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Seeing Love in a Different Light

1 May 2008 (All day)
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Daiqin Li/The National University of Singapore

Seeing is believing.
Jumping spiders (Phintella vittata) can see ultraviolet B light, the first animals shown to have this capability.

For people, ultraviolet B (UVB) is an invisible, cancer-causing ray to be blocked with sunscreen and dark glasses, but for a species of jumping spider, the light sets a romantic mood. In the first evidence of an animal having the ability to see UVB, researchers have found that the ornate jumping spider uses the light in mating displays. The finding raises the possibility that other animals can see this wavelength, which researchers have long assumed no creature could detect.

Several insects, crustaceans, birds, fish, and mammals can see ultraviolet A light, but researchers took it for granted that no animal could perceive UVB because their eyes do not appear capable of detecting this slightly shorter wavelength, which packs more energy and can cause skin cancer and eye damage. However, while working last year on a species of jumping spider that uses UVA in mating displays (Science, 26 January 2007), a team led by Daiqin Li, an arachnologist at the National University of Singapore, made the surprising discovery that males of another jumping spider species (Phintella vittata) in China have patches on their abdomens that reflect UVB light and are used in mating displays. Could these spiders see the light, so to speak?

To put this notion to the test, Li’s team placed 20 male spiders in glass cages lined with light filters. Female spiders in cages next to the males spent more time ogling the guys’ mating displays when UVB light was allowed in than when the wavelength was blocked. In a separate experiment, the researchers took 14 pairs in which the females had responded positively to the male’s courtship display under normal lighting conditions and placed them in cages separated by the UVB-blocking filter. Eleven of the 14 females now no longer showed an interest in the males’ come-hither dance, confirming that seeing UVB light--and not the male’s particular display--was essential, the researchers report online today in Current Biology. Li’s team doesn’t yet know how the spiders see UVB or whether their eyes are somehow protected from the damaging effects of the light.

Carlos Ballaré, an ecologist at the University of Buenos Aires in Argentina, says the spiders’ UVB vision may have evolved as a way of private communication that predators, such as birds, can’t detect.

Eileen Hebets, a behavioral ecologist at the University of Nebraska in Lincoln, says that although the spiders appear to be able to sense UVB light, she’s not convinced that it’s used in mating because the males and females weren’t allowed to carry through and actually breed. Still, the findings may be the beginning of understanding how these spiders and other animals use UVB in communication, she adds.

Li says other animals that may be able to see UVB light include another species of jumping spider and the domestic pigeon, both of which have outer surfaces that reflect UVB. Insects called thrips and two species of poison-dart frogs in Costa Rica avoid areas in their natural habitat with high levels of UVB, suggesting they also may be able to detect the wavelength.

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