Ultraviolet (UV) light—the band of electromagnetic radiation nestled between visible light and x-rays—seems to cast a particularly amorous glow on the animal world. For instance, the budgie, an Australian parrot, is known to respond negatively to potential mates whose plumage has been stripped of its UV-induced fluorescence (wherein ultraviolet light is absorbed and light of a different, usually visible, wavelength is emitted). And although we humans cannot see UV light, as birds and many other animals can, we have incorporated lamps that produce it into some of our modern courtship rituals—just ask anyone who has ever hit the tanning beds in hopes of snaring a mate or any teen whose idea of setting the mood involves shining a black light on a Pink Floyd poster (which then, like the plumage of a budgie, fluoresces visible light).
The most illuminating example of the potential of ultraviolet romance, though, just might come from a jumping spider. As described in a January 26 paper in Science, researchers have shown that the Cosmophasis umbratica spider not only needs UV light (a constituent of sunlight) to instigate normal mating behavior, but that males and females of the species respond to it in physiologically distinct ways.
C. umbratica males feature scales on their face and body that reflect ultraviolet light, whereas the females do not. (Jumping spiders possess UV receptors in their retinas, so they can detect its emission or reflectance.) The females do, however, possess something the males lack: the ability to fluoresce bright green under UV illumination. Having recognized this distinction, the researchers decided to examine what role this gender-specific physiology plays in mating. So they blocked the ultraviolet wavelengths and observed what might be the arachnid equivalent of a cold shower. "It kind of ruined their sex life, really," says Michael Land, professor of neurobiology at the University of Sussex in England and one of the authors of the Science paper.
The sexual preferences of this species are easily observable, because an interested C. umbratica "has, like many jumping spiders, a fairly colorful mating dance," Land notes. "The males do this kind of Highland fling in front of the females." Females, for their part, have their own come-hither protocol: "They either stay still or they go for a little run and then come back again," he says. Under UV-blocked light, the authors report, "a large proportion of the same pairs that successfully interacted in the presence of UV failed to show intersexual behavior in its absence." Most males "failed to court the female when she lacked fluorescence," and most females similarly snubbed males not reflecting UV.
While the aforementioned budgies also disdain suitors in the absence of UV-induced cues, "it just seems to be rather like having a shirt at the disco under UV light, which glows if you wash it in the right washing powder," Land says. "But that's both sexes and just seems to be a property of the yellow pigments. So this is different. This is a sexual badge, if you like." Whether that badge acts as an aphrodisiac or merely a prerequisite identifier remains unclear. "I don't know how you distinguish between the two. Because if they identify [another spider] as the wrong species, this is obviously not going to be very aphrodisiacal," Land explains. The two roles, he adds, are "obviously going to go together, and it's almost impossible, I think, to disentangle them."
Even if the presence of UV had no effect on C. umbratica courtship, and even if the spiders were unified in their UV-induced signaling—that is, if both sexes reflected UV or both fluoresced green under UV—the species would still be in elite company: "Ultraviolet reflectance is not particularly common in animals," says Thomas Cronin, a vision researcher and professor of biological sciences at the University of Maryland, Baltimore County. And as for fluorescence, Cronin says, "it's not thought to be that common; we don't have very many examples of it." Still, he adds, "it would be easy to miss, because you kind of have to look for it rigorously."
Land says his colleagues have "big, big plans" to do just that—to look at other species and determine whether this type of signaling is unique to C. umbratica or more widespread. Whatever these further experiments reveal, this much is already clear: one species, for ultraviolet light at least, seems to have reached some sort of consensus on the age-old lights on–lights off debate.