Monkeys rubbing pungent onions on their fur is a fascinating behavior that has long intrigued scientists. This peculiar action, where monkeys vigorously apply plant matter to their bodies, prompts inquiry into its purpose. Understanding this practice sheds light on the complex strategies animals employ for their well-being.
The Peculiar Practice
Capuchin monkeys, particularly tufted capuchins, are known for “anointing” behavior. They bite into an onion and vigorously rub its exudates over their bodies, sometimes sharing the substance with other group members. This behavior is not limited to onions; these monkeys also use other strong-smelling plants like citrus fruits, wild peppers, tobacco, or even millipedes.
The rubbing action involves crushing and tearing the plant material to release its juices, which are applied to their fur and skin. This can be done individually or socially, where individuals rub the materials onto their group mates. This communal aspect allows monkeys to reach otherwise inaccessible body parts. The behavior occurs primarily during the wet season, often associated with an increase in insects and the risk of bacterial and fungal skin infections.
Unraveling the Theories
One prominent theory suggests that compounds in onions and other pungent plants deter parasites like ticks, mosquitoes, and fleas. Onions contain sulfur-containing compounds, such as allicin, known for their insect-repelling properties. By rubbing these substances on their fur, monkeys create a protective barrier against ectoparasites.
Beyond repelling insects, these plant compounds also possess antimicrobial and antifungal properties. The sulfur compounds can combat skin infections or fungi that thrive in humid environments. This suggests a direct medicinal benefit, helping monkeys maintain healthier skin and fur.
The scent of these substances might also play a role in social signaling and communication within the monkey group. Anointing behavior can mediate social relationships and strengthen social bonds, similar to social grooming. Studies show monkeys increase proximity and spend more time in groups when engaging in fur rubbing with onions, suggesting a communal aspect.
This behavior can also be viewed as self-medication, where animals instinctively use plants for therapeutic purposes. This encompasses both preventative measures, like repelling parasites, and therapeutic actions, such as treating existing skin ailments. The observed “onion washing” in capuchins appears to be a carefully selected adaptation.
Beyond Onions: Broader Animal Self-Medication
The practice of monkeys rubbing onions on themselves is an example of zoopharmacognosy, the study of how non-human animals self-medicate using natural substances. This field explores behaviors where animals select and ingest or topically apply plants, soils, or insects with medicinal properties to prevent or reduce harmful effects.
Numerous other animal species exhibit similar self-medication behaviors. Chimpanzees, for instance, eat specific leaves like Aspilia to expel intestinal parasites. These leaves contain compounds that act against nematodes, and chimpanzees may swallow them whole to scour their intestines. Birds like babblers and weavers engage in “anting,” rubbing ants on their feathers and skin. The formic acid secreted by these ants acts as an insecticide, miticide, fungicide, or bactericide, protecting the birds from infestations.