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Fish Adjusts Its Shape to Lure Hungry Females

By NICHOLAS BAKALAR
Published: July 16, 2012

The male of a small freshwater fish, the swordtail characin, tempts females with an ornamental lure that looks like food. Females react by biting at the stalk that has an enlarged tip and extends from the male fish’s body, positioning themselves in a way that allows the male to transfer sperm. Now researchers have found that the shape of the lure evolves depending on what kind of food is available.

             @characin: any small carnivorous freshwater cyprinoid fish

The variations in shape are evident in different populations of the fish in its native Trinidad, but the researchers tested their theory with captive fish. They raised females on ants, and then presented them with ornaments of males with antlike lures and males with lures that looked like beetles.

The experiment, published online last week in Current Biology, found that even after a food habituation period of only 10 days, the ant-eating females were more attracted to the antlike male ornaments.

In other words, the style of communication between these fish evolves in response to local environmental conditions — in this case, the male flag ornament develops to mimic the most commonly available food, making males with antlike lures most likely to reproduce.

In nature, two conditions have to be met for this “signal divergence” to happen: the fish have to live in areas where the food supply varies by location and is consistent over time, and contact between different populations must be limited. Both conditions are met in the streams where the charicins live.

“The main finding is that variation in the food that the females eat can lead to variation in male ornament that can in turn lead to the formation of new species,” said the lead author, Niclas Kolm, an evolutionary biologist at the University of Uppsala in Sweden. “This is the first time so many pieces of the puzzle have been put together.”

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