The animal with the biggest sperm is the Drosophila bifurca, a species of fruit fly.
While "biggest" can refer to various aspects (size, volume, etc.), in this context, it refers to the length of the sperm cell. Male Drosophila bifurca produce sperm cells that are a staggering 5.8 cm (2.3 inches) long when uncoiled. This is over twenty times the length of the fruit fly's body itself. The sperm is mostly tail and is delivered to the female in a tangled, coiled form.
Why such long sperm? The exact reasons are still being researched, but it's believed to be related to sperm competition. Females often mate with multiple males, so the sperm from different males compete to fertilize the eggs. Longer sperm may have an advantage in navigating the female's reproductive tract or in displacing the sperm of previous mates.
In summary, although other animals may have larger sperm volume, Drosophila bifurca holds the record for the animal with the longest sperm.
