Why wild guppies are still orange after all this time...

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Wild guppies have had over half-a-million years to evolve, enough time for males to dramatically change their colours. Curious then, that after all this time males continue to hold onto their characteristic orange patch, and maintain it at the same hue of orange.

But researchers have now determined the reason why, and it’s simply that it is the colour that females prefer!

"Sometimes populations have to evolve just to stay the same," said Greg Grether, a UCLA professor of ecology and evolutionary biology and co-author of the study published in the online edition of the journal Proceedings of the Royal Society B: Biological Sciences.

"In this case, the males have evolved back over and over again to the colour that females prefer," said Grether, noting that there are many examples in which there is less variation among populations of a species than scientists would expect.

The orange patches consist of yellow carotenoids mostly obtained from algae, and red drosopterins which are produced by their bodies.

Lead author Kerry Deere, of UCLA, conducted experiments whereby she presented female guppies with a choice of males who had varying levels of drosopterin to see which the females preferred, the range was wider than typically found in the wild.  More than 100 mate-choice trials were conducted and the females displayed a strong preference for the males whose patch was the right hue of orange – not too red, and not too yellow.

"The females preferred the males with an intermediate drosopterin level by a highly significant margin," Deere said. (See pics below, taken by Kerry Deere, UCLA)

"Males that are closer to this preferred hue probably have more offspring," Grether said.

It might be expected that the orange coloration is dependent on the ingested carotenoids, but this doesn’t hold true, as in those locations where algae is abundant the males are genetically predisposed to producing more drosopterins in order to maintain the correct hue of orange.

"A pattern I discovered 10 years ago, which was mysterious at first, is that in locations where more carotenoids are available in their diet, guppies produce more of the drosopterins," Grether said. "There is a very strong pattern of the ratio of these two kinds of pigments staying about the same.

"To human eyes at least, as the proportion of carotenoids in the spots goes up, the spots look yellower, and as the proportion of drosopterins goes up, the spots look redder. By maintaining a very similar ratio of the two pigments across sites, the fish maintain a similar hue of orange from site to site. What is maintaining the similar pigment ratio across sites and across populations?

"The reason for the lack of variation is that genetic changes counteract environmental changes. The males have evolved differences in drosopterin production that keep the hue relatively constant across environments. As a result of Kerry's experiment, we now have good evidence that female mate choice is responsible for this pattern."

There are many cases in nature where genetic variation in a trait masks environmental variation, but there are very few examples where the cause is known.

"I originally assumed if there was variation among populations in drosopterin production, it would be the populations where carotenoid availability was lowest that were producing more of these synthetic pigments to compensate for the lack of carotenoids in their diet. But we found the opposite pattern," Grether said.

"They're not using drosopterins as a carotenoid substitute; they're matching carotenoid levels with drosopterins. Why they are doing that was a mystery. The answer appears to be that it enables them to maintain the hue that female guppies prefer."

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