Wild spider monkeys now have a new tool under their proverbial belt: a body scratcher that may release medicinal compounds, according to a study published in the latest issue of the journal Primates.
The study is the first to report this spider monkey scratcher. Lead author Stacy Lindshield told Discovery News that two other instances of the use of objects as tools by the social monkeys have been documented.
"Spider monkeys have been observed rubbing crushed and chewed leaves on their bodies," said 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."
"Second, spider monkeys are known to break off branches and drop them on or near human observers," she added, "so it's not a good idea to be directly beneath these guys!"
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, but they "are not extractive foragers so we wouldn't expect them to employ tools, such as termite fishing wands or hammers and anvils to crack open nuts."
Spider monkeys also have another major drawback — no thumbs — "which may make it more difficult for them to handle tools relative to other primate tool users."
Julio Mercader, a University of Calgary archaeologist, believes that, due to so many recent new discoveries of tool use by a wide variety of wild primates, a new inter-disciplinary field of primate archaeology should be established to examine tool use by these animals in a long-term, evolutionary context.
Mercader said, "We used to think that culture and, above anything else, technology was the exclusive domain of humans, but this is not the case."