1 Department of Physiology and Behavior, Federal University of Rio Grande do Norte, Natal, Rio Grande do Norte, Brazil.
2 Postgraduate Program in Ecology and Biodiversidade, Federal University of Mato Grosso, Cuiabá, Mato Grosso, Brazil.
3 Department of Geography, Planning and Environment, Concordia University, Montreal, Quebec, Canada.
4 Butantan Institute, São Paulo, Brazil.

Parasitism is one of the primary causes of biotic stress in several taxa, and behaviors resembling self-medication have been documented in numerous species. Anointing involves the application of chemical substances derived from animals, plants, mud, soils, and minerals, often emitting a pungent odor, onto an animal's body. We examined the circumstances surrounding 34 bouts of anointing with millipedes in a group of blond capuchin monkeys (Sapajus flavius) inhabiting an Atlantic Forest fragment in northeastern Brazil. Over 412 h of observation, we collected behavioral data through photographs, video recordings, and ad libitum field notes. We collected and identified the millipedes to the species level. We tested three non-mutually exclusive hypotheses on the function(s) of this behavior: self-medication, social bonding, and opportunistic use. We analyzed data in R using nonparametric tests due to low sample size. Anointing occurred at a rate of 8 episodes every 100 h. Millipedes used during anointing are from species that produce benzoquinone. The behavior was more frequently observed in the inferior strata, during midday, while the group is mostly foraging, in periods of higher rainfall, when millipedes emerge from the ground, and during the high fruit productivity season, when the capuchins' activity budget is less constrained. Although anointing occurred at similar rates in solitary and social contexts, adult males were more actively engaged in anointing bouts, indicating sex difference in this behavior, and a possible social function. The observed pattern suggests anointing is a multifunctional behavior, combining elements of self-medication, opportunity, and social interaction.