New World monkeys applying strong-smelling, often irritating substances to their fur is a behavior known as self-anointing or fur rubbing. This practice is most famously observed in Capuchin monkeys, but also appears in species like Owl monkeys and Spider monkeys. They seek out items with a pungent aroma, ranging from wild onions and citrus fruits to toxic millipedes. Scientists believe the act serves a complex, dual purpose rooted in biological self-care and social interaction. The specific motivation for this behavior remains a subject of detailed scientific investigation.
The Mechanics of Fur Rubbing
Fur rubbing is a deliberate sequence of motions designed to maximize the transfer of the substance’s chemical compounds onto the monkey’s coat. The monkey first acquires the material, often crushing or chewing it to release potent volatile compounds, such as the sulfur-containing molecules in wild onions or garlic. They then grasp the macerated material, sometimes mixing it with saliva, and rub it vigorously into their fur. They often concentrate on areas difficult to reach with normal self-grooming.
Capuchin monkeys (genera Cebus and Sapajus) are the most frequent practitioners of this behavior. The materials they utilize are extensive, including plants like Piper species (which have a licorice scent) and artificial items like tobacco in captive environments. When using millipedes, the monkey carefully handles the invertebrate to cause it to excrete defensive secretions, which are then applied across the body. This focused application aims to achieve maximum saturation of the fur with the irritating, bioactive compounds.
Chemical Defense and Parasite Control
The most supported scientific hypothesis is that the pungent compounds provide pharmacological benefits, acting as self-medication against ectoparasites. Many of the plants and invertebrates selected contain secondary metabolites known to have insecticidal and antiseptic properties. For instance, the sulfur compounds in the Allium genus (onions and garlic) are effective at repelling insects.
Millipedes, another popular choice, release benzoquinones, a chemical defense that acts as a deterrent to mosquitoes and other biting arthropods. This application is common during the wet season, when high heat and humidity increase the risk of parasitic infestation and skin infections. By rubbing the substances into their fur, the monkeys are applying a natural insecticide and antifungal agent. The compounds are thought to repel external parasites like ticks and lice, and may also improve the condition of the skin and fur by mitigating microbial ailments.
Social Signaling and Learned Behavior
While biological defense is the primary driver, the behavior also carries social implications within the monkey troop. Fur rubbing is often a synchronized, communal activity, sometimes referred to as “social medication.” When monkeys anoint together, they can rub the substance onto areas of their groupmates’ bodies they cannot reach themselves, such as the back and shoulders. This achieves better coverage for the entire group.
Younger monkeys acquire the technique by observing adults, demonstrating that the specific choices of anointing material can be a socially transmitted tradition within a troop. However, the social impact is complex. While some studies suggest a temporary increase in proximity during the act, the intense smell of substances like onion can interfere with the subtle olfactory cues used for social regulation. This sometimes leads to short-term increases in aggressive interactions among tufted Capuchins.