Ectothermy
Ectothermy is when an animal relies on the heat around it to warm or cool its body.
A block of butter sits on a tray in the kitchen. Near the warm stove, it softens and spreads. On the cold counter, it firms back up. The butter can not make its own heat, and neither can an ectothermic animal.
Explaining ectothermy by grade level
A lizard sits on a warm rock to heat up its body. On a cold day, the lizard moves slow and its skin turns dark. The dark color soaks up more sun. The lizard needs the sun because it can not make its own heat inside.
Projects that explore ectothermy
Ants depend on outside warmth to stay active. You can see this directly by filling a glass jar with soil to make an ant observatory and watching the ants move at room temperature. After ten minutes in a refrigerator, they move much slower. As the temperature rises back up, they become active again — the same ants, responding entirely to the heat around them.
Ectothermy controls more than movement speed. It also affects how fast an animal breathes. Cooling a toad's water in ten-degree steps and counting breaths at each step reveals a direct link between outside temperature and breathing rate.
Because caterpillars rely on outside warmth to power their body's processes, warmer air accelerates their transformation. You can test this by raising Painted Lady caterpillars in three transparent tanks at 18°C, 24°C, and 30°C and watching the chrysalis stage each day. The caterpillars kept at 30°C finish the change the fastest. The group at 18°C takes the longest. Recording how many days each caterpillar takes to emerge as a butterfly shows how directly outside temperature controls the pace of metamorphosis.
Some ectotherms use color change to manage how much heat they absorb from their surroundings. Anoles are cold-blooded reptiles that cannot generate their own warmth. Lowering the temperature in their terrarium to 70 degrees triggers a visible shift in skin color, revealing how these lizards use pigment to regulate body temperature.