If had to guess what animal had the longest sperm, would you think of the fruit fly?

The scientific community has always known that smaller animals have bigger sperm, and vice versa.

A study published today indicates not only the sperm length, but also how many they have, are determined by an animal's size.

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The Drosophila bifurca (left) fruit fly's sperm, although tightly coiled, was discovered to be 5.8 centimetres in length (right), over 20 times the length of its body.

FACTS ABOUT FRUIT FLY SPERM Flies develop huge testes that make up almost 11 percent of its body weight in order to produce the giant sperm. Producing such sexual organs consumes so much energy that it delays the male insects' sexual maturity. The insects have about 800 sperm in their reproductive tracts at one time. Each sperm looks like a rolled-up ball of twine and is transferred to a female as a large mass that slowly loosens. The fly sperm is sluggish and relying more on some transport mechanism within the female to move toward its goal. Advertisement

A mouse ejaculation contains about 9.5 million sperm - each about 124 micrometers long - and that of an elephant (56 micrometers) over 200 billion sperm, they said.

Researchers from the University of Manchester and the University of Zurich recorded species-specific values of sperm length, total number of sperm in ejaculates, combined testes mass and male and female body masses of 1000 mammalian species.

Only sperm counts from samples that were obtained by natural ejaculates were used in the study.

In some cases, the total sperm number was not provided but it could be calculated as the product of ejaculate volume and sperm concentration, as reported in the study.

In species where there is no monogamy, sperm have to compete in order to fertilize an egg.

In small animals, larger sperm seem to have an evolutionary advantage, while in big animals it is smaller sperm in higher numbers which prevail.

The length of a sperm varies from about 30 micrometers, as in some whales, to about 350 micrometers for the honey possum.

The Drosophila bifurca fruit fly's sperm, although tightly coiled, was discovered to be 5.8 centimeters.

Its sperm 20 times the length of its body and 1,000 times that of human sperm.

A new study says not only sperm length, but also numbers, are determined by an animal's size. A mouse ejaculation contains about 9.5 million sperm—each about 124 micrometers long—and that of an elephant (56 micrometers) over 200 billion sperm

The team speculated that the trade-off between sperm size and numbers might have something to do with the size of female plumbing.

'Since elephants are bigger than mice, it seems that their sperm have a higher risk of being diluted or lost in the bigger female reproductive tract,' study co-author Stefan Luepold of the University of Zurich said.

'In other words, sperm number becomes far more important than sperm size'.