Maybe you’ve never stuck around long enough to get a close look at a tarantula, but most of them are blue. And according to a new study published in Science Advances this week, that vibrant hue is driven by natural selection – not sexual selection like we might expect.
The diversity of colors displayed by animals – from peacock feathers to butterfly wings – is produced by the absorption of certain wavelengths of light by pigments or by light scattering from nanostructures on those colorful parts (or a combination of the two). While sexual selection typically increases color diversity, natural selection usually reduces it. Prey species relying on remaining hidden, for example, often have similar background-color matching. But it’s not always so black and white, if you will: Colors might perform practical functions like heat regulation, and iridescence is sometimes the byproduct and not the intended result. Distinguishing between the effects of sexual selection and natural selection on colors can be challenging. That’s why animals with limited visual capacities provide a unique opportunity to investigate how color evolves through natural selection – without the confounding influence of sexual selection.
Most tarantulas are nocturnal ambush predators that rely on stealth to capture prey, not speed. They typically live in retreats and burrows in the understory of tropical forests, and they navigate using chemotactile senses. Even though they have eight eyes like most other spiders, they have poor vision. Yet, many also boast a bright, cobalt blue coloration that’s produced by nanostructures in the individual hairs attached to the exoskeleton that reflect blue light.
A team led by University of Akron’s Bor-Kai Hsiung surveyed the colors of tarantulas from 53 genera using digital images obtained through Google and Flickr searches. Forty of those genera had blue coloration. The team then analyzed specimens from eight species (purchased from private sellers) using electron microscopy and a technique called reflectance spectroscopy. That blue color, they found, evolved at least eight times independently in tarantulas via three different mechanisms. Blue coloration in even distantly related tarantulas peaks within a narrow 20-nanometer region around 450 nanometers.
But unlike many other flashy animals, these spiders don’t show off their blue in visual courtship; they instead rely heavily on vibratory and chemical cues. Not to mention, the blue shows up early in sexually immature spiders, and some adults even lose their blue when molting. Furthermore, the spiders can’t even see blue light wavelengths well at all. Together, these findings suggest that the blue hue couldn't have been sexually selected.
A front view of a gooty sapphire ornamental tarantula. Michael Kern/www.thegardensofeden.org
Image in the text: The underside of a gooty sapphire ornamental tarantula. Michael Kern/www.thegardensofeden.org