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Typically, a male turtle will be more colorful and more outgoing than a female.   Males have a flat or concave plastron (bottom shell), and females have a flat or convex plastron, which allows more space for eggs. Females will also almost always be larger.  Among water turtles, size is also a good indicator:  males are smaller than their female counterparts.  Also, male sliders have long claws on their front legs. When it comes to box turtles, males generally have bigger, fatter tails, and a vent (cloaca) that's further from the shell than the female's. Turtles are not able to breed until they've reached sexual maturity.  When it comes to water turtles, males should be at least three years old, and females need to be about five years old.  Neither male nor female box turtles will breed before they're at least five years old. Don't plan to breed a recently acquired turtle.  Wait at least a year. To increase the chances for successful breeding, it's suggested that you cool your turtles.  Nesting season can run from March through June, so the cooling period actually needs to take place January into February for water turtles and December through February for box turtles.  Keep turtles at a temperature between 50 and 60 degrees Fahrenheit for six to eight weeks for water turtles and eight to 12 weeks for box turtles. Leave the turtles alone during this period.  You can make food available, but they'll eat little or nothing at all. If your turtles live in an outdoor pond, you can take advantage of the natural cooling that wintertime offers and allow them to hibernate. After the cooling period has ended, return your turtles' habitat to its regular temperature. It's particularly important that your turtles eat well during the breeding season.  In addition to their regular diet, be sure the females get enough calcium and Vitamin D3.  A healthy diet for a water turtle will contain some combination of:  earthworms, snails, well-washed butter lettuce, melon, bananas, strawberries, blueberries, vegetable scraps, peas, tomatoes, cooked sweet potatoes, dandelion flowers and leaves and mulberry leaves. A box turtle has the same basic diet as a water turtle, but the menu could also include  whole feeder goldfish, crickets, kale, boiled egg, corn on the cob, chopped and steamed broccoli and lots of green leafy vegetables. You can meet your turtles' need for calcium but putting a Cuttle fish bone to be nibbled on in the their habitat, or you can feed them calcium supplements. Turtles that are kept outdoors don't need additional Vitamin D3; they make plenty of their own.  But indoor turtles need exposure to a full spectrum reptile light or a D3 supplement.
Be sure you have a male and a female turtle. Be sure your turtles are sexually mature. Cool your turtles. Feed your turtles well.