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Why Are Flowers Brightly Colored ?

Karl von Frisch, a Nobel Laureate, one of the founding fathers of Ethology, in the second instalment of the series on how to design experiments in animal behaviour asked a question: Why are flowers brightly colored ?

He surmised that flowers must be brightly colored in order to attract honey bees. But this contradicted the work of an authority in visual science, the ophthalmologist C von Hess who concluded on the basis of his own experiments, that all invertebrates including honey bees and even fishes were color blind. The idea that flowers must be brightly colored to attract honey bees and that bees, therefore, must have color vision, was so convincing for Frisch that he started with his own experiments to re-examine the question of color vision in honey bees around the year 1910.

Error in Sir von Hess’s Views

Sir Von Hess placed honey bee workers into experimental chambers, where experimental factors could be totally controlled, and presented them with two light spots simultaneously, each of a different color and varying in light intensity. In this situation, the bees were invariably attracted to the brightest spot, whatever the color was, which seemed to prove that bees are color blind and reacted only to different light intensities.

Von Frisch’s Experiment

Frisch on the other hand ensured that he trained and tested his bees in their natural habitat. He tested them in a situation where they would be expected to be motivated to learn and to display their ability to distinguish different colors if they have the capability to do so. Ironically, Frisch’s experiment was much simpler than that of von Hess’s. Here’s a description of it:

“By the scent of a little honey it is possible to attract bees to an experimental table. Here we can feed them on a piece of blue cardboard, for example. They suck up the food and, after carrying it back to the hive, give it to the other bees. These bees come back repeatedly to the rich source of food which they have discovered. W e let them do so for some time, and then we take away the blue card scented with honey and , put out two new, clean pieces of cardboard at the site of the former feeding place – on the left a blue card and on the right a red one. If the bees remember that they found food on the blue, and if they are able to distinguish between the red and blue, they should now alight on the blue card. This is exactly what happens.”

The above work of Frisch was not only simple and elegant that the original results have been repeated several times but that it opened a whole new field of Experimental Behavioral Ecology in which people have further tried to test the sensory and learning abilities of many animals under many different conditions.

Bees have Trichromatic Colour Vision

Not only did von Frisch’s evidence from behavioural evidence show without doubt that bees have color vision, but subsequent experiments by Frisch and others showed that honey bee color vision is quite different from that of the humans, and that bees are blind to red color. Many years later in 1962, a psychologist H Autrum obtained direct evidence of trichromatic color vision in bees by inserting microelectrodes into single photoreceptor cells in bees’ eyes and recording their electrical in response to lights of different wavelengths. Like humans recognize only three primary colors – red, green and blue, bees also recognize three primary colors, but these are ultraviolet, green and blue.

Why do Bees have Color Vision ?

We have already seen that Frisch set out to prove his hypothesis that the flowers are brightly colored so as to be attracted by honey bees, so the bees should have color vision. Conversely, we can ask why bees have color vision and hypothesize that it must be to locate flowers efficiently and harvest pollen and nectar. The idea that color vision in bees and brightly colored flowers co-evolved, reinforcing each other, making bees with better color vision better at seeking out flowers and brightly colored flowers better at attracting bees – a win-win situation – is indeed an attractive one.

Professor Lars Chittka’s Solution

Research by Professor Lars Chittka, of Queen Mary University, London, has shown us that an attractive hypothesis is not necessarily a correct one. By mapping the spectral sensitivities of the photo receptors of different insects onto a phylogenetic tree of arthropods, he has proved that the spectral sensitivities of arthropod lineages that diverged from those of bees even before flowers had evolved are very similar to to those of bees.. In other words, bees had evolved trichromatic color vision even before flowers evolved. Thus flowers seem to have evolved to match the spectral sensitivities of bees.

But why did bees acquire color vision in the absence of colored flowers is still a mystery to be solved ?

Reference article: How to design experiments in Animal Behaviour? – Resonance, October 2018 issue

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Akshat Mishrahttps://www.scilynk.in
Akshat Mishra is a Physics Undergraduate at Centre for Excellence in Basic Sciences, Mumbai. He feels the need to explore the depths of the not-so-dark universe while at the same time watch the quanta in action. Electronic Music is what puts him in the thinking zone.

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