Many tortoises hibernate each year when the weather gets cold and the plants they eat are in very short supply or no longer available. A tortoise that is about to hibernate needs to have built up enough fat reserves in order to survive through winter until the spring. For a specific tortoise species, the chart shows the arithmetic mean ratio of carapace (upper shell) length to body weight prior to hibernation, as well as the minimum length-to-weight ratio that the tortoise needs in order to ensure the likelihood of surviving until spring.