Have you ever noticed how much time cats spend cleaning themselves? I’m sure they believe that “cleanliness is next to Godliness,” but spending 15% of their day grooming seems a bit excessive! Nevertheless, cats are not the only ones obsessed with hygiene, and many other animals regularly clean their fur, scales, feathers, and skin. For some animals, personal hygiene is not the only goal, and they will spend additional time grooming other members of their group. This is called social grooming and is characteristically seen in primates. The partnerships formed during social grooming are long-lasting, much like the relationship you have with your best friend. It might be unsurprising, then, that social grooming in primates serves primarily a social purpose, allowing animals to bond and build relationships. So what links grooming to social bonding?
Social grooming involves gentle touches as well as stroking, scratching, and massaging. The “feel-good” C-fibers that Anita discussed on Monday are activated during petting and light touching and give rise to a pleasant sensation via the limbic system and orbitofrontal cortex. Sometimes, grooming can be quite rough, but like a massage, it is initially painful and gradually becomes pleasant. How does this change happen? Well, being groomed results in the release of endorphins, which cause pleasure and relaxation and act as a pain-reliever. Studies have shown that animals given endorphin blockers groom excessively, as if they are unable to get enough grooming, while animals given endorphins act as if they are satisfied and are disinterested in either giving or receiving grooming. These experiments show that endorphins are key to the motivation that primates have to groom one another.
Recently, researchers at the Max Planck Institute showed that oxytocin is also involved in the formation of social bonds during mutual grooming. (This is the same neuropeptide that Juan discussed last week in his article about pair-bonding in prairie voles.) In this study, the researchers found that oxytocin levels were higher in chimpanzees who had been grooming with a “bond partner” compared to a “non-bond partner,” whom they did not share a cooperative relationship. Interestingly, these oxytocin levels were similarly high after grooming with kin and non-kin bond partners. This was a pivotal finding because oxytocin had never been implicated in non-kin relationships! Moreover, oxytocin is known to act on the neural reward and social memory systems, so this study shows that oxytocin plays a key role in maintaining social relations beyond genetic ties and in keeping track of social interactions with multiple individuals over time.
So what does this mean for us humans? We are also social animals, and we maintain relationships for long periods of time. If you think about it, we are frequently involved in social grooming. We shake hands with acquaintances, hug our friends, play fight with our siblings and cuddle with our significant others. Though we may not realize it, these forms of social grooming are fundamental to our ability to connect with others. Humans are particularly unique in that social grooming can take on a completely non-physical form due to our extensive spoken and written language and is probably the dominant form of social grooming. A few kind words are often all the “grooming” it takes to strengthen social relationships with others. More recently, the modern age has allowed us to develop completely non-physical and non-lingual forms of social grooming. Facebook and Twitter allow us to “poke,” “tag,” “tweet,” or “re-tweet” our friends over the internet, which doesn’t include language per se, but still maintains similar social “feel-good” grooming. Regardless of how we humans experience social grooming, it is quite likely that the fundamental mechanisms of endorphins and oxytocin are working in a very similar way in our brains as in a primate’s or even a prairie vole’s!
I’d like to think that we humans still prefer to have our C-fibers directly activated by those we love, but when vast distances separate bond partners, #groom may do the trick!
Crockford C., Wittig R.M., Langergraber K., Ziegler T.E., Zuberbuhler K. & Deschner T. (2013). Urinary oxytocin and social bonding in related and unrelated wild chimpanzees, Proceedings of the Royal Society B: Biological Sciences, 280 (1755) 20122765-20122765. DOI: 10.1098/rspb.2012.2765