A very particular shade of blue hair has evolved independently on eight separate occasions and in at least three different ways in tarantulas, a new study finds. And scientists are having a hell of a time figuring out why.
Like most animals with a blue hue, tarantulas don’t get their color from pigments. Rather, that striking cobalt fuzz comes from nanoscale lattices bending and scattering light in specific ways. So called “structural colors” are used for everything from attracting mates to warding off predators. Usually, the function of these dazzling displays is obvious; in tarantulas, it’s an ever-deepening mystery.
As reported this week in Science Advances, a team of researchers has completed a detailed optical, morphological, and evolutionary analysis of blue haired tarantulas. At least forty species in eight distinct lineages are blue—and not just any ol’ shade. Across those eight groups, hair color converges within an extraordinarily narrow band of wavelengths, centered around 450 nanometers. That suggests natural selection is playing a big role. But what are the spiders adapting to by going blue?
Color (LM), morphology (SEM), and nanostructure (TEM) of blue hairs in eight different tarantulas (A-H). Image via Hsiung et al. 2015
Perhaps “tarantula blue” is a side-effect of some other desired physical property, for instance, the ability to repel water. But if that were the case, we’d expect each type of tarantula hair to have a similar molecular architecture. Instead, the researchers found at least three distinct nanoscale configurations, including multi-layered sheets of cells, and semi-ordered spongy structures.
Mate attraction also seems to be off the table, seeing as tarantulas have very poor color vision and don’t perform any conspicuous courtship displays. The most likely explanation seems to be that tarantulas are signaling another species—perhaps a predator—but at present “the receiver of that signal remains unclear.”
In any case, it’s a fascinating illustration of how natural selection can produce similar outcomes from very different starting points. The ways of evolution are often mysterious, but there is a method to the madness.
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