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 charachin (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 charachin’s main fare is aboreal ants.
For male charachins, size is besides the point: they don't even have an external sex organ, but they still need to do their thing by somehow fertilizing females internally. So how do you get a female fish to sidle on over so you can slip her your genetic goods? The evolutionary answer turns out to be a fishing line and lure. Over the eons, the male charachins 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's in close-enough range and a good position for the male to do the deed.
But how important is the appearance of a male's lure? A team of researchers, led by Niclas Kolm, of Uppsala University, tested just how well targeted the male's ornaments were. The lab tested captive-raised females that had never encountered ants. One group of adult females started on an ant-filled diet, and the other group continued to eat other food. After just 10 days of these diets, the females then met two ornament-bearing males—one that came from an ant-eating community and another that came from an area where ants made up a small amount of the diet (and thus had a decidedly un-ant-like appendage). Ant-fed females were much more likely to get lured in by the male with the ant-like ornament, suggesting that it was "more efficient at attracting the attention of females that are habituated to eating ants," the researchers wrote in their study, which was published online July 12 in Current Biology.
"This is a natural example of a fishing lure designed to maximize the chance to catch a fish," Kolm said in a prepared statement. "In this case, it is not just any fish, however—it is a fish of the opposite sex."
Does the female understand the game: does she think she's just catching a meal and then is surprised to wind up with a mate? The researchers admit that their findings "blur the distinction between female food preferences and female mate preferences." But charachin dinner dating strategies seem to work for both him and her. As an adaptive function, males with an effective food-like lure will likely pass along the enabling genes for any male offspring to reel her in, while also increasing the odds that the hooked female's genes will get passed on to the next generation.
These male fish are not the only ones that seem to use the promise of food to find a female, Kolm and his colleagues pointed 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 certain frequency similar to small copepods that the females eat. Food as a way to, um, you know, is perhaps even more entwined evolutionarily than we realized.