A tiny fruit fly has the bizarre distinction of possessing the longest sperm of any animal — 20 times the length of its own body and 1,000 times that of human sperm. Not longer in proportion to its body size, just longer. Period. It is a freakish extreme, but as a general rule, scientists have long known that the smallest animals have the biggest sperm, and vice versa. In a study published Wednesday, researchers said 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 micrometres long — and that of an elephant (56 micrometres) over 200 billion sperm.