SCI-TECH & AGRI

Bats scan rainforest with UV vision

Flower visiting bats seem to need UV-vision, as the flowers they visit in the rainforest are characterised by a strong reflection of UV-spectrum light at night.

Flower visiting bats seem to need UV-vision, as the flowers they visit in the rainforest are characterised by a strong reflection of UV-spectrum light at night.  

BATS FROM Central and South America that live on the nectar from flowers can see ultraviolet light. This was discovered by York Winter, of Munich University and the Max-Planck-Research Centre for Ornithology. As they lack cone pigments in their eyes, flower bats capture ultraviolet light with rhodopsin of their rod pigments. The researchers discovered this ability while keeping the bats in an environment with computer controlled artificial flowers equipped with small signal lights. Flower visiting bats seem to need UV-vision, as the flowers they visit in the rainforest are characterised by a strong reflection of UV-spectrum light at night.

Modern mammals lost their ability to see ultraviolet in the course of evolution, contrary to birds and lower vertebrates. Of the originally four cone pigments of ancestral vertebrates, the higher mammals have retained only two.

Night-active bats lost functional cones altogether and retained only the rods as photoreceptors. At night, compared to daylight, the colour spectrum is shifted towards short, UV-wavelengths. The flowers that are pollinated by bats in the rainforest use this fact by having their petals strongly reflect UV radiation.

The flower-visiting bats use their rod receptor for UV-perception and catch the UV-photons with the so-called beta-band of their photoreceptor, a peak of minor sensitivity for light absorption. In these mammals, therefore, only a single photoreceptor is responsible for the perception of light radiation over the whole wavelength spectrum.

The researchers discovered that bats could see UV-light in the psychophysical experiments that involve behavioural observation. The animals learnt over several months in a computer-controlled artificial environment that only flowers with a small signal light would also give food. The researcher made use of the bats' ability to react to the lit flowers by changing the wavelengths of the signal lights and varying their intensity. This showed that the bats could still see the signal lights on the flowers far into the UV range.

Light receptors are less sensitive under bright light. The scientists used this fundamental property to investigate the cause for their UV-sensitivity. They immersed the artificial environment of the bats in a one colour, monochromatic background light.

At the same time, the researchers reduced the intensity of the signal lights at the artificial flowers and were thus able to measure, at which light intensity the bats could still see the lights. That bats can see UV is also due to the fact that a UV-filter is lacking from their eyes' lenses. Normally, the UV-absorbing lens protects a mammal's eye from UV-radiation. UV-light not only damages the retinal cells but it also causes an optical problem. The angle of light refraction depends on the wavelength of the light.

A point of light is refracted at the lens, the refractive surface of the eye. As different wavelengths are refracted at different angles, a light of many colours such as one containing UV, will lead to an out-of-focus image on the retina of the eye.

But the smaller the size of the eye, the less disturbing this effect will be. Thus UV-vision should only be expected in small, nocturnal mammals such as the bats with their small, 2 mm eyes.

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