for National Geographic News
Ladies: If you're looking for diamonds on Valentine's Day, be careful that your beau isn't giving out fakes. Scientists have found that, even in the animal kingdom, males hand out so-called nuptial gifts that seem to be worthless.
Nuptial gift giving is a well-known phenomenon. Gifts frequently consist of food like nutritious prey items or dried insect fragments. The female typically eats her gift while copulation takes place.
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"Insects and spiders are well known for giving presents," said Steven Heydon, senior museum scientist and collections manager at the Bohart Museum of Entomology at the University of California, Davis. "Some species always give a gift, some never give a gift. Some males just give a drop of saliva; it's basically just a little treat to keep her busy while mating goes on."
However, scientists have also observed that among some species in which males bring a nuptial gift they're getting away with bringing large, highly visible, but worthless gifts like seed tufts and silk balloons.
"Males take some risk in hunting for a gift; they could be eaten by a bird or become prey themselves," Heydon said. "Also, the time they spend hunting can't be spent mating, so the evolution toward useless presents is more of an advantage to the male."
Two biologists at the University of St. Andrews in Scotland set up a series of experiments using a species of dance fly, Rhamphomyia sulcata, to try to figure out why females would require a worthless gift as a precondition to mating.
They found that the female dance flies that received the largest nutritious gifts copulated for a significantly longer amount of time than when given either a small nutritious gift or a larger worthless one. Longer copulation time would increase the male's chances that his sperm would be used to fertilize her eggs.
The females copulated for a shorter time when given a small nutritious gift. But the researchers found that worthless tokens were worth as much as small valuable gifts, at least from the male perspective. Each of these two types yielded the same amount of mating time.
Assuming the shorter copulation time still allowed the male to transfer sperm, it may be to his advantage to bring an easily obtainable worthless gift and mate more frequently, albeit for shorter amounts of time.
"These findings suggest that female behavior in genuine gift-giving species is susceptible to the invasion of male cheating on reproductive investment," Natasha R. LeBas and Leon R. Hockham conclude in a recent study published in the journal Current Biology.
Building a Scientific Experiment
Lebas and Hockham needed to make sure that the female dance flies were responding to the biggest nutritional gifts, and not some evolutionary response to mate with males who have the largest bodiesand can therefore bring the largest gifts.
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