Sunday, April 1, 2012

Fingertip Evolution and Wet-Induced Wrinkles



It’s a common experience: stay too long in the shower, and your fingers become wrinkly. But could these wrinkly fingertips have evolutionary significance? A recent paper suggests that they could, pointing out that cutting a sympathetic nerve to a finger eliminates the wrinkling response. This means purely osmotic interactions do not fully explain these wrinkles.

The wrinkles, as observed in humans (a) and macaques (b).

The authors analyzed pictures of wrinkled fingers from different people, and discovered that these wrinkles have a structure typical of drainage networks. Drainage networks include river basins, and, more importantly, rain treads on tires. These rain treads were specifically designed to reduce slippage when driving on wet roads. Thus, the structure of the wrinkles could help maintain a good grip in wet conditions – on tree bark, for example, which would have been essential for our primate ancestors.
Rain treads on tires.
The paper’s authors offer only the broadest sketch of the possible water-wrinkle effect benefits, and make little attempt to quantify how well the structure improves grip in wet conditions. As such, I am reluctant to accept their idea unequivocally. However, it certainly would be interesting to look at different primates to see whether or not they also posses the water-wrinkle response. At what point in the evolutionary tree did this character evolve? Are primates in wetter climates more likely to have this response? If this drainage network is so efficient, as the authors suggest, what other examples can we find in biology? These are interesting questions that could strengthen the case for the evolutionary significance of these wrinkled fingertips.


Reference:

Changizi M, Weber R, Kotecha R, Palazzo J (2001). "Are wet-induced wrinkled fingers primate rain treads?" Brain Behav Evol 77:286-290

4 comments:

  1. This is so interesting! I had never considered the idea that there might be some evolutionary process involved with fingers wrinkling when in water. As far as comparing to other primates, I found one site (http://www.nature.com/news/2011/110628/full/news.2011.388.html) that Japanese macaques, a primate that is famous for bathing in hot springs, also display this wrinkling pattern.

    However, something I've just noticed in my life is that the pruning is by no means immediate. Rather, the wrinkles get deeper the longer I stay in the water. For this to really be an efficient way to grip in wet situations, shouldn't the wrinkling effect be quicker?

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  2. Yeah, that's one possible criticism. Like I said in the post - this is basically just a hypothesis with minimal evidence. I will say, though, that evolution isn't necessarily perfect. It may be that the wrinkling effect isn't "optimal" because of the time it takes to wrinkle, but it's certainly better than the alternative :).

    Also, the first picture on the right is a macaque's hand (probably should have made that clearer...). I think it's still important to expand the number of species as much as possible, to see if the trend really holds.

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  3. Thinking back on that, though...

    I wonder how the time to wrinkle varies among species? Do macaques take less time to wrinkle? This particular feature probably hasn't played a huge role in very recent human evolution. We could have relaxed selection on the time to wrinkle, when compared with other primates.

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  4. Haha I thought that was an exceptionally hairy human! I didn't look at the caption I guess. My mistake lol But yeah, that's an interesting thought. I wonder if there is a difference in wrinkling time. I agree with you, though, that some quantitative evidence to support the gripping benefit hypothesis would really help convince me.

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