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Here's Why Fruit Flies Have Giant Sperm

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Scientists found that a type of male fruit fly has giant sperm. The reason and the implications of such trait were explained in a new study.

In animals, sperm are typically smaller than the eggs of females. This means that males can produce far more sperm than females can generate eggs.

The bigger the sperm, the greater the chance to fertilize an egg, particularly if the female has mated with several males. The competition among sperm rises as the female mates with more and more males, simply because the number of sperm candidates increases. In this competition, the female reproductive tract favors the best-quality sperm.

Quality, Not Quantity

Looking at how fertilization occurs, it looks as if the more sperm a male produces, the more likely he is to win. This is why experts were surprised to find that males of some species generate fewer but very large sperm. In particular, Drosophila bifurca (D. bifurca) was found to be only a few millimeters long yet its sperm measure nearly 6 centimeters (2.36 inches) in length.

The case of D. bifurca poses a contradiction to the typical male selection concept that more sperm means more chances for fertilization. This is because having bigger sperm means less sperm can be accommodated, thus relaxing the competition.

Effects Of Sperm Size To Sexual Selection

In the new study, experts found that sexual selection has a big effect on the length of sperm. The authors presented the explanation as to why sperm develop into a gigantic size. They used quantitative genetic, comparative and experimental data to study the sexual traits of different Drosophila species.

Findings of the study showed that at some point, the female reproductive system prefers longer sperm. However, the longer the sperm, the fewer the sperm that can be transferred and reproduced. As a result, females have to mate more frequently to make sure that eggs get fertilized.

Each mating produces a chance for sexual selection via sperm competition. In the case of fruit flies, longer sperm are good at outdoing competitors.

"Sexual selection thus favors longer sperm," says first author Stefan Lüpold.

What Females Favor

The team also found that female preference before mating is influenced by the length of sperm. Since small males also have low sperm production rates, they just use their reserved sperm during chances for copulation. The large males, on the other hand, are able to produce more sperm, despite the increased energy expenditure needed for their longer sperm. This gives the larger males better abilities to engage in more frequent mating - a character most favored by females.

More male fruit flies are expected to have giant sperm because more offspring are being born to fathers with longer sperm. Such trend may heat up sexual selection via sperm competition.

The study implies that sexual selection research must be broadened. The changes in sperm are derived from similar mechanisms and other male sexual features such as the use of horns to get rid of competitors and use of ornaments to appeal to females.

Lüpold thinks, however, that the sperm of the fruit fly are possibly the most extreme example among animals. For D. bifurca, in particular, sperm can reach up to 20 times longer than the body itself, and are transferred as coiled balls.

The study was published in the journal Nature on May 26.

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