What They Did
The researchers collected tawny owl (Strix aluco) feathers from nine populations throughout Europe and analyzed the degree to which feather traits varied by population and by climate. They found that the length of the plumulaceous portion (the fluffy part at the base) of feathers from the owls’ backs was inversely correlated with winter temperatures: the colder it was, the longer the plumulaceous part of the feathers. This finding did not hold for feathers on the front of the body. The researchers hypothesize that the insulating function is more important on the back because of the position of the lungs and because the front of the body has more insulating fat deposits.
They also obtained DNA samples from some of the owls in order to compare the differences in microsatellites (repeated sequences of DNA that are usually non-coding) among the populations; this give an estimate of the amount of divergence expected through genetic drift. When comparing the differences in feather traits to the microsatellite differences, the researchers found that the density of feather barbs varied among populations for the front feathers but not the back feathers. The density of barbules varied for both front and black feathers, as did the length of the plumulaceous portion of both front and back feathers.
Although all these traits varied among populations, only the length of the plumulaceous part of the back feathers was directly correlated with climate variables. The other variations may reflect adaptation to some other aspect of the environment for each population.
Further Exploration
Feathers have kind of a fractal structure: the rigid shaft of the feather, called the rachis, has barbs growing off perpendicular to it. The barbs are what you see if you gently pull apart a feather. Each barb has barbules growing off, perpendicular to the barb, so parallel to the rachis. The barbules interlock to keep the whole feather together. (see https://academy.allaboutbirds.org/feathers-article/).
The linked article also discusses the hypothesized steps in the evolution of feathers, from simple hollow tubes (that’s why we can put ink in them for quill pens!) to fuzzy barbs all attached at the base. The ends of the barbs eventually grew together to form a longer rachis, and the barbs also developed barbules. Finally, the barbules evolved the hook structure that holds the feather together, and feathers began developing different shapes for different purposes.
What did they evolve from? It can look like feathers replaced scales as non-avian dinosaurs evolved into birds, but a feather isn’t just a modified scale. Feathers and scales are entirely different structures that develop from the same type of thickened skin regions in the embryo. (see https://evolutionnews.org/2023/05/fossil-friday-a-dinosaur-feather-and-an-overhyped-new-study-on-the-origin-of-feathers/). The linked article also notes that the scales on bird feet are more chemically similar to bird feathers than to the scales of other reptiles, suggesting that sometime after feathers had evolved, scales re-evolved in birds. It would be interesting to look into the specific genetic and biochemical pathways that allow scales and feathers to develop in embryos, but that’s a rabbit hole for another day!
Image credit: Peter Trimming
https://commons.wikimedia.org/wiki/File:Tawny_Owl_%285946073610%29.jpg
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