New research suggests that body size is the key to why mice have longer sperm than elephants.

Sperm competition that occurs when multiple males mate with the same female is important in the evolution of sperm size, the University of Zurich reported. Longer sperm are generally more competitive, and are mysteriously most common in small rodents. The sperm of mice and rats is about twice as long as what is seen in large carnivores and even whales.

To solve the mystery of rodents' unusually large sperm, a team of researchers compared the influence of sperm competition on the evolution of sperm in 100 mammalian species. They looked at both sperm size and number of sperm in ejaculate. The longer each individual sperm is, the less that can be produced in quantity.

New meta-analytical methods allowed the researchers to discover species battling significant sperm competition, such as rodents, must invest more in their ejaculates than more monogamous larger animals. Whether the length or the number of sperm is more important was found to heavily rely on the size of the animal.

Larger animals have a greater selection pressure on the overall investments in ejaculates, making sperm number more important than size because of a more "voluminous female reproductive tract" where sperm may become lost or diluted. In smaller species, the length the sperm must travel to reach the egg is reduced, making the sperm more likely to get lost and the size more important.

"As a result, you tend to find the most complex sperm forms in small species, not in large ones. For instance, small fruit flies have the longest sperm ever described, not whales, whose sperm are less than a tenth of a millimeter long and almost a thousand times shorter than those of the flies," the researchers stated.

The findings were published in a recent edition of the journal Proceedings of the Royal Society of London B.