The mating game. Individuals of many species, including insects and humans, evaluate potential mates using gifts.

Worthless Gifts Get the Chicks

Males seeking a mate often give gifts. Whether it's a guy with a bouquet of roses or a fly offering a tasty dead insect, the message is the same: the better the gift, the better the guy. But how can a sincere male protect himself from a gold-digging female who takes the goods and runs? A new model suggests the best solution is for males to give gifts that are expensive for them, but worthless to the female.

Gifts can either help the receiver or convey information. Giving food to someone who is hungry is a helping gift; giving a fruit and cheese basket to your child's teacher is a sign of appreciation. When males are on the prowl, they give information gifts, which are designed to show either the male's quality or his intentions to stay with the female and provide for her offspring.

In this week's online issue of the Proceedings of the Royal Society of London, Peter Sozou, a theoretical biologist, and Robert Seymour, a mathematician, both at University College London, demonstrate a solution to the sincerity problem. Borrowing a technique from economics, they set up a game model in which males could offer three kinds of gifts: a valuable gift that was expensive for the male and useful to the female, a cheap gift worth nothing to either, and an extravagant gift that was expensive for the male but worthless to the female.

The model judged strategies as successful if they produced the same results over time. According to this standard, the best strategy is for a male to selectively offer extravagant gifts. The gifts' expense demonstrates the male's sincerity, but their material worthlessness means insincere females won't take them. The finding explains animal antics like those of the male empid fly, which sometimes offers females elaborate silk packages that contain nothing of value.

"It's the wrapping that counts," says Mike Ritchie, an evolutionary biologist at St. Andrews University in Fife, U.K. "The female is getting valuable information," he says, while the males avoid "gold diggers." Humans do the same thing, say the researchers. A man offering a pretty rose bouquet is playing the same game as an empid fly offering an empty silk balloon.

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