If the enzyme, or the cell it is in, is too cold , collisions between enzyme and substrate occur infrequently, and metabolic processes slow to rates which may no longer support life. If the enzyme, or the cell it is in, is too hot, the sensitive proteins denature and their function is lost, usually permanently. Without functioning enzymes, life is not possible. For these reasons, animals have evolved with a variety of ways to maintain their body core temperature within functioninal tolerance limits . They need to gain heat from the environment when they become cool, and they need to lose heat to the environment when they become too warm.
Ectotherms rely on specific behaviours to regulate their heat gain and heat loss.
But in the heat of the day, when temperatures in the sun may be high, the same snake would be found sheltering under plants or rocks, and the frog would be in cool water. In general, these behavioural mechanisms for temperature regulation are not always available to an ectotherm, so it is fortunate that these animals have a wider range of temperature tolerance limits than most endotherms. The Table below summarises these responses. Endotherm's responses to temperature variation. Functional response Internal temp.