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Metabolic rate

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Metabolism of endotherms and ectotherms. Basal metabolic rate and standard metabolic rate. How metabolic rate varies with body size and activity level.

Key points:

  • Metabolism is inefficient and produces heat. Endotherms use metabolic heat to keep a stable body temperature, while ectotherms do not.
  • The "baseline" metabolic rate of an animal is measured as the basal metabolic rate (BMR) for an endotherm or as the standard metabolic rate (SMR) for an ectotherm.
  • Among endotherms, smaller animals tend to have higher per-gram basal metabolic rates (a "hotter" metabolism) than larger animals. The same is true among ectotherms, though we can't compare between the groups.
  • Metabolic rate varies with activity level. More active animals have a higher metabolic rate than less active animals.
  • Some animals enter a state of torpor in which their metabolism slows. Hibernation in the winter and estivation in the summer are forms of torpor.


You may be used to thinking about metabolism in terms of human eating patterns. For instance, a person who has to eat constantly to keep from losing weight may say they have a “fast metabolism,” while a person who eats only a little and still gains weight may say they have a “slow metabolism.”
However, metabolism isn't something that's unique to humans. In fact, when you get right down to it, metabolism just refers to the sum total of the biochemical reactions that take place in an organism’s body. So, every living thing has a metabolism, from a bacterium to a plant to you!
What, exactly, is the rate of an organism’s metabolism? Broadly speaking, metabolic rate refers to how quickly fuels (such as sugars) are broken down to keep the organism’s cells running. There are general differences in metabolic rate among species, and the environmental conditions and activity level of an individual organism will also affect its metabolic rate.
In this article, we’ll take a closer look at the basics of metabolism and see how metabolic rate can vary among species and depending on circumstances.

Metabolism and heat production

It's probably not news to you that animals (such as humans) need food as a source of energy. But why is this the case?
The molecules in your breakfast, lunch, or dinner have energy stored in their chemical bonds. Some of your body's metabolic reactions, like the ones that make up cellular respiration, extract this energy and capture part of it as adenosine triphosphate (ATP). This energy-carrying molecule can, in turn, be used to power other metabolic reactions that keep your cells running.
Image modified from Overview of metabolic reactions: Figure 2 by OpenStax College, Anatomy & Physiology, CC BY 4.0
Molecules from food are also used as building blocks for the structures of your body. For instance, proteins from your food are broken down into their component parts (amino acids) and may be used to build new proteins in your own cells. If you eat more than enough food to replenish the energy you use, food energy may also be stored as glycogen (a chain of linked glucose molecules) or as triglycerides (fat molecules) for later use.
The business of extracting energy from fuel molecules and using it to power cellular reactions is not a perfectly efficient process. In fact, no energy transfer can be perfectly efficient – that's a basic law of physics. Instead, each time energy changes forms, some amount of it is converted into a non-usable form. In the reactions of an animal's metabolism, much of the energy stored in fuel molecules is released as heat.
This is not necessarily a bad thing! Some animals can use (and regulate) their metabolic heat production to maintain a relatively constant body temperature. These animals, called endotherms, include mammals, such as humans, as well as birds. Ectotherms, on the other hand, are animals that don't use metabolic heat production to maintain a constant body temperature. Instead, their body temperature changes with the temperature of the environment. Lizards and snakes are examples of ectotherms.
Left panel based on data from Cannon and Nedergaardstart superscript, 1, end superscript, Figure 2, and on similar figure in Purves et al.squared Right panel based on theoretical graph from Meekcubed, Figure 1 and on Akinstart superscript, 4, end superscript, Figure 1.

Metabolic rate

The amount of energy expended by an animal over a specific period of time is called its metabolic rate. Metabolic rate may be measured in joules, calories, or kilocalories per unit time. You may also see metabolic rate given as oxygen consumed (or carbon dioxide produced) per unit time. Oxygen is used up in cellular respiration, and carbon dioxide is produced as a by-product, so both of these measurements indicate how much fuel is being burned.
In some cases, metabolic rate is given for the entire animal. In other cases, metabolic rate is given on a per-mass basis – for example, how much energy 1 start text, g, r, a, m, end text of the animal's tissues use per unit time. Per-mass metabolic rates help us make meaningful comparisons between organisms of different sizes.
The "baseline" metabolic rate of an animal is measured as the basal metabolic rate (BMR) for an endotherm or as the standard metabolic rate (SMR) for an ectotherm. Both the BMR and SMR are measures of metabolic rate in animals that are at rest, calm/unstressed, and not actively digesting food (fasting).
  • For an endotherm, the BMR is also measured when the animal is in a thermoneutral environment, that is, one where the organism does not expend extra energy (above baseline) to maintain temperature.
  • For an ectotherm, SMR will vary with temperature, so any SMR measurement is specific to the temperature at which it's taken.
Endotherms tend to have basal high metabolic rates and high energy needs, thanks to their maintenance of a constant body temperature. Ectotherms of similar size tend to have much lower standard metabolic rates and energy requirements, sometimes 10, percent or less of those of comparable endothermsstart superscript, 5, end superscript.
What about humans? Human adult males typically have a BMR of 1600 to 1800 start text, k, c, a, l, slash, d, a, y, end text, and human adult females typically have a BMR of 1300 to 1500 start text, k, c, a, l, slash, d, a, y, end text. That doesn't mean that's all the calories you should eat, though! Most people have a higher metabolic rate than this just from carrying out daily activities like standing up, walking around, and working or studying.

