Why do fingers become pruney? - Mark Changizi

Key Takeaways

[Music] there are all sorts of things that happen every day that might make you ask why why do we drive on a Parkway and park on a driveway why does my hair get frizzy when it's humid and why do my fingers get all pruny when they're wet often we simply can't know the answers to these questions but progress can often be made and when it comes to your pruny fingers scientists have a few interesting ideas the story here begins with an observation made in the 1930s in the emergency room doctors noticed that patients with nerve damage to their hands did not get pruny fingertips like most of us do in the bath which might make you wonder why would primates want wrinkly fingers presumably when it's rainy or dewy what could this trait be for what what if these wrinkles functioned like rain treads think about car tires when the ground is dry it's generally best to have smooth tires and race cars do smooth tires means more rubber or surface area in contact with the road which gives them better grip but in rainy and muddy conditions it's a different story and here's where Treads come into play Treads reduce the amount of rubber on the road but they help Channel out water when it rains lowering the risk of hydroplaning well if your fingers really are like rain treads you should be able to predict the optimal shape for them right so what would the predicted wrinkle shapes be that is are our prunes actually the right shape to be rain treads let's take a detour into rivers when we think of river networks we usually imagine lots of smaller River channels joining to make larger River channels Downstream which is what happens in concave basins in these cases the divides the regions between the river segments are disconnected from one another and diverge away from one another uphill but River networks look fundamentally different on convex promontories or protrusions here the river channels are disconnected from one another and diverge away from one another Downstream not very river-like in these cases it's the divides that link together to form a tree with its trunk uphill at the top of the Promontory here it's the divides that look more traditionally river-like if our pruning fingers are drainage networks designed to channel out water when we grip then we expect to find similar shapes on our fingers as we find out there among Rivers there should be a tree network of divides or ridges with their trunk near the top of the fingertip and with their more leafy Parts reaching out downhill away from the tip the channels themselves through which the water is channeled during a grip should not connect to one another and instead should diverge away from one another downhill if pruny fingers are rain treads then they should look like the river networks on convex promontories in fact that's exactly the morphology we find among pruny fingers when we grip then our pruny wrinkles really do Channel out the water but does it actually help us grip new behavioral experiments have shown that they do in a task where subjects had to grasp wet Marbles and move them through a hole and out the other side those with pruny fingers finished the task significantly faster than those with smooth fingers pruny fingers not only ought to enhance grip in wet conditions but do so pruny fingers may be a crucial part of the primate repertoire maybe once primates lost their claws in favor of fingernails rain treads were needed to deal with the especially challenging often wet gripey Forest habitats by looking closely at the mysteries in our world and trying to find things that look similar like our wet fingers and rain treads we can come up with ideas is about what's happening and that's a good way to think about all sorts of questions in life too [Music]