hair roots to supplement their diet. Over time, this behavior becomes a habit that resembles catching fleas.