Biologists from the University of Ghent discovered that bird skin becomes darker towards the equator. Tanned skin protects birds from harmful UV light. This evolutionary pattern was already know for humans and reptiles. This pattern is most apparent in bald birds and birds with white plumage. The scientists investigated no less than 2,250 bird species from museum collections.
Bird plumage has been extensively investigated, but the underlying skin, has surprisingly, been under no such scrutiny. Biologist from the University of Ghent examined the skin color of more than 2.250 birds from natural history collections, including that of the Royal Belgian Institute of Natural Sciences. The specimens represent all known bird family and a quarter of all known bird species.
Darker towards the equator
Five percent of the investigated bird genera has dark skin. This dark skin has evolved over 100 times, and thus independently in different groups ranging from gannets to birds-of-paradise. The biologists observed that the proportion of dark skin increases as their habitat was located closer to the equator. This pattern – Gloger’s rule – is also observed in humans, reptiles and bird feathers.
One of the most plausible explanations for this pattern is that melanin, the black pigment that darkens skin and feathers, protects against harmful UV light. However, on first sight, this explanation does not hold for bird skin. They already have a first line of defence – their feathers and the melanin that is often present within. However, the researchers found that darkness increased in birds without feathers, including chicks. Furthermore, birds with white feathers, containing no melanin and offering only limited protection, showed a similar increase in dark skin.
“For the first time we could show that black skin in birds, like for humans, serves as protection against harmful UV light, even in situations where the skin is not visible”, concludes Michaël Nicolaï (UGent and RBINS), first author of the study in Nature Communications.
Some birds even appear to “tan”. During a short period of time some birds have a dark skin: for example when they molt many birds are temporary bald on their head and thus less protected.
Exploring the collections
The research was special for Nicolaï. “Even as a kid I was fond of the Museum of Natural Sciences in Brussels. Spending weeks in natural history collections, looking at stuffed birds, was a dream come true. Some specimens were almost 200 years old, others were collected by famous names such as Alfred Russel Wallace or during the first Belgian Antarctic expedition led by Adrien de Gerlache.”
