On a female turtle, the tail serves no purpose, but for male turtles, the tail houses the reproductive organs. Unlike with male turtles, the cloaca, which handles urinary, fecal and reproductive functions, is located under the tail and not housed inside the tail for females.
Due to the tail on male turtles housing reproductive organs, these tails are much longer than the tails on females. When a male turtle reaches maturity, it goes through a growth spurt. During this phase, the size of the tail grows drastically and eventually extends past its back flippers. The tails on female turtles never go through such a growth spurt. As a result, one of the quickest ways to tell the sex of a turtle is to examine the size of its tail.
Although female turtles do not have a use for their tails, scientists assume female turtles still have tails for the same reason human males have nipples. No matter the sex of a human baby by the end of the pregnancy, all babies start off as females in the womb. As a result, all humans are born with nipples despite the fact males have no need for them. Scientists believe that, like humans, the gender of a turtle might not be decided until later in the incubation period.