The function of a frogs fat bodies is to assist them in producing energy so that they can survive and function during hibernation. Frogs cannot regulate their own bodies temperatures, as this is determined by how hot or cold their environment is. Without fat bodies, they wouldn't be able to survive the several months of hibernation they endure during cold seasons.
The fat bodies of frogs are also responsible for helping frogs float easily in water, as fats do not sink. The reason as to why fat floats in water is because the molecular structure of fat is not as dense as water. The fat body its self can be found behind the frogs intestines and is a strange-looking yellow substance that is not difficult to locate and identify.