for National Geographic News
Many fish, reptiles, birds, and insects are able to see ultraviolet light. Some even use pigments that reflect it to attract mates and communicate. But most mammals have lost the ability to see ultraviolet light and lack the cellular machinery necessary to detect it.
Now, a new study suggests that urine plays such an important role on the rodent communication grapevine that it may explain why some species have retained ultraviolet (or UV) sensitive cells and visual pigments and other mammals haven't. The urine of many species of rodent strongly reflects ultraviolet light, says the study published in a recent edition of the journal Investigative Ophthalmology and Visual Science.
The ultraviolet messages left with urine could be dangerous, says the study. Other research has shown that UV-vision might be used by birds of prey to pinpoint the location of their next meal.
Visual pigments are found in specialized retinal cells at the back of the eye. They react to different wavelengths of light, or colors, and allow humans to detect them. As mammals, we are totally ignorant to the ultraviolet hues that adorn both the plumage of brightly colored birds like blue tits, and attract bees and butterflies to the petals of many common flowers. In fact most mammals have just half as many types of color-receptive cells as the eyes of many other vertebrates.
One theory is that tens of millions of years ago when dinosaurs filled the jobs of most of today's large mammals, small scavenging mammalian ancestors were forced to become entirely nocturnal, said neurobiologist Leo Peichl at the Max Planck Institute for Brain Research in Frankfurt Germany. In the night-time environment, both UV and color vision are functionally useless. Without a use for UV vision, mammals eventually lost the ability.
"Mammals had to cope with what they had after this, and for many this seemed sufficient," said Peichl.
Unique among mammals however, many rodentssuch as rats, mice, and gerbilshave retained the ability to see UV light, though the reasons behind this have remained mysterious.
Guinea Pig Relative
To answer this question, scientists in Chile and Germany have been studying the color vision capabilities and environment of the South American degu (Octodon degus), a relative of the guinea pig.
Lead scientist Adrian Palacios at the University of Valparaiso in Chile, Peichl, and co-authors examined the cellular make-up of the degu retina. The retina is a layer at the back of the eye packed with light-detecting cells. Experiments revealed the same cells that have also been discovered in rats, gophers, gerbils, mice, and hamsters. The cells are sensitive to light in the ultraviolet part of the spectrum.
A quick scan of the animal's natural environment revealed that food plants, soil, and stone all reflect ultraviolet light poorly. However several things did reflect ultraviolet light very well, and one of these was degu urine.
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