Energy requirements related to body size

Which one has a higher basal metabolic rate: a mouse or an elephant? If we look at the metabolic rate of the entire organism, the elephant is going to win – there is way more metabolizing tissue in an elephant than in a mouse. If we look at per-mass metabolic rate, however, the situation flips. A gram of mouse tissue metabolizes more than 10 times faster than a gram of elephant tissue!
Curiously enough, this is a very general relationship in nature. Among endotherms (animals that use body heat to maintain a constant internal temperature), the smaller the organism's mass, the higher its basal metabolic rate is likely to be. The relationship between mass and metabolic rate holds true across many species, and even follows a specific mathematical equation.
Image credit: "Animal form and function: Figure 3," by OpenStax College, Biology, CC BY 4.0. “Mouse”: modification of work by Magnus Kjaergaard; “Elephant”: modification of work by “TheLizardQueen”/Flickr.
Why is this the case? The short answer is that we don't know for sure! Part of the explanation may relate to animals' surface area-to-volume ratio and how it varies with size. Just as a small cell has more surface area relative to its volume than a large cell, so a small animal has more body surface relative to its volume of metabolizing tissue.
Since animals exchange heat with their environment across their body surfaces, small animals will tend to lose heat to a cooler environment faster than large animals. Because of this, a smaller animal would need more energy and a higher metabolic rate to maintain a constant internal temperature (in an environment below its body temperature).
However, this probably isn't the full explanation for the relationship between body mass and metabolic rate. Why not? For one thing, the metabolic rates of ectotherms also tend to scale with body mass just like those of endothermsstart superscript, 6, comma, 7, end superscript. This is difficult to explain with relation to heat retention and heat loss, since ectotherms don't maintain a body temperature different from their environment. The real cause of the relationship between metabolic rate and body mass remains an unsolved mysterystart superscript, 6, comma, 8, end superscript.

Energy requirements related to levels of activity

The basal metabolic rate (BMR) or standard metabolic rate (SMR) is a measure of an animal’s metabolic rate when it is quiet, not stressed out or excited, and not doing anything active. I don’t know about you, but most of the time, that doesn’t describe me!
The more active an animal is, the more energy must be expended to maintain that activity, and the higher its metabolic rate. For instance, the hamster running on its wheel in the picture below would have a higher metabolic rate than a similar hamster snoozing in the corner.
Image credit: Phodopus sungorus - Hamsterkraftwerk by Roland Meinecke, CC BY-SA 3.0
This is something we humans are familiar with from everyday life. For example, if you spend your day going for a long hike or playing sports with friends, you are likely to get pretty hungry (reflecting that you’ve used up a lot of energy and need more fuel). If, on the other hand, you lie in bed all day reading or watching TV, you’ll likely be less hungry because you’ve used up less energy.
For a typical animal, the average daily rate of energy consumption is much higher than the animal's BMR – by about 2 to 4 times. We humans are more sedentary (less active) than the typical animal, so we have an average daily metabolic rate of only about 1, point, 5 times our BMR.
An animal’s metabolic rate determines how much food it must consume to maintain its body at a constant mass. If an animal doesn’t eat enough food to replace the energy it uses up, it will lose body mass (as glycogen, fats, and other macromolecules are burned for fuel). On the other hand, if an animal eats more food than it needs to replace the energy it uses, there will be leftover chemical energy that is stored by the body as glycogen or fat. This is the basis of weight loss and weight gain in humans as well as other animals.

Torpor, hibernation, and estivation

Some animals respond to environmental cues by slowing down their metabolic processes and reducing their body temperature, entering what’s known as torpor. Torpor is a state of decreased activity and metabolism that allows animals to survive unfavorable conditions and/or conserve energy.
Torpor may be used over long periods. For instance, some animals go into hibernation, a state in which they slow their metabolism and maintain a reduced body temperature during the winter. Cues that cause animals to enter hibernation include drops in temperature and the shortening of daysstart superscript, 9, end superscript. The photograph below shows a Norway bat in its winter hibernation.
Image credit: Eptesicus nilssonii hibernating, by Magne Flåten, CC BY-SA 4.0
Different animals have different hibernation patterns. For instance, the abdominal temperature of a hibernating ground squirrel may drop as low as 0, °, start text, C, end text (32, °, start text, F, end text), but the squirrel must wake up periodically during its hibernation period – possibly to sleep, eat, or do other body maintenance. In contrast, a bear’s internal temperature stays higher, at 31, °, start text, C, end text (88, °, start text, F, end text) or above, but the bear can hibernate for its entire winter period without needing to awakenstart superscript, 10, end superscript.
Some animals enter an extended period of torpor during the summer months, when there are high temperatures and little water. In this case, the extended torpor is called estivation. Some desert animals estivate in response to dry conditions, and this shift helps them survive the harshest months of the yearstart superscript, 9, end superscript. The snails in the photo below climb to the tops of fence posts to estivate.
Image credit: Kadina snails climb fence, by Vladimir Menkov, CC BY-SA 4.0
Torpor can also last for short periods. Daily torpor can be sporadic, in response to unfavorable conditions, or can repeat in a predictable pattern. For instance, some small endotherms such as dormice reduce the amount of energy they need (and thus, food they must consume) by entering torpor during the part of the day that is coldest, when they would otherwise need to use a lot of energy to produce metabolic heat and maintain body temperature.

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