Metabolic rates in animals
Metabolic rates in animals refer to the rate at which organisms convert food into energy, a vital process for sustaining life and facilitating growth, movement, and other activities. This metabolic activity involves various biochemical reactions, collectively known as metabolism, and is often measured by the amount of oxygen consumed by the organism. While oxygen consumption is a common metric, it does not fully capture energy conversion, as some animals can metabolize without oxygen in anaerobic conditions.
Factors influencing metabolic rates include temperature, body size, age, and sex. Poikilothermic animals, such as fish and amphibians, have metabolic rates that vary with environmental temperature, whereas homeothermic animals, like birds and mammals, maintain a constant body temperature and metabolic rate across a range of conditions. Interestingly, larger animals tend to have lower metabolic rates per kilogram of mass than smaller ones, though small animals often exhibit faster heart rates and shorter lifespans.
Metabolic rates can be assessed through various methods, including calorimetry and respirometry, which measure heat production and gas exchange, respectively. Understanding metabolic rates provides insights into an animal's dietary needs, energy adaptations, and ecological distribution, highlighting the complex interplay between physiology and environment in the animal kingdom.
Metabolic rates in animals
All living things require energy to sustain life as well as to carry out normal activities, develop, and grow. All the chemical reactions that allow energy acquisition and use are collectively called metabolism. Animals usually obtain energy by breaking down food in the presence of oxygen, so the amount of oxygen they use can be considered a measure of their rate of metabolism. Oxygen consumption is not always a good indicator of rate of energy conversion and use. Some organisms, such as fish, that live in oxygen-poor environments gain energy without using oxygen through anaerobic metabolism. Since all energy used by living things is eventually converted to heat, metabolic rate can be defined as the total amount of heat produced by an organism in a certain period.
Rates of metabolism measured on whole organisms can be used to study the effects of factors such as temperature, size, age, and sex on rates of energy use. Studies of metabolic rates of different species of organisms are used to determine food requirements and energy adaptations in different environments. Metabolic rate can also be measured on isolated tissues, cells, or cell organelles to study the different biochemical reactions that occur in tissues and cells.
Factors Affecting Metabolic Rate
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 animals, 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 poikilotherms, 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 homeotherms. 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 kilocalories 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, a cat uses ten times the energy of a kilogram of an 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.
Measuring Metabolism
Rate of metabolism can be measured as the amount of heat produced by an organism in a time period. The traditional unit of heat is the calorie; a kilocalorie is one thousand calories. The two terms are frequently confused in popular literature. In the international system of units, heat is measured in joules, and one calorie is equal to 4.184 joules.
Metabolic rate can be determined from the energy budget of an animal. If the total energy excreted in urine and feces is subtracted from the total energy in food eaten during a period of time, the result would be a measure of metabolic rate. The energy content of food and waste products can be determined by burning these materials in a calorimeter. The amount of heat produced is used to raise the temperature of a known amount of water. This method assumes that the organism is not growing or changing the amount of fat stored during the measurement period. It is also difficult to control metabolic activity of gut microorganisms. Although this technique is cumbersome, it may be the best way to assess energy metabolism in a normally active state for animals in their natural habitat.
More controlled measures of basal and standard metabolism can be made by isolating the animal in a calorimeter and directly measuring heat produced. This method is more accurate than the energy budget approach but still assumes that no new molecules are being produced and no activity or work is being performed. This technique is most useful for birds and small mammals that have relatively high rates of metabolism. Normal behavior and function may be altered by the confined conditions.
Indirect calorimetry is the most often used method in assessing metabolic rates of whole organisms, isolated cells, and cell components. Some factors related to energy use, such as oxygen consumed or carbon dioxide produced, is measured as an index of energy use. For aerobic metabolism, the amount of heat produced is related to oxygen use by the organism. Respirometry is the method used to monitor the oxygen used and carbon dioxide produced by an organism in a closed chamber. Oxygen consumption can be measured by absorbing carbon dioxide with soda lime and measuring the change in gas pressure in the closed system by a manometer. Oxygen electrodes can also be used to measure the decrease in oxygen concentration in water within the chamber if the animal is a water-dweller. For air-breathing animals, oxygen in the gas phase can be measured by a mass spectrometer. Carbon dioxide can also be measured by an infrared unit. In such closed systems, the gases can be monitored in the air as it enters and leaves the chamber. Respirometry can also be accomplished in open systems if the animals are fitted with breathing masks and the respired air is collected and analyzed. Oxygen consumption is a good index of metabolic rate for most animals at rest, since most of their metabolism is aerobic. Animals that live in oxygen-poor environments, such as internal parasites and mud-dwelling invertebrates, and all animals under extreme exercise often metabolize anaerobically.
To translate the amount of oxygen used into heat produced, one must know the proportion of fat, carbohydrate, and protein in the diet, since the amount of heat produced for each liter of oxygen consumed differs. In practice, it is usually assumed that only carbohydrates are being used.
Assimilating and digesting food causes large increases in metabolic rate. This increase reaches a maximum about three hours after a meal and remains above basal level for several hours in birds and mammals and up to several days in poikilotherms. Foods differ in the amount of increase in metabolic rate. Proteins, for example, cause about three times the increase in rate compared to carbohydrates or fats. The increase partially results from increased activity in cells of the digestive tract and partly from higher activity of liver and muscle cells preparing these foods for storage. Basal metabolism must then be measured after a twelve-hour fast to minimize this effect.
Since body temperature, body size, and activity affect metabolic rates of organisms, one can see that available food supplies, oxygen levels, and environmental temperatures limit the physiology of energy metabolism in different habitats. Scientists study metabolic rates in whole organisms to explain their food habits and their distributions in different habitats and to calculate energy requirements for raising animals under different conditions.
Principal Terms
Adenosine Triphosphate (ATP): The primary energy storage molecule in cells; links energy-producing reactions with energy-requiring reactions
Anabolism: A series of chemical reactions that builds complex molecules from simpler molecules using energy from ATP
Basal Metabolic Rate (BMR): The rate of metabolism measured when the animal is resting and has had no meals for twelve hours; used to compare different species
Catabolism: A series of chemical reactions that break down complex molecules into simple components, usually yielding energy
Electron Transport Chain: A series of electron carrier molecules found in the membrane of mitochondria; oxygen is used and ATP is made at this site
Hormone: A chemical messenger molecule within organisms; acts as a regulator of cell activities
Mitochondrion (pl. mitochondria): A cell organelle found in plants, fungi, animals, and protists; the site of most aerobic metabolism
Specific Metabolic Rate: The rate of metabolism per unit body mass (calories per gram per hour)
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