Nature & Environment

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Studying mammals: The insect hunters

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# 5.2 Body size and metabolic rate

Figure 6 is a slightly more complex graph than those used in S182_1. In particular, the masses of the mammals that are plotted on the horizontal axis vary so much that a normal scale would squeeze together all the small and medium-sized ones, in order to be long enough to get the very large ones on the scale. So, as explained in the figure caption, a logarithmic scale is used here to get them all to fit. In this series of units, you won't come across plots like this very often, so it's not worth spending time struggling at this point to understand what a logarithmic scale means, if you are not familiar with one. You should still be able to read off the values of the plotted points as normal. As a general rule, you should be prepared to 'put to one side' complications like this that are not central to your learning and focus on the key points - here, the relationship between BMR and body mass. Don't forget to check the figure caption and look carefully at the units in which the measurements have been made.

It has long been known that the metabolic rate of mammals is closely related to body size. Look at Figure 6 which shows how BMR decreases in mammals as body size increases. (The way the units of body mass are marked onto the horizontal axis may well be unfamiliar to you, but you should find it possible to use the scale to work out the value of plotted points, most readily those between 1 and 10 and 10 and 100 kilograms.)

Figure 6: Adapted from Schmidt-Nielson (1975), Animal Physiology, Adaptation and Environment, Cambridge University Press and reprinted in Randall et al. (1997) Eckert Animal Physiology: Mechanisms and Adaptations, W. H. Freeman and Company ©
Figure 6: Adapted from Schmidt-Nielson (1975), Animal Physiology, Adaptation and Environment, Cambridge University Press and reprinted in Randall et al. (1997) Eckert Animal Physiology: Mechanisms and Adaptations, W. H. Freeman and Company
Figure 6 Graph of basal metabolic rate (measured as cubic centimetres of oxygen taken up per gram of body mass per hour) against body mass for various mammalian species. Body mass has been plotted on a scale known as a logarithmic scale, which allows a wide range of values for body mass to be included. To interpret the graph, use the values provided on each axis. For example, the cat with body mass of 3 kg (shown on the horizontal axis) has a metabolic rate of about 0.5 cm3 O2 g−1 h−1

## SAQ 6

From Figure 6 read off the BMR of the shrew, which weighs around 0.01 kg, and of the sheep, which weighs around 40 kg, and compare the values you obtain. Explain how your comparison relates to the energetic lifestyle of the shrew and what you know about the lifestyle of the sheep.