The body temperatures of some animals (especially aquatic animals) are coupled to environmental temperature. The body temperature of a fish in a pond, for example, is the same as the water temperature, which might range from 4°C in winter to 24°C in summer. If we bring that fish into the laboratory in the summer and measure the rates of any of its physiological or biochemical processes such as oxygen consumption, we will demonstrate a Q₁₀ relationship. On the basis of the Q₁₀ relationship, we would predict that the fish’s oxygen consumption—a measure of its metabolic rate—would be lower in the winter pond because the water temperature is lower. However, if we bring that fish back into the laboratory in the winter and measure its metabolic rate at winter pond temperature, we find that rate to be higher than we predicted. The fish’s biochemistry and physiology will have acclimatized to the seasonal change in water temperature so that it can remain active at winter temperatures. What could be the mechanism of this acclimatization? Many organisms express multiple forms of enzymes (isozymes) that have different temperature optima. In winter, organisms express the forms that show maximum substrate affinity at low temperatures, and in summer they express the forms that have maximum substrate affinity at high temperature. Another mechanism of temperature acclimatization is a change in the composition of cell membranes designed to maintain optimum fluidity despite changes in temperature (see Key Concept 6.1). Acclimatization in animals essentially means that their metabolic functions are less sensitive to long-term changes in their environment than to short-term changes.