Competitive Swimmers

Experts have long thought, but haven't been able to prove that the fastest sperm have the edge when it comes time to fertilize an egg. But a group of researchers from Uppsala University were able to demonstrate that the females of the cichlid fish species can affect the speed of male sperm when they are unfaithful. It seems that when there are sperm from different sources, competition ensues and larger faster sperm are the result. These findings have been published in the scientific journal known as Proceedings of the National Academy of Sciences (PNAS). To sum up: it seems that size does count.

Evolutionary Force

Uppsala University researcher Niclas Kolm says that the competition that occurs among the sperm of various men as they try to fertilize the female egg serves as an important evolutionary force in influencing the characteristics of the sperm, for instance speed and size. Kolm's study was a collaborative effort including scientists from many universities. The team studied the mating habits of 29 types of Tanganyika cichlids. "For the first time, we can show a strong link between the degree of sperm competition and the size and speed of the sperms. Males with promiscuous females develop faster and larger sperms than the monogamous species," says Niclas.

Niclas says his study was unique in that the researchers used a very large research base of fish from several species. The fish came from African lakes. Kolm says this type of fish is unusual in that there are numerous species, each with its own set of mating methods and behaviors. Some species have monogamous males while others may have females that mate with a large number of males.

Size Counts!

Kolm's research team was able to prove a correlation between the size and the speed of sperm. Larger sperm were found to be faster. One reason for this is the increased size of the sperm flagellum, but faster sperm also rely on larger stores of energy. These large energy stores may, in turn, generate larger sperm.

The scientists on this project used novel methods for analysis through which they were able to show the order of sperm development. The study found that the sperm become faster first, and then they become larger. All of this follows in the wake of increasing female promiscuity within a species. Kolm says that until this time, no one had been able to determine the order of what happens in this mating system. This team showed a clear link between female promiscuity and sperm characteristics. 

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