Ever wondered about the phenomenon of wrinkling in fingers when exposed to water? Glabrous skin on fingers, palms, toes, and soles uniquely responds to water, forming wrinkles after a few minutes. While some attribute it to osmosis and a biochemical reaction, it was recognized a century ago that this reaction is not a straightforward reflex.
A baby's fingers wrinkled as a result of staying in the bath
Osmosis Wrinkles Skin Submerged in Water
Osmosis, the movement of water molecules through a semipermeable membrane, leads to wrinkling when skin is submerged in water. This effect is prominent in areas with the thickest skin layers, like the inner hands and feet due to constant friction. Researchers attribute skin wrinkling to the autonomic nervous system, outlining two essential cases.
In Case 1, the skin's outermost layer is coated with sebum oil, acting as a water-resistant barrier. Prolonged water exposure removes sebum, enabling water penetration and causing wrinkles.
In Case 2, the skin's high keratin content, especially in fingers and toes, absorbs water during prolonged contact, leading to aquatic wrinkles. Digital vasoconstriction, narrowing blood vessels, plays a role in forming wrinkles by pulling the epidermis downward.
The hands and feet have more keratin cells than other body parts, resulting in less skin bunching and, consequently, no wrinkling in those areas. Nails and hair, also made of keratin, absorb minimal water during bathing, remaining unaffected.
Wrinkling is absent in fingers or toes with a severed sympathetic nerve, even with prolonged soaking, as the nerve function loss prevents this reaction. Individuals with disorders or replanted amputated fingers experiencing nerve function loss will similarly exhibit no wrinkling when immersed in water.
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Evolutionary of Wet Wrinkly Skin
A century ago, scientists recognized that the wrinkling response in fingers wasn't a simple reflex or a result of osmosis. Surgeons discovered that cutting certain nerves in the fingers eliminated the wrinkling response, indicating an intact nervous system. This wrinkling response has been considered a way to assess the functionality of the sympathetic nervous system in unresponsive patients.
While there is consensus on the physiological aspects, the evolutionary purpose of the wet-wrinkling response remains a debated topic. Neurobiologist Mark Changizi proposes that wrinkled fingers might be an adaptation for optimal grip in both wet and dry conditions.
In 2011, Changizi and colleagues found evidence that wrinkled fingers act as rain treads, channeling water away during wet conditions, enabling primates to maintain tighter grips.
The pattern of wrinkled fingers resembles an inverted river drainage system, with raised interconnected "trees" of bumpy skin and diverging disconnected sunken channels.
The ridges on wrinkled fingers force water away, and the response is more pronounced with freshwater exposure, suggesting its potential evolutionary origin in primates. While the wet-wrinkling response may not have explicitly evolved for gripping, studies indicate its usefulness in manipulating wet objects.
Changizi suggests that the wet-wrinkling response may support body weight, particularly in activities like tree gripping. He proposes conducting experiments with parkour experts in various conditions to assess the impact of wrinkled skin on locomotion rather than dexterity.
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