How do turtles reproduce and how do you tell a male turtle from a female turtle?

The turtle lives ‘twixt plated decks
Which practically conceal its sex.
I think it clever of the turtle
In such a fix to be do fertile.

The poet Ogden Nash was right about the turtle’s external ambiguity and fertility. Turtles are not only enthusiastic breeders, they also have external sexual characteristics that often make it hard for creatures other than turtles to determine which is which.

The male is sometimes distinguishable by an indentation or curvature in its plastron, or lower shell, which fits over the back of the female; females have a flat or convex plastron.

To fertilize the female’s eggs, the male turtle conceals a sexual organ inside the cloaca, or waste removal chamber.

The male positions itself over the female and often grasps the upper shell, or carapace, with its claws, then curves its tail until the vent contacts the female’s vent; the penis emerges for fertilization. The often dozens of eggs develop internally and are then usually laid and buried in sandy soil.

Fertilization is sometimes preceded by elaborate courtship rituals, with hours of demonstration followed by a few minutes of copulation. The female can store sperm to fertilize its eggs, sometimes years later.