Science puts wrinkled fingers to the test

By Jonathan Amos
Science correspondent, BBC News

image captionThe wrinkles may act like the tread on tyres

Science may be getting closer to explaining those prune-like fingers and toes we all get when we sit in a hot bath too long.

UK researchers from Newcastle University have confirmed wet objects are easier to handle with wrinkled fingers than with dry, smooth ones.

They suggest our ancestors may have evolved the creases as they moved and foraged for food in wet conditions.

These involved asking volunteers to pick up marbles immersed in a bucket of water with one hand and then passing them through a small slot to be deposited by the other hand in a second container.

media captionDr Tom Smulders: Wrinkly fingers helped us to 'get a grip'

Volunteers with wrinkled fingers routinely completed the task faster than their smooth-skinned counterparts.

The team found there was no advantage from ridged fingers when moving dry objects. This suggests that the wrinkles serve the specific function of improving our grip on objects under water or when dealing with wet surfaces in general.

For a long time, it was assumed that the wrinkles were simply the result of the skin swelling in water, but recent investigations have actually shown the furrows to be caused by the blood vessels constricting in reaction to the water, which in turn is a response controlled by the body's sympathetic nervous system.

That an active system of regulation is at work led scientists into thinking there must be some deeper evolutionary justification for the ridges.

"If wrinkled fingers were just the result of the skin swelling as it took up water, it could still have a function but it wouldn't need to," said Dr Tom Smulders, from Newcastle's Centre for Behaviour and Evolution.

image captionThe tests involved handling wet objects with wrinkled and un-wrinkled fingers

"Whereas, if the nervous system is actively controlling this behaviour under some circumstances and not others, it seems less of a leap to assume there must be a function for it, and that evolution has selected it. And evolution wouldn't have selected it unless it conferred some sort of advantage," he told BBC News. (Listen to Tom Smulders also on BBC Health Check)

US-based researchers were the first to propose that the wrinkles might act like the tread on tyres, and even demonstrated how the patterns in the skin resembled those of run-off channels seen on the sides of hills.

Wet trees

What the Newcastle team has now done is confirm that prune-like fingers are indeed better at gripping wet objects.

"We have tested the first prediction of the hypothesis - that handling should be improved," Dr Smulders said.

"What we haven't done yet is show why - to see if the wrinkles remove the water, or whether it's some other feature of those wrinkles such as a change in their stickiness or plasticity, or something else. The next thing will be to measure precisely what's happening at that interface between the objects and the fingers."

Our ancestors might not have played with wet marbles, but having better gripping fingers and feet would certainly have been advantageous as they clambered about and foraged for food along lake-shores and by rivers.

It would be interesting to see, observed Dr Smulders, just how many other animals displayed this trait - in particular, in primates.

"If it's in many, many primates then my guess is that the original function might have been locomotion through wet vegetation or wet trees. Whereas, if it's just in humans that we see this then we might consider something much more specific, such as foraging in and along rivers and the like." and follow me on Twitter: @BBCAmos