Getting a grip on your marbles

I found today's post by Ed Yong fascinating because of the interaction he provoked between two scientists on Twitter. The subject is the evolution of finger pads that become wrinkly when wet, as humans' fingertips do:

But evolutionary biologist Mark Changizi has an intriguing hypothesis about the origin of pruney fingerstheyre an adaptation that allows us to grip wet surfaces. Like the rain treads on tyres, when pressed down, pruney fingers create channels that let water drain away, allowing them to make better contact with damp surfaces.

Yong points to a new experiment by Kyriacos Kareklas and colleagues Kareklas:2013, where they find that wrinkly wet fingers are better at picking up wet marbles underwater than unwrinkled fingers, and that wrinkling makes no difference at all to picking up dry marbles.

This is one of stories to which people seem to have a love-it/hate-it relationship. It's trivial. Most people have some personal experience with the anatomy in question. We don't have much comparative data about other primates, because the scenario in which humans show the trait is not one most primates face. And it has to do with water. That last one always brings out strong feelings because of the aquatic ape theory.

I'm pointing to the story because of Yong's next step:

I engineered a debate between [T. Ryan] Gregory and [Mark] Changizi over the pruney fingers hypothesis on Twitter, and I think its a fascinating case study in how to think about evolutionary hypotheses.

It's a study in 140-character chunks of the spandrel versus adaptationist positions. Which I must say, is about the longest acceptable length for this debate as applied to water-wrinkled fingertip pads.