Bats Follow Ultraviolet Light to Nectar, Study Suggests

John Roach
for National Geographic News
October 27, 2003
Halloween decorators take note: Reflected ultraviolet light lures bats to succulent treats. Fortunately, these bats live in the rain forests of Central and South America.

According to a team of German and Guatemalan researchers, rain forest flowers that reflect ultraviolet light may help guide the color-blind bat Glossophaga soricina to their nectar like a harbor beacon guides a ship to shore at night.

The bat's sensitivity to ultraviolet light is one facet of an evolved symbiotic relationship between the bat and the flowers. The flowers provide the bats food in the form of nectar, while the bats help pollinate the flowers, much like a honeybee, allowing the plants to reproduce.

The bats also use echolocation (akin to sonar) and the flowers' distinctive smell to find nectar-rich blooms in the rain forest, said York Winter, a biologist at the University of Munich in Germany.

Winter and his colleagues Otto von Helversen of Germany's Erlangen University and Jorge Lopez of the University of San Carlos in Guatemala reported on the ultraviolet light sensitivity of Glossophaga soricina in the science journal Nature earlier this month.

Elizabeth Dumont, a biologist at the University of Massachusetts at Amherst and member of the North American Symposium on Bat Research, said the study makes sense and may reflect a co-evolution of bats and plants.

"It is well-known that many flowers that rely on bats for pollination services are pale in color. This has been thought to make the flowers contrast with the surrounding vegetation and thus be more easily located by foraging bats. Since darkness obscures color and contrast, it makes sense that bats might use UV cues to locate flowers," she said.

Single Receptor Vision

Contrary to many fish, reptiles, birds, and insects, most modern mammals, including primates such as humans, lost the ability to see ultraviolet light over the course of evolution, according to Winter.

Most mammals are dichromatic, meaning they have the use of two types of light-receiving cells, called cones, in their eyes that allow them to distinguish two of the four primary color phases, giving them limited color resolution.

Primates, including humans, have three cones and can thus distinguish three of the primary color phases, giving us trichromatic vision, or high color resolution.

Ultraviolet vision was discovered in mammals just over a decade ago. Some rodents and marsupials, for example, can detect ultraviolet light through a distinct cone in their eyes, said Winter.

Nocturnal bats lost the function of cones altogether. Instead, they have rods, which handle vision in low light. Rods are also present in human eyes for black and white vision in low-level light.

Since bats have lost the cone that other ultraviolet-light-sensitive mammals have retained, they use their lone rod receptor to perceive light radiation over the wavelength spectrum from about 310 nanometers to 600 nanometers.

Ultraviolet light ranges from 100 to 400 nanometers and visible light ranges from 380 to 770 nanometers, thus Glossophaga soricina's receptor is sensitive to light in both the ultraviolet and visible spectrum, according to the researchers.

Winter and colleagues suggest that this unique visual system likely evolved to help Glossophaga soricina find the ultraviolet-light-reflecting flowers at twilight when the light spectrum is shifted towards these short wavelengths.

All plants can reflect light across the entire spectrum. This makes plants visible to humans, because we can see all colors in the visible spectrum.

"But because we have a strong UV filter in our lens, we cannot see the UV component," said Winter. "It is like looking through a pair of sunglasses at night. Bats have taken off their UV glasses—i.e. the corresponding filter in the lens—and thus get more of the light's spectrum into their eyes."

Glossophaga soricina is one of several closely related nectar-feeding bats in the family Phyllostomidae, and it is possible that ultraviolet sensitivity is characteristic of the whole group, said Dumont.

"In that case, UV sensitivity may be associated with at least some of the radiation of nectar-feeding bats in the neotropics," she said. "The other species would, or course, need to be tested in the same rigorous way in order to make firm conclusions about that."

Observational Experiments

Winter and colleagues reached the conclusion that Glossophaga soricina bats can see ultraviolet and visible light through a single receptor via a series of so-called psychophysical experiments, which involved behavioral observations.

The bats were placed in a computer-controlled environment and were trained over several months to learn that only flowers with a small signal of light would give them food. The researchers then varied the wavelength and intensity of the signal light and gauged the bats' reactions.

Based on these observations, the researchers concluded that the bats could see well into the ultraviolet spectrum but were color blind.

In a separate experiment, the researchers made the background light of the bats' artificial environment one color. At the same time they reduced the intensity of the signal lights at the artificial flowers and measured at what intensity the bats could still see the lights. This experiment was repeated with different background colors.

The results showed that independent of the color of the background light, diminishing visual sensitivity of the animals was uniform over the whole spectrum of wavelengths. This is the case when only a single photoreceptor is active in the eye, said Winter.

Winter and colleagues speculate that larger mammals are likely to be unable to detect ultraviolet light because as eye size increases ultraviolet light would become more scattered, making clear, focused vision more difficult.

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