Model offers fish eyes' view of colour

By Mark Kinver
Science and environment reporter, BBC News

Image caption,
During the breeding season, male sticklebacks' colouration becomes more vivid

A maths model that mimics how a fish sees colour offers an insight into how markings help females choose mates.

A team of scientists say sticklebacks' eyes are sensitive to ultraviolet light, invisible to humans, that is reflected by marking on males.

The model provides clues how more colourful markings, produced by pigments known as carotenoids, help females select the most suitable mate.

The findings have been published in the journal Functional Ecology.

"Females typically use carotenoid colours to assess the quality of a potential mate, with more colourful males generally being regarded as the most attractive," explained lead author Tom Pike from the University of Exeter.

Carotenoids are naturally occuring yellow, red and orange pigments that colour plants and animals. For example, flamingos' pink features, blue tits' yellow breast feathers and carrots' orange flesh.

However, the vast majority of animals are unable to produce their own carotenoids, so they consume them in their diet.

Eye of the beholder

Writing in the British Ecological Society journal, the team observed: "During the breeding season, male three-spined sticklebacks (Gasterosteus aculeatus) develop a region of intense, carotenoid-based colouration that is used by females when deciding a mate and during male-male competition."

However, they added: "The [colours] appear red and yellow to human observers, but also have an ultraviolet component to which humans are not sensitive."

The researchers said that previous studies had used photographic techniques and limited analyses to the human-visible spectrum, so did not take into account the difference between human and stickleback eyes.

"The major difference between stickleback vision and our own is that they can see ultraviolet light, which is invisible to humans," Dr Pike said.

"This may be important because carotenoids reflect ultraviolet light as well as the reds, oranges and yellows that we can see."

However, the reason why females were attracted to more colourful males was based on more practical reasons than just good looks.

"Variation in carotenoid concentration may indicate individual variation in the ability to assimilate (or in the wild, locate) dietary carotenoids, and so may provide honest information on foraging ability," the team suggested.

They concluded: "We demonstrate that the visual system of sticklebacks is acutely sensitive to variations in both the total concentration of carotenoids in the male's nuptial signal and the relative proportion of its constituents.

"This may allow sticklebacks to accurately assess male quality and thereby inform mate choice and intrasexual competition decisions."

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