Something weird can happen to the ears of people who hang out in the icy cold of the ocean's embrace. Small spurs of bone start growing in the ear canals, triggered by the irritation the cold temperatures cause on the inner ear. It's a condition seen a lot in surfers, so it's known as surfer's ear (although it's also common in divers and kayakers).
According to ancient bones, our human ancestors and relatives, including modern humans and Neanderthals, were also prone to this ailment. But although these growths - called exostoses - have been noted, not a lot of research had been conducted into their prevalence, and how they might have been obtained.
According to new research, Neanderthals seem to have been more prone to surfer's ear than any other ancient group. And since we haven't found a lot of Paleolithic surfboards laying around, they probably didn't get it by hanging ten.
Which means they were likely doing something else in the cold water. One possibility could be harvesting aquatic resources, maybe even fishing. Until recently, such tasks were thought to be beyond the capabilities of Neanderthals, based on the levels of coordination and intelligence we assumed they had.
"It reinforces a number of arguments and sources of data to argue for a level of adaptability and flexibility and capability among the Neanderthals, which has been denied them by some people in the field," paleoanthropologist Erik Trinkaus of Washington University in St Louis told AFP.
In their new paper, Trinkaus and his colleagues - Sebastien Villotte and Mathilde Samsel from the University of Bordeaux - have examined and analysed the skulls of 77 hominins with well preserved ear canals.
Of those skulls, 23 belonged to Neanderthals, dating between 100,000 and 40,000 years ago; around half of those Neanderthal skulls had aural exostoses, ranging from mild to severe.
That was almost twice the frequency of any other group in the study.
But there are a few problems with the fishing hypothesis. First, the inland geographic locations the skulls were found in have not been correlated to sources of water, nor cooler climates. Second, isotope analysis of Neanderthal remains - including two of the skulls with exostoses - found that they weren't eating a lot of freshwater fish.
And the Neanderthal prevalence of external auditory exostosis was also right at the upper limits of the prevalence we see in human populations today - among people who repeatedly and for long periods of time expose their ears to chilly water.
So, another possibility is that Neanderthals had a genetic predisposition to bone growths in their ears.
We're still unsure if there's a genetic component to the condition, although a 1998 study suggests otherwise. Researchers examined over 1,000 skulls from Chile, from between 7000 BCE to 1500 CE, and found over 30 percent of the skulls from the coast had aural exostoses - but skulls from the highlands weren't affected at all.
So, the most probable reason for the condition in Neanderthals, the researchers said, is probably foraging (or maybe they really liked swimming). And there could be other factors at play that we have yet to determine.
"It remains likely that the high level of external auditory exostosis among the Neanderthals [..] is due in part to the exploitation of aquatic resources," they wrote in their paper.
"However, the Neanderthal frequency is at the upper limits of recent human population values and is matched only by those who experienced cold water maritime climates. It is therefore likely that, as with eastern Eurasian later archaic humans, multiple factors were involved in their abundance of external auditory exostosis."
The research has been published in PLOS One.