Beauty in the deepest dark: Peacock spiders seduce females using “super-black” patterns
For male peacock spiders, flirting means risking getting eaten, so they have evolved to impress as much as possible. Special hairs on the male’s rear absorb over 99.5% of all light — making nearby colors ‘pop’ even more.
When someone thinks of spiders, their love affairs may not the first thing to come to mind. Yet, these creatures, like other animals, reach a point in their life where they must seek a mate and reproduce. There are nearly 50,000 species of spiders, and the vast majority spend most of their lives alone. All spiders are predators, and many will attack and eat members of their own species if they can. So, spiders often avoid each other.
Mating is one of the rare times when two spiders of the same species can have a peaceful interaction. But safety is not guaranteed.
When males are courting females, they have to get two very important messages across: tell the female that he’s not food, and to convince her to mate with him. Different spiders have different courtship tactics: orb-weavers send tactile signals by plucking the female webs; other spiders use chemical substances; others yet drum on the soil to signal their intentions.
Humans are visual animals, and thus we can more easily appreciate the courtship of spiders that, like us, also interact with the world through their eyes. This may be why jumping spiders (Salticidae) are so popular on the internet.
Luckily for us, jumping spiders are the most diverse group of spiders, with more than 6000 known species. They can be found on all continents except Antarctica. Jumping spiders do not rely on a web to catch their prey, and instead stalk them actively on the ground or the vegetation. As their name suggests, they are able to jump short distances to catch their prey.
Jumping spiders’ excellent vision helps them calculate their jump trajectories, and spot hiding prey. In bright light, they can see about a sharply as a cat or pigeon, despite being many times smaller! Many species have also evolved the ability to see in color. The next time you find a jumping spider, take a moment to appreciate the fact that when you look at it, it looks back at you.
And, just like us, jumping spiders like to see beautiful sights. In many species, males are bright and colorful, and perform elaborate dances when courting a female.
In the Australian bush, we find the jumping spiders that have taken these dances to an extreme: peacock spiders (genus Maratus). You might not be able to tell from seeing the females — they are camoflaged with flecks of brown and grey, like in many jumping spiders. But males have bums (their abdomen) covered in colorful hairs (setae)!
Like their bird namesakes, peacock spiders keep the real show under wraps wait until they find a female. When a lady is in sight, males will lift their abdomens, unfold two lateral flaps, and make and elaborate dance involving movements of the abdomen and of the third pair of legs. Remarkably, scientists have discovered around 85 species of peacock spiders, and in each of them the male has a different pattern (as you can appreciate in this photo gallery) and dance.
Some males take being colorful a bit further — by using black. A team led by Dakota McCoy, a graduate student at Harvard University, noticed that in some species males have a large, black patch near the colorful areas. In a new study published in the Proceedings of the Royal Society B, they discovered the secret behind the hairs that make up these patches.
Looking through an electron microscope, the scientists found that the hairs in these black patches have a very special shape (a, below). Each individual hair is covered in small barbs, and lies just above a layer rich in melanin, a pigment responsible for absorbing light. When light goes through the barbs, it gets scattered and filtered multiple times, and finally absorbed by the melanin layer (b, below). This design is incredibly efficient in absorbing the light, and less than 0.5% gets reflected back (c, below) — this is as good as “Vantablack,” the darkest material we humans have been able to make. As a result, the regions covered by these special hairs appear super-black to the female.
But why? Well, colors can appear even brighter than usual if they are next to something that is much darker. So males’ colorful patches might appear to light up even more if they are next to these super-black hairs. Male peacock spiders bearing these super-black patches probably appear more interesting to females.
Interestingly, peacock spiders were not the only animals to “invent” super-black patches. In birds of paradise, which also have colorful males perform elaborate dances to females, the males’ feathers have a similar structural design and also reflect very little light. This might mean that, despite their many differences, males of both species have evolved under the same type of selection.
How did peacock spiders — and birds of paradise — evolve such a diversity of shape and colors? As it turns out, it’s because females are extremely choosy. Most females mate with a single male, and thus they need to choose their partner very wisely. With their powerful eyes, females can distinguish small differences between different males’ abdomens, and choose accordingly. There is something about colorful, elaborate patterns that appeals to females, and she has kept choosing fancier males.
This is sexual selection: traits evolve because they help an organism be chosen as a mate and reproduce. This is different from natural selection, where a trait is evolves because it helps an organism survive (for example, by catching food mor easily, or by helping it escape its predators). Natural and sexual selection are often in conflict: perhaps a colorful peacock spider has a better chance of seducing a female, but it is also more visible to its predators.
Sexual selection is powerful — humans only invented Vantablack in 2017, but peacock spiders have been sporting their super-black hairs for at least hundreds of thousands of years. In this case, a choosy mate is the mother of invention.
