Stamina is your other lifeblood in Improbable Island, besides hitpoints.
Almost every action in the jungles and while traveling1) or flying will cost you stamina. If your stamina goes too low, you will drop of exhaustion and be thrashed to death by butterflies or feathers or Conservative Party advocates. This is most embarrassing be trim and fit for exercising well. Alternatively, if your stamina goes too high, your circulatory system will become so hyperactive that you will collapse in a fit of twitches. But there is a small possibility that you will begin to levitate from the pure energy, and will suddenly find yourself on a completely different part of the Island.
Stamina is acquired by eating.2) There are several ways to eat. You can go and buy meals in restaurants throughout the island. You can clean and cook your kill while you're in the jungle or while you're travelling. You can also buy Rocket brand energy drinks in eBoy's Trading Post. (Be sure to drink them after you've bought them.)3)
Each day you start off with 100% stamina, but if you've been packing on the cheeseburgers in NewHome, you might get fat and lose stamina. If you've been eating nothing but ration packs, you might get too skinny and lose stamina. If you've been eating nothing but brains, you're probably a zombie. If you've been eating healthy, and are trim and fit, you'll gain stamina for the day. If you've been sleeping well4) you'll gain stamina. Oh, and if you've been donating diligently along with the rest of the flock, you'll gain boatloads of stamina if Extended Play gets activated.5)6)
Some items at Sheila's may help you get additional stamina.