Yesterday I was riding the T, Boston’s subway system, and had the pleasure of sharing my car with a class of children on the way home from a field trip. They were probably in 5th grade or so, clearly excited to be on the train, and had a particular fondness for hanging off of the straps that standing passengers hold on to. (The chaperone admonished them for doing this, but didn’t seem to mind enough to actually stop them.)
It was fun noticing that they loved this simple act so much, and, not coincidentally, how much they looked like monkeys when they were doing it. Most of the people reading this can probably see where I’m headed: we’re tree climbers, or descendants of tree climbers, thus we must, for the purpose of adaptation, love hanging and swinging from anything that remotely resembles a branch. This explains why children love monkey bars almost as much as the slide, and why hippies and foresters get a rush out of scaling trees. This is all true, but it’s a tale of ultimate mechanism, telling us about the past environments that might have shaped such a trait. It is a sound theory, but doesn’t quite tell us how it works.
As the children leaned back, hanging from the strap, they were having a lot of fun—there was something rewarding about this behavior, something in their physiology was responding to the act. Maybe it was the way it pulled out the shoulders, stretched the back, relaxed the neck. Maybe all of those things combined. Could it be that their bodies were releasing some neurochemical—maybe endorphins, maybe serotonin—to soothe the body, to reward it for being in this posture? And if it was, it could even be because it is a restorative pose in some sense, effectively releasing and massaging muscles that are typically engaged.
Returning to the ultimate mechanism, and the theory of why our attraction to hanging in this way would be a consequence of our evolved physiology, why would this system work in exactly this way? Why would it be adaptive for a tree climber to feel this sort of reward for hanging. Well, there’s the simple answer I alluded to earlier, those who create a niche for climbing will have systems rewarding climbing because that is their livelihood. The reward system impels us to do what brought us food, once upon a time.
I want to go a little further though, into the specific act of hanging. Why would it relax and soothe in comparison to the act of swinging from branch to branch? An important difference between the two is that one is in fact a callisthenic activity, the other is more passive, and this may be where the answer lies. For example, when one is running, and tires, they can walk or sit. A climber cannot walk instead, and sitting requires pulling up onto the branch and taking an extended pause. Thus, climbing probably is punctuated by all sorts of resting moments, and our physiology has evolved to utilize them and respond to them efficiently. They are important to the act of climbing, and an animal built for climbing should be able to extract maximum benefit from hanging.
With that all said, it makes the chaperone’s insincere attempt to forbid the kids from hanging off of the subway straps even more amusing. She didn’t really want them doing it. She claimed it was because they would lean back and hit people—which they occasionally did. But, in the end, she let them do it. Maybe because it really was a pretty benign act on a pretty empty train. Maybe because she remembered how fun it was for herself as a kid. Or maybe because there’s something about just hanging from a branch, or a branch-substitute, that feels good, and we all kindav know that.