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Fish Swap Dinner for Sex

Unendowed males lure females with treats

The promise of a nice dinner might not always win over a woman, but for some male fish a tasty-looking lure seems to get the girl pretty reliably. The trick is to make sure the offering resembles the local cuisine, and then they can reel in the ladies, hook, line and sinker.

Swordtail characins (Corynopoma riisei) that live in the rivers of Trinidad feast mostly on hapless bugs that plop into the water from surrounding vegetation. In areas where streams flow mostly through forests, the characins' main fare is arboreal ants.

Characins are unusual in the fish world in that they rely on internal fertilization. For male characins, however, size is beside the point: they do not even have a penetrating organ. Still, they need to do their thing by somehow getting their genetic goods inside the female. How do they do it? The evolutionary answer turns out to be a fishing line and lure. Over the eons the male characins have developed a thin cord that extends from their gill area, on the end of which is an ornament of sorts. When a female bites onto this piece of flesh, she is in close enough range and a good position for the male to do the deed.

Yet how important is the appearance of a male's lure? A team of researchers, led by Niclas Kolm of Uppsala University in Sweden, found that ant-fed females were much more likely to get lured in by the male with the antlike ornament.

Does the female understand the game, or does she think she is just catching a meal and then is surprised to wind up with a mate? The researchers, who published their study online in July in Current Biology, admit that their findings “blur the distinction between female food preferences and female mate preferences.” But characin dinner-dating strategies seem to work for both him and her.

These male fish are not the only ones that seem to use the promise of food to find a female, Kolm and his colleagues point out. Male orchid bees bathe themselves in the scent of flowers that females frequent for nectar. And male water mites have been documented vibrating their legs at a frequency similar to the vibrations made by the small copepods that the females eat. Food as a way to, um, you know, is perhaps even more entwined evolutionarily than we realize.

Adapted from Observations at blogs.ScientificAmerican.com/observations

COMMENT AT ScientificAmerican.com/sep2012

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