English: Body temperature poikilotherm (green line) changes with ambient temperature, while homeotherm (blue line) regulates its own body temperature which remains constant regardless of ambient temperature (red line). All organisms must thermoregulate in some way. Poikilotherms adapt their temperature to the ambient temperature. This means that if the ambient temperature fluctuated greatly, the temperature of the organisms would also fluctuate. Sometimes organisms have alternative ways of changing temperature, such as warming up (lizards) or swimming in shallower waters (fish), but they are still dependent on the resources provided by the environment. Therefore, they are referred to as ectotherms (metabolic conformers). On the other hand, endotherms (metabolic regulators) can regulate their own body temperature and are independent of the environment, like all mammals and birds. Regardless of the ambient temperature, these organisms maintain a constant internal body temperature (they are homeothermic), which allows them to be more active at any time of the day.