Energy Budgets, Endotherms, and Ectotherms

Every organism takes in a limited amount of energy from food, and that energy has to cover everything the body needs: staying alive, building new tissue, and making offspring. This is the energy budget. Maintenance comes first — basic survival costs like repairing cells, running organs, and moving around — and whatever is left over gets split between growth and reproduction. How much is left over depends heavily on how the organism handles one huge expense: temperature.

Endotherms, like mice and other mammals and birds, generate most of their own body heat through metabolism, burning energy constantly to hold their internal temperature steady regardless of the surroundings. That internal furnace is expensive to run, so a huge slice of an endotherm's energy budget goes straight to maintenance, which is why a mouse eats almost its own body weight in food every week. Ectotherms, like snakes and lizards, do not generate meaningful metabolic heat. They get warm by basking in the sun or cool off by retreating to shade, so their body temperature rises and falls with the environment. That does not mean their blood is actually cold; a lizard sunning on a rock can be warmer than the air around it. Because ectotherms skip the cost of internal heating, their maintenance costs are far lower, leaving more energy free for growth and reproduction relative to their size.

This budget connects directly to population size and survival strategy. When food intake drops, maintenance still gets paid first since a dead organism cannot reproduce, so growth and especially reproduction get cut back or shut down entirely. That tradeoff is why energy availability limits how many offspring a population can produce, linking individual energy budgets to the broader question of what caps population growth in an ecosystem.