It is often difficult to determine the sex of a pet turtle, especially if you did not purchase it from a breeder that hatched them from controlled temperature environments. 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 size and appearance between the sexes.
Using Shell Size and Shape to Sex Turtles
The size differences between most male and female turtles 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 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 its 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. These shapes enable male turtles to more easily mount a female during mating, and it give females more room to hold eggs internally.
Using Claw Length to Sex Turtles
Female turtles often have claws on their front feet that are short, stubby, and the same length as their toes. Males (and specifically red-eared sliders and other aquatic turtles) have much longer claws on their front feet than 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 its tail. Female turtles have short and skinny tails while males sport long, thick tails, with their vent (cloaca) positioned closer to the end of the tail when compared to a female. It is, of course, easiest to sex a turtle when looking at its tail length if you have multiple turtles of both sexes to compare.
Using Markings and Coloration to Sex Turtles
All red-eared sliders have predominantly green bodies suffused with bright yellow streaking which won't help distinguish males from females. But there are other color indicators that may. 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 it is pregnant. This change can be used as another indicator that a turtle is a female, but until and unless it lays eggs, appetite shouldn't be used as the sole method for determining its gender.
Edited by Adrienne Kruzer, RVT