One of the most important factors that affects metabolic rate is the temperature of the organism, since within limits all chemical reactions of metabolism proceed faster at higher temperatures. The internal temperature of most invertebrate an imals, fish, and amphibians is the same as the temperature of the environment in which they live. Such organisms are called poikilotherms. In poikilothermic organisms, metabolic rate increases as the environmental temperature increases. Such organisms move slowly and grow slowly when the temperature is cold, since their metabolic rate is very low at cold temperatures. To compare the metabolic rates of different poiki-lotherms, one must measure their rate of metabolism under standard conditions. Standard metabolism is usually defined as the rate of energy use when the animal is resting quietly, twelve hours after the last meal, and is at a temperature of 30 degrees Celsius; however, for small invertebrates, protists, and bacteria, only temperature is usually controlled. Most reptiles, birds, and mammals can maintain their body temperature at a constant level even when the environmental temperature changes greatly. Such organisms are called homeo-therms. Birds and mammals can maintain their body temperature through internal heat production (endothermic homeothermy), while reptiles must acquire the necessary heat from their environment by changing their behavior, body posture, or coloration (ectothermic homeothermy). Most endotherms can maintain a constant body temperature over a range of temperatures (thermal neutral zone) without affecting their rate of metabolism. At temperatures outside the thermal neutral zone, metabolic rate increases to maintain constant body temperature. At colder temperatures, increased muscular activity and shivering require increased metabolic rate. Sweating and panting can increase the rate of metabolism at high temperatures. To compare the metabolic rates of endothermic homeotherms, scientists measure the basal metabolic rate (BMR), which is also referred to as the energy cost of living.
Body size (mass in grams or kilograms) is another major factor that affects basal or standard metabolic rate. A 3,800-kilogram elephant has a metabolic rate of about 1,340 kilocalories per hour, while a 2.5-kilogram cat has a rate of 8.5 kilo-calories per hour, which means an elephant needs about 150 times as much food as a cat each day. A
different picture emerges if one looks at energy use per kilogram of mass. For each kilogram of mass, the cat actually uses ten times the energy of a kilogram of elephant. Metabolism per unit of mass (specific metabolism) decreases as body size increases for all organisms. Since small organisms or cells have a larger surface area relative to their total volume than large ones, they can lose more heat from the surface. For two organisms of the same mass, the taller or thinner organism will have a larger surface area and higher BMR than a shorter, fatter one. More oxygen, food, and waste products can diffuse across the larger surface area; thus cell size in single-celled organisms and in different types of cells in multicellular organisms is limited by rate of energy metabolism. Small animals move faster, breathe faster, and their hearts pump faster. A mouse has a heart rate of six hundred beats per minute, while an elephant's heart rate is thirty beats per minute. Even the length of life appears to be related to the faster metabolic rate of these small creatures. Mice live only two to three years, while an elephant can live sixty years or longer.
Age and sex also influence basal metabolism. Young animals that are growing rapidly have a higher BMR than adults. As adults age, the proportion of skeletal muscle decreases and the BMR declines. Muscle tissue is metabolically very active even at rest, contributing to the higher BMR in males as opposed to females, since males have a higher proportion of body mass that is muscle. Physical or emotional stress can increase metabolic rates by increasing the catabolism of fats through the action of the hormones epinephrine and norepinephrine.
Skeletal muscle activity causes rapid short-term increases in metabolic rate. In humans, for example, a few minutes of vigorous exercise causes a twentyfold increase in the rate of metabolism, and the metabolic rate remains high for several hours. Walking, swimming, running, and flying require more energy than sitting still; however, each of these activities influences metabolic rate differently. Water is denser and has higher viscosity and resistance to movement compared to air, so more energy must be expended to swim than to walk at a given speed. Running also increases energy use, and the faster one runs, the more energy is required. Large animals, however, increase their rate of metabolism less per kilogram of mass than do small animals, so there is a metabolic advantage to large body size. Intriguingly, for the same size animal, flying is less energy-expensive than running.
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