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turtles and the pig-nosed turtle are the most specialized for swimming. Their front limbs have evolved into flippers while the shorter hind limbs are shaped more like rudders. The front limbs provide most of the thrust for swimming, while the hind limbs serve as stabilizers. Sea turtles such as the green sea turtle rotate the front limb flippers like a bird's wings to generate a propulsive force on both the upstroke and on the downstroke. This is in contrast to similar-sized freshwater turtles (measurements having been made on young animals in each case) such as the Caspian turtle, which uses the front limbs like the oars of a rowing boat, creating substantial negative thrust on the recovery stroke in each cycle. In addition, the streamlining of the marine turtles reduces drag. As a result, marine turtles produce a propulsive force twice as large, and swim six times as fast, as freshwater turtles. The swimming efficiency of young marine tur tles is similar to that of fast-swimming fish of open water, like mackerel. Compared to other reptiles, turtles tend to have reduced tails, but these vary in both length and thickness among species and between sexes. They are especially large in snapping turtles and the big-headed turtle. The latter uses it for balance while climbing. The cloaca is at the base of the tail, and the tail itself houses the reproductive organs. Hence, males have longer tails to accommodate the penis. In sea turtles, the tail is longer and also somewhat prehensile and males use it to grasp females when mating. Several turtle species have spines on their
