With so many predators gunning for rabbits, the peaceful herbivores need mechanisms to survive. The skeletal muscles of rabbits include fast-twitch and slow-twitch muscle fibers. Fast-twitch fibers, as the name implies, are used for fast reactions such as escaping a hungry, running fox, and usually fatigue quickly, depending less on aerobic respiration for cellular energy. Slow-twitch muscles are for more long lasting exercise such as a sustained chase, contract slower than fast-twitch skeletal muscles, and usually have more mitochondria to meet the high oxidation demands for making ATP (cellular energy source). In some cases, muscle fibers can switch from slow to fast-twitch (or the reverse), depending on the frequency, duration, intensity, and type of exercise exposure.