By Stephen Beech
Our fingers always wrinkle in the same pattern every time we spend too long in water, reveals new research.
The patterns remain constant after multiple immersions, say American scientists.
And they believe their findings could help solve murder cases where a body has been submerged in water for a long period.
Researchers at Binghamton University in New York previously investigated why human skin wrinkles when people stay in the water too long.
Received wisdom held that the water swelled the skin and made our fingers wrinkly, but little research had been done to prove that.
Professor Guy German and his team at the Biological Soft Matter Mechanics Laboratory found that blood vessels beneath the skin actually contract after prolonged immersion, and that's where the wrinkles come from.
Now a follow-up study, published in the Journal of the Mechanical Behaviour of Biomedical Materials, shows that the topography patterns remain constant after multiple immersions.
German said: "Blood vessels don't change their position much - they move around a bit, but in relation to other blood vessels, they're pretty static.
"That means the wrinkles should form in the same manner, and we proved that they do."
The researchers put participants' fingers in water for 30 minutes, took photos and then repeated the immersion under the same conditions at least 24 hours later.
By comparing the images, the team found the same patterns of raised loops and ridges after both immersions.
German revealed that they also made another discovery.
He said: "We've heard that wrinkles don't form in people who have median nerve damage in their fingers.
"One of my students told us, 'I've got median nerve damage in my fingers.' So we tested him - no wrinkles."
German says the findings could have applications in forensics, such as fingerprinting at crime scenes and identifying bodies found after prolonged water exposure.
He said his father, a retired British police officer, faced some of those challenges during his law enforcement career.
German said: "Biometrics and fingerprints are built into my brain.
"I always think about this sort of stuff, because it's fascinating."
Now German is keen to further explore questions about skin immersion.
He added: "I feel like a kid in a candy store, because there's so much science here that I don't know."