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New research indicates that the size of the body is the reason why mice's sperm is longer than elephants'. There is an inverse proportion between the body size and the length, according to ameripublications.

Strangely, when multiple males mate with the same female , the sperm competition becomes important for evolution of sperm size, says the University of Zurich .

The longer sperm is always more competitive, especially in smaller rodents. Hence, in mice and rats, the sperm size is twice as long as those of the larger carnivorous animals, including whales.

To solve the mystery of rodents' unusually large sperm, a group of researchers compared the impact of sperm competition on its evolution, taking note of the habits of 100 mammalians. Checking the size and quantity in ejaculation, they found that the longer each sperm is, the less number is produced.

New meta-analytical methods enabled the researchers to fight the challenges of sperm competition in mammals such as rodents. They found that such mammals need to put in more investment in their ejaculates, as compared to monogamous, larger animals. The size of the body tended to affect the length as well as the quantity of sperm.

In large animals there is a huge selection pressure on the investments in ejaculates. Hence, the quantity of the sperm becomes much more important than size due to a more "voluminous female reproductive tract", in which the sperm could get lost or diluted. On the other hand, smaller species show a sperm length such that its journey travelled to reach the egg is much less.

"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 study was published in a recent edition of the journal Proceedings of the Royal Society of London.