Galápagos tortoise are the giants of the tortoise world. Males can weigh more than 500 pounds, and females average about 250 pounds. They have thick, sturdy legs to hold up all that weight, but they still spend a lot of time lying down to conserve energy. There are two types of Galapagos tortoises: domed, with big, round shells; and saddlebacks, with shells that curve up in front like a saddle. The shells are not solid; instead, they are made up of honeycomb structures that enclose small air chambers. This makes it possible for the tortoise to carry the weight. A tortoise cannot "walk out" of its shell, as you may see in cartoons—its ribs are attached to the shell, and its lungs are located on the top, right under the shell.