Many different kinds of turtles are cared for as pets. It is often difficult to determine what sex these turtles are, especially if you did not purchase them from a breeder that hatched them from controlled temperature environments (since the temperature during incubation is what determines whether an embryo will become a male or female, cooler incubation temperatures produce males and females develop in warmer temperatures).
Thankfully there are some types of turtles that make it easier than others to distinguish a male from a female without knowing their incubation temperature. Red eared sliders, for example, demonstrate sexual dimorphism and have distinct differences in their size and appearance between the sexes.
Using Shell Size and Shape to Sex Turtles
There are fairly distinct differences in size between most male and female turtles but these differences may not be obvious until the turtle reaches sexual maturity (and the diet can also play a role in the size of a turtle). For male red eared sliders, sexual maturity is about the time they reach four inches in length (and at about two to five years old). Females are sexually mature when they reach six to seven inches in length (which may take five to seven years). Many kinds of turtles, including even sea turtles, are sexually dimorphic due to the size difference of males and females, but in order to reliably use shell size as a factor in determining the sex of a turtle you must also know their age.
The bottom of a turtle's shell (called the plastron) is also used as an indicator for sexing turtles. Male turtles have a concave (curved in) plastron while females have a flat one. This enables male turtles to more easily mount the female and it gives females more room to hold eggs internally.
Using Claw Length to Sex Turtles
If you take a look at the claws on a turtle's front feet you'll notice that females often have claws that are short, stubby, and the same length as their toes.
Males on the other hand (and specifically red eared sliders and other aquatic turtles) have much longer claws on their front feet than those of females. This is because males utilize their claws when they are attempting to woo females to breed. During mating, the males will also grab the female's upper shells by using their claws.
Using Tails to Sex Turtles
The most common way people sex a turtle is to look at the length of their tail. Female turtles have short and skinny tails while males sport long, thick tails, with their vent (cloaca) positioned more towards the end of their tail when compared to a female. It is of course easiest to sex a turtle when looking at their tail length if you have multiple turtles of both sexes to compare it to.
Using Markings and Coloration to Sex Turtles
Both male and female red eared sliders have predominantly green bodies suffused with bright yellow streaking which won't help your quest in determining who is a male and who is a female. But there are other color indicators on them that may. The bottom shell, also called the plastron, is yellow with uneven, dark markings that are paired while the tail, legs, and head are green with thick yellow stripes.
As red eared sliders get older, many turn to a dark, almost black color and may obscure some or all of their yellow markings. This darker coloration is more common in male red eared sliders.
Ornate box turtles are another kind of turtle that are sexually dimorphic. Mature male ornate box turtles have red eyes while female eyes are brown. The males also have greenish colored heads with red or orange leg scales and females have brown heads with yellow leg scales.
Using Diet to Sex Turtles
Red eared sliders are omnivorous and they hunt and scavenge for their food. Both males and females of this species of turtle eat a variety of decaying or live organic matter such as fish, snails, crayfish, worms, crickets, insects and aquatic plants. However, the food preferences of a female will change when they are pregnant.
This change can be used as another indicator that a turtle is a female but should not be relied on solely.
Edited by Adrienne Kruzer, RVT