2 You are what you eat
In this section, you will meet some new units, the units in which energy is measured. Nowadays, there are internationally agreed units (called SI units) that are often used in combination with a prefix to show the scale of the measurement. The SI unit for energy is the joule (pronounced 'jool', and with the symbol J) but from reading food packaging and books on diets you may be more familiar with measurements in the non-SI unit, the calorie (cal), or more usually the kilocalorie (kcal). Food labelling gives the equivalent in kJ as well, just as Table 1 does. 1 kcal is equal to 4.2 kJ (1 cal = 4.2 J). Notice that when you write a number with its units, you should always leave a space between the numerical value and the unit. Note too that the full names of most units begin with a lower case letter, but the abbreviation is an upper case letter; for example, 'joule' becomes 'J'.
Table 1 The energy yields of different nutrients
|Nutrient type||Energy content|
|/kcal g−1||/kJ g−1|
In Table 1, the values in the columns give you the energy content of various sorts of food (carbohydrate, fat and protein) in kcal per gram and kJ per gram. But notice how 'per gram' is written as 'g−1'. This representation is scientifically correct, but you will probably find it easier, at least at first, to continue to read 'g−1' as 'per gram'.
You will have probably noticed too, that there is a division sign (/) before the units. Again, this is for reasons of mathematical accuracy. Consider the value for fat of 9 kcal g−1 (read as 'nine kilocalories per gram'). If we want to enter this in the table as just 9, then we have to divide the value by kcal g−1. To show that all the values in that column are divided by kcal g−1, the column headings are written as 'Energy content/kcal g−1'. You will see the same representation in Table 2, where the percentages of various components of seeds are given and the columns are headed 'Water/%', etc.
I've already mentioned that rodents make use of a wide variety of foodstuffs, but both LoM and the TV programme emphasise the importance of seeds.
There are problems inherent in using seeds as your main source of food. Both are described in some detail in LoM and the TV programme. In your own words explain the problem.
In a nutshell, the answer is that if you eat everything there is nothing for tomorrow.
Biting the hand that feeds you is one thing, killing it off quite another; clearly it is important that there is a balance between exploiting a source of food and destroying it beyond the point from which it can make a recovery. Relying on seeds is therefore a high-risk strategy because a seed cannot recover from being eaten; the