Sperm are the most diverse cell type despite their homologous function across taxa of securing male fertility 3
The sperm-production gene is apparently so critical to life that it hasn't changed since every animal's common evolutionary ancestor—likely just a blob of cells—arose some 600 million years ago, the researchers conclude. 2 The gene is found in a wide range of sexually reproducing creatures, including flies and humans.
Why do Fruit Flies Have Giant Sperms?
When mating time rolls around, most male animals offer a glut of tiny sperm. Fruit flies operate a little differently. They deliver a few gigantic sperm. In fact, male Drosophila bifurca flies can produce sperm that are 5.8 centimeters long—20 times longer than their own bodies.
My comment: That means, if human sperms had the same size compared to average male height, a human sperm would be 36 meters long. ( A human sperm cell consists of a flat, disc-shaped head 5.1 µm by 3.1 µm and a tail 50 µm long 1×10−6 or 1meter has 1,000,000 ) That means, one meter contains 18.100 sperms. That means, a sperm of Drosophila bifurca is proportionally 655 thousand times longer than human sperm.
The size range alone is huge across the animal kingdom: the porcupine's 28-μm sperm contrasts dramatically with the current world record holder Drosophila bifurca, whose spermatozoal leviathans stretch to 6 cm. Maintaining the species-specific sperm size range seems to be vital: nutritional limitation that halves the resources available for males to invest in gametogenesis leads to a halving in the number of sperm produced but no reduction in the size of the sperm. Why this 2,000-fold variation in sperm size across species, when all have the same fundamental task?
These extravagant sperm are puzzling to scientists because flies have to invest a lot of energy into making them. Long sperm in males requires consequently long reproductive tract in females.