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Summary: The sex of red-eared slider turtles can be determined by looking at the length of their nails and their chest cavities. Look for long claws and chest indentations in male turtles with tips from a reptile specialist in this free video on turtle care.
"Hi, I'm James Dix from Reptile Rescue Service in Salt Lake City, Utah, and we're going to show you how to determine the sex between red eared sliders. This is a real common aquatic turtle in the pet stores, most people have for pets. The one on my left here is a male, and if you look up close, you can see he has very long nails. If you can look right there and see that. Very long claws. The female is larger than him, and she will get much larger than that, but she has very short ones. If you can see right there on her foot, they're very short little claws. The male, once again, has very long ones, as you can see right here. Almost three times the length of a female. Okay, on this species here, an African spur-thighed tortoise, or a Secada tortoise, this is a female. The males have a big indentation cavity in the chest here, and that's to help 'em when they get on top of the female from not sliding off, or he would just slide right off. Another sign is, the wide opening back here, 'cause she has to drop eggs. If this was narrow, she wouldn't be able to drop eggs, and then of course, the short tail is a sign of a female."
eHow Article: How to Determine the Sex of a Turtle