Studying mammals: A winning design
Studying mammals: A winning design

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Studying mammals: A winning design

5.2 Metabolism

Scientific data are often presented in the form of a table, with the data arranged in columns (running vertically) and rows (running horizontally). You will find Table 1 below. As with diagrams, a table has a title and you should read that carefully. Look at the headings of the rows and columns. The units in which the values are measured are usually given in the column (or row) headings, rather than being written beside each value.

Metabolism is the technical name given to the sum total of all the chemical transformations inside cells. Many such changes involve building up complex chemicals - for example, the proteins and fats I've already mentioned - from simpler building blocks. In animals, the energy that such 'building-up' processes require has to come from the process of breaking down foodstuffs - the progressive breaking down in our bodies of the complex macromolecules that comprise our diet. (The final stages of breakdown and release of useful energy take place in the mitochondria, like those you saw in Figure 1d.) None of these energy-yielding or energy-consuming chemical transformations is 100% efficient and inevitably heat is released as a by-product. So an animal with a high level of metabolism, i.e. a high metabolic rate, produces a greater amount of heat than one operating at a lower level.

The chemical transformations we've talked of depend ultimately on the animal's intake of oxygen. In very simplified terms, oxygen is used in the mitochondria to complete the final stages of the breakdown (or more technically, the oxidation) of small energy-rich molecules - products of the chemical fragmentation of macromolecules mentioned earlier. In this process, usable energy (and some heat) are released.

Measuring how much oxygen is consumed by an animal over a period of time, such as a minute or an hour, is a pretty good measure therefore of the intensity of metabolism (i.e. the metabolic rate). But if we simply measure oxygen consumption in two animals that are very different in size - say a mouse and an elephant - we'd find that overall the elephant consumed much more oxygen over a minute than did a mouse, simply because a massive elephant has so many more cells in which metabolism is whirring away. If our interest is in metabolic rate, we'd have to take into account differences in the size by calculating the volume of oxygen consumed for a particular amount (or mass) of animal. Volumes of a gas such as oxygen, would be measured in millilitres (ml) or, more usually, the numerically equivalent unit, cubic centimetres (cm3), and the mass of the animal in grams (g), or perhaps in thousandths of a gram, i.e. milligrams (mg). In practical terms, oxygen consumption by a mouse of known mass would be measured over a period of time, say 15 minutes, and the values recalculated as so many 'cubic centimetres of oxygen per gram of mouse per hour'. (A more scientifically correct way of expressing the same thing is cm3 O2 g−1 h−1, and I'll have more to say about using this type of scientific notation in S182_3 Studying mammals: chisellers) In principle, the calculation for the elephant would be done in the same way, though one suspects with a few practical problems along the way.

Table 1 Metabolic rate (expressed as volume of oxygen, O2, consumed, in cubic centimetres per gram of body mass per hour) in a range of animals, at rest and during maximum activity

Metabolic rate/cm3 O2 g−1 h−1
Species at rest at peak activity
salmon 0.08 0.60
monitor lizard 0.08 0.38
turtle 0.03 0.64
humming-bird 2.80 42.00
mouse 2.50 20.00
dog 0.33 4.02
human 0.23 3.20
You might notice from these few data an implication that small mammals, e.g. the mouse, have a higher metabolic rate than larger mammals, such as the human. This trend is borne out by many other measurements, but I'll ignore it for the moment and return to it in S182_2 Studying mammals: the insect hunters.)

Question 6

Look at Table 1, above, which shows the metabolic rates of a number of animals. What is the most striking difference between the values for mammals and those for fish, reptiles and a bird? (Concentrate on the 'at rest' values.)

Answer

The metabolic rates for mammals (and the humming-bird) are much higher than those for the fish and reptiles. This is true of both the 'at rest' levels and, as you'll see if you now look at the far right column, also the 'peak activity' values; the mouse, dog and human (and the humming-bird) values are higher than the remainder, often considerably so.

S182_1

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