Physiological adaptations include colour. Dark colours absorb more radiation. The Namaqua chamelion is black in the morning but pale grey during the hotter day.Ectotherms have much lower metabolic rates than endotherms and the metabolic rate rises with the ambient temperature since the rate of enzyme action is directly related to temperature.The American wood frog (Rana sylvatica) toes start to freeze below 0°C. Within 10-15 minutes of freezing, glycogen stored in the liver is converted into glucose which is released into the bloodstream and moves into the tissue fluid. The glucose acts as an anti-freeze, lowering the freezing point of blood and tissue fluid and preventing the formation of ice crystals in the cells.