How to Know a Water Turtle's Gender

The easiest way to tell the gender of a red-eared slider (Trachemys scripta) or painted turtle (Chrysemys picta) is to look at the animal's fingernails. These species feature female-choice mating systems, in which males vie for the attention of a female. The males will use their fingernails to stimulate a female's cheeks and test receptivity. Accordingly, while females have relatively short fingernails, males have extraordinarily long fingernails to better facilitate this behavior.

Though interpreting the gender of a turtle by examining its tail is not as easy for beginners as some other methods, it is the most consistent indicator across a variety of species. Most male turtles will have longer, thicker tails than females will to contain the male's phallus. Additionally, the vent or cloacal opening will be located closer to the base of the tail in females, closer to the tip of the tail in males.

Males and females have different growth rates and adult sizes in some turtle species, and this can be used to offer clues to gender. Females of many aquatic species, like red-eared sliders and soft-shell turtles (Apalone sp.), grow larger than males, so very large animals are invariably female. Unfortunately, this isn't always a helpful criteria; northern populations of stinkpot turtle (Sternotherus odoratus) feature larger males than females, while southern populations show the opposite trend. Other species, like snapping turtles (Chelydra serpentina), feature larger males than females in most cases.

Males of terrestrial species, like box turtles (Terrapene carolina) and tortoises (Geochelone sp.), generally have a concave plastron that helps during mating activities. This is not generally the case with aquatic species that breed while swimming, like sliders. There are some exceptions, though, particularly among species that are semi-aquatic, like the Blanding's turtle (Emydoidea blandingii), or those that walk along the bottom of the water, like the stinkpot turtle.