"Spider monkeys have been observed rubbing crushed and chewed leaves on their bodies," said lead author Stacy Lindshield, a researcher in Iowa State University's Ecology and Evolutionary Biology Graduate Program.
She explained that the smelly practice may "play a role in olfactory communication."
She and co-author Michelle Rodrigues collected observational data on wild spider monkeys at El Zota Biological Field Station in northeastern Costa Rica.
They documented three instances where the monkeys used the scratcher tool.
The first to scratch was an adult female. Holding a small, leafy branch in her hand, she scratched her chest and abdominal regions.
The second, another adult female, used a detached stick lacking side branches and leaves to scratch her left side.
She chewed the tool tip between bouts.
The third individual, a juvenile female, first chewed the distal tip of a stick before scratching the underside of her tail and her genital region.
The scientists think that by modifying the scratcher tip, the monkeys could be providing "more relief and comfort during scratching."
The chewing alteration could "also be related to the chemical properties of the selected plant, as research on fur-rubbing and self-medication indicates that some primates select plants or invertebrates with chemical properties for this reason."
Like a human slathering on scented ointment, the plants may then be providing soothing compounds.
Since the monkeys aren't just scratching hard-to-reach spots, they could also be stimulating their own scent production glands, which are involved in nose-detecting communication.
Chimpanzees, orangutans and capuchin monkeys are known as being the most prodigious non-human tool users, but generally their tools are used for foraging or feeding.
Self-directed and social tools appear to allow for a bit more "innovation and creativity," which seems to hold true for the scratcher tool.
"Spider monkeys are an interesting case because they fit some, but not all, of the general characteristics shared by primate tool users," Lindshield said, explaining that the monkeys have a large brain relative to their body size. (ANI)