MessageToEagle.com - Many animal species are able to perceive light wavelengths beyond those visible to humans.
The infrared and ultraviolet portions of the spectrum are invisible to humans.
While numerous species are additionally sensitive to short wavelengths (UV), long wavelengths such as the
near-infrared spectrum (NIR) are supposed to be unsuitable for visual perception.
The cichlid fish Pelvicachromis taeniatus, for example, displays a clear foraging response towards NIR reflecting prey.
This was thought to be unlikely until now.
Pelvicachromis taeniatus are among the most colorful of the West African dwarf cichlids. Photo credits: Montana Exotic Tropicals
A research team in the work group of Prof. Dr. T. C. M. Bakker at the Institute for Evolutionary Biology and Ecology,
University of Bonn, has been studying the biology of the African cichlid fish Pelvicachromis taeniatus for years.
They discovered that fish can detect prey using infrared light and seeing in the infrared range is apparently
helping fish to hunt in shallow African rivers.
Researchers investigated the ability to see in the infrared range using a classical prey choice experiment. P. taeniatus
also feeds on small crustaceans, such as freshwater shrimp.
These prey animals reflect near infrared radiation.
The researchers used this fact to examine the perception of infrared light. In a dark room a prey selection
experiment was set up illuminated by infrared lamps.
In front of the water basin containing the fish freshwater shrimp were offered in two separate chambers. One of the
chambers with the prey was covered with a filter blocking infrared wavelengths. The other chamber was covered with
a filter that would let only infrared light pass.
"Consequently, the fish were only able to perceive the freshwater shrimp in one chamber in the near infrared range"
explains Dr. Sebastian Baldauf, one of the scientists involved
in the study, published online.
The experiment showed that the fish spent more time and were more frequently in front of the chamber that let
infrared light pass. "The fish detect their prey based on infrared radiation alone" reports the biologist from
the University of Bonn.
"Until now, physiologists thought that noise levels in the near-infrared range were too high
to allow visual perception." As the experiment has shown, the fish were capable of perceiving prey in a wavelength
range above 780 nanometers. It is well-known that snakes can perceive far infrared radiation at longer wavelengths
above 2,000 nanometers.
"But they don't use their eyes for this purpose; instead they have a heat-sensitive pit organ,"
says Dr. Baldauf. Human eyes are not capable of seeing infrared radiation.
The advantage of the fish’s ability to see infrared may become obvious when you look at its natural habitat.
The shallow rivers of West Africa have a relatively large amount of infrared radiation. "That's exactly why it makes
sense to use infrared cues for detecting prey organisms," explains Dr. Baldauf.
"It is a clear selective advantage if you can perceive additional signals that others cannot perceive." It is
quite likely that other animals also have evolved a perception of near-infrared radiation, e.g. for hunting or
orientation, such as other fishes or birds. The researchers from the University of Bonn now want to study more
closely the physiology of infrared vision, and to what extent infrared radiation is relevant in other contexts.
When performing color measurements on these fish the researchers found that certain regions of the cichlid body
reflect the light in the near-infared range.
"We found that females reflect infrared radiation from their belly
region, and males from their fins" says Dr. Baldauf. The female belly is important for mate choice, and the
fins are displayed during aggressive encounters between males.
"Perhaps near-infrared signals play a role in visual communication in this species" says the biologist.
"And that's what we additionally would like to study in further experiments."
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