Not Exactly Rocket Science

Ultraviolet (UV) radiation lies beyond the violet end of the rainbow. Our eyes aren't equipped to see it and its presence only becomes visually apparent when enough of it hits our skin and causes a painful, red patch - a sunburn. But not all animals have eyes that are so ill-equipped. The females of the jumping spider Phintella vittata not only see UV light, they also find it sexy.

UV light may be invisible to us but many animals can see it and use it to communicate. Sometimes, this is deliberate, as in the case of blue tits using UV patches to seduce females. It can also be inadvertent and downright unfortunate, as in the case of voles that give away their position to hovering kestrels through the UV reflections from their urine. Even though UV-based messages are widespread, those of the jumping spiders are unique, for they use a specific type of ultraviolet light called UVB.

The UV-sensitive cells in animals that can detect ultraviolet light almost always respond most strongly to UVA, the type that lies closest to violet and has the longest wavelengths. It's usually been assumed that even though ultraviolet vision is very common among animals, none of them can see into the UVB range. Until now, only three exceptions have been recently found - a species of thrip (a tiny insect) and two species of poison-arrow frogs. And all of these detect UVB so that they can avoid it.

There's good reason to do so for UVB causes a variety of harmful effects. It has just the right properties for damaging DNA and it does so by fusing two molecules of thymine, one of the four component nucleic acids that make up DNA. These merged molecules distort the DNA helix and the information it contains, in the same way that sticking two pages of a book together would hinder your reading experience.  The end results are mutations and potentially cancer, and in humans, overexposure to UVB is the main cause of skin cancer.

Despite these dangers, Jingjing Li and colleagues from Hubei University in China found that one species of jumping spider can indeed see UVB and signals with it during courtship. By collecting jumping spiders from the Xishuangbanna Tropical Botanic Gardens, he found that both males and females of Phintella vittata have patches on their backs and abdomens that reflect UVB but not UVA. The patches are displayed during courtship and those on the males shine more brightly.

Spider-sense tingling...

Li used a specially constructed box to allow two males to compete for the attentions of a female. The males couldn't see each other but the female had full view of both her suitors. Separate lights shone over the chambers that housed the males and by placing filters over these, Li could remove the UVB reflecting off their backs.

At first, the females were kept alone to make sure that they had no preference for the ambient light levels. The males were then added, and allowed to strut their funky stuff by arching their legs and vibrating their palps. Li found that the females used the UVB reflections of the males to select the sexiest mate. Males that shone brightly with UVB were more likely to draw the notice of females and kept their rapt attention for three times as long as males housed under a UVB filter.

To rule out the possibility that some males were just naturally sexier than others, Li ran a different experiment where he transferred the same male from the normal chamber to the UVB-filtered one.  Almost 80% of females that fancied a male in the first box ignored him when his UVB reflections were rendered invisible. Li also used more general filters to show that it was the presence of UVB rather than the brighter quality of non-filtered light that really kept the females' attentions.

Jumping spiders have very complex eyes, which they use to accurately judge the position of prey and coordinate their long-distance pounces. It's likely that all of these hunters can see UV light and most rely on UVA.  But When Li compared the attractiveness of males housed under UVA or UVB filters, he found that females preferred those that only reflected UVB over those that only reflected UVA.

It's clear that Phintella vitatta uses UVB for communication, but how it does it is a mystery. How does it detect light at this wavelength and how does it protect its eyes from the harmful effects that UVB can cause? And how common is the use of UVB signalling in the rest of the animal kingdom?

Only more research will say for sure. Only two other species are known to have patches that reflect UVB - another jumping spider and the common rock pigeon - but how they use these is a mystery. But if any other animal can see and communicate with UVB light, there's a pretty good chance it's the mantis shrimp. These critters have the most incredible eyes of any animal and they have four different UV receptors. While all four are tuned to different parts of the UVA range, at least one straddles the divide with UVB.

Reference: LI, J., ZHANG, Z., LIU, F., LIU, Q., GAN, W., CHEN, J., LIM, M., LI, D. (2008). UVB-Based Mate-Choice Cues Used by Females of the Jumping Spider Phintella vittata. Current Biology DOI: 10.1016/j.cub.2008.04.020

Images: from Current Biology