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.