For many social animals that live in colonies, scent marking is an important process, helping the animals to follow commonly used paths through vegetation and reinforce the status of more senior animals. However, it's difficult to quickly localize the direction a scent is coming from, said Peichl, suggesting that ultraviolet visual cues might make it easier for animals to find their way quickly. Fresh urine also reflects more strongly than old. That could indicate how fresh a path is, what came past, when, and what sex the animal was.
The authors also noticed that the degu has white markings on its chest region, which are revealed when the animals raise up on their hind legs to make alarm calls warning of possible danger. These markings also reflect ultraviolet light, said Peichl.
The importance of communicating with urine could have favored the retention of UV- vision in the degu and other rodents, says the study.
"It wasn't realized until fairly recently that rodents had ultraviolet vision," commented David Hunt a molecular geneticist at University College London's Institute of Ophthalmology in the United Kingdom.
Ultraviolet light damages the eye, in much the same way ultraviolet rays from the sun are bad for the skin, said Hunt. This might explain why some species lack the ability to see it. Many mammals have filters in the lens of the eye, which block UV rays.
The importance of ultraviolet vision for communication with urine could explain why the degu has retained the ability to detect light at that wavelength, said Hunt. But more evidence would be required to claim that this explains ultraviolet vision in other rodents, he said.
Admittedly some of the rodents that possess the ability, such as rats and mice, are almost completely nocturnal. Peichl agreed that it's a total mystery why a nocturnal species might need ultraviolet vision.
However, communicating with ultraviolet light reflecting urine does have some drawbacks, said Peichl. One recent study showed that kestrels in Finland improve their hunting success by using the freshness of the urine to distinguish used from abandoned vole trails. "The same may be true for the degu's native predator in Chile," he said.
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