The crane is good at nurturing the spirit, so it lives for a thousand years. The tortoise is good at nurturing energy, so it can survive a century without food.
What I realize as I observe this is the Tao of prolonging life. If people are able to humbly lower themselves, to be yielding, plain, and sincere, not wasting energy but always nurturing energy, then they will be full of energy. If people can be free from cogitation and rumination, have few desires and little ambition, not belabor their spirit but always maintain their spirit, then their spirit will be complete. When energy is full and the spirit is complete, the root is stable and the foundation is secure. Thus you can extend your life span, prolonging life without deterioration. The crane and the tortoise can live long, even though one only keeps its spirit complete and one only keeps its energy complete; how much the more so when both spirit and energy are kept complete – how could you fail to live long?
Awakening to the Tao Liu I-Ming translated by Thomas Cleary p. 12