Color at night: geckos can distinguish hues by dim moonlight.

Of all the vertebrates, a gecko has just become the first to ace behavioral tests for seeing color in very low illumination.

People, for example, go color-blind in light equivalent to dim moonlight, but helmet geckos, Tarentola chazaliae, don't. They can still tell a blue from a gray of the same intensity, report Lina S.V. Roth and Almut Kelber, both of the University of Lund in Sweden, in an upcoming Biology Letters.

Earlier physiology had shown that most vertebrates deploy two systems of light-sensitive cells in their eyes. Two or more types of cone cells work together to sense color in abundant light, and a single type of rod cell detects light more sensitively, but only in black and white. Thus, when the seeing gets tough, people forgo color vision and rely on their rods.

Lizards, however, lack rods, presumably because they evolved for a long period as strictly daytime creatures.

To learn how an animal uses its eyes, says Roth, scientists have to test its behavior rather than just analyze the rods and cones. Famous nighttime prowlers, such as cats and owls, show color vision, but they've been tested only in ample light, Roth says. Their physiology suggests that they might use the humanlike strategy of tuning to black-and-white in murky conditions.

But night-cruising hawkmoths, during tests in Kelber's lab, have done better than people do at distinguishing colors even in light as dim as starlight (SN: 11/30/02, p. 350).

Gecko eyes intrigue researchers because the helmet geckos of Africa and most others have given up lizard-style daylight living and returned to darkness. To devise a test for color vision, Roth and Kelber used previous studies of gecko eye pigments to match eight shades of blue with grays expected to look the same to a gecko except for the hue. The researchers made cards with a checkerboard of grays or blues.

Roth trained a male and a female gecko to snatch crickets from forceps labeled with one of the cards. With blue cards, she offered a prime cricket. With gray cards, she presented a cricket made unpalatable by being dipped in saltwater. In low light, Roth couldn't distinguish the cards, but the geckos snapped at crickets associated with blue cards more than twice as often as at those tagged with gray ones.

Jim Bowmaker, an animal-vision specialist at University College London, welcomes the paper as "exciting." He agrees with the authors' speculation that similar powers of dim-light color vision might turn up in nocturnal frogs.

"The question is really not so much about nocturnal animals," he adds. Animals that live underground and deep-sea fish would also he interesting to test. Some deep-sea fish do have two kinds of photoreceptors--both rods--and, he speculates, might use color vision to distinguish other sea creatures' blue-green and red bioluminescence.