Enzymes have an optimum temperature: the temperature at which they work best. Not all animals have a body temperature of 37 °C; this temperature is a characteristic of humans, while other mammals, birds, and cold-blooded animals have different body temperatures. For example, dogs have a higher normal temperature (around (38.5 °C), birds are warmer (around (41 °C), and cold-blooded animals' temperatures vary based on their environment.