Now scientists have an idea why.
Eggs are expensive; sperm is cheap. That’s just the way it is in the animal kingdom. Eggs need to be large enough to hold all of the machinery and nutrients required for a newly fertilized cell to grow. For males, on the other hand, producing small sperm enables them to make a lot of it, thus boosting their chances of fathering offspring.
But rules in nature are often broken. For example, rather than making lots of tiny sperm, some males produce a few gigantic sperm. "The record holder is Drosophila bifurca: Although this fruit fly is only a few millimeters long, its sperm reach an impressive length of almost six centimeters," said evolutionary biologist Stefan Luepold from the University of Zurich, in a press release.
A study, led by Luepold and published in the journal Nature, now provides the first explanation for how such giant sperm might have evolved.
The researchers found that when male fruit flies produce only a few sperm, females have to mate more often to ensure that their eggs get fertilized. However, having highly promiscuous females poses a problem for males because it pits each male’s sperm against each other’s. And this is where longer sperm can come in handy.
As Luepold explains, in fruit flies, “longer sperm are really good at displacing their competitors from the female reproductive tract, which gives them an advantage in the competition for fertilization. Sexual selection thus favors longer sperm.”
Female fruit flies were also found to prefer mating with males who produced longer sperm. The vast amount of energy that goes into manufacturing sperm that is 20 times longer than the male itself is a good indicator to a female that she is getting a strong, healthy mate — traits that any female would want to pass onto her offspring.
Both sperm competition and female preference appear to have promoted the evolution of giant sperm in these fruit flies, who hold the title for the longest sperm cells of any organism in the world.
Such long sperm may seem cumbersome for the females on the receiving end. But this is not an issue for male fruit flies, who have devised a clever delivery system in which the massive sperm is transferred to females as a tightly coiled package.
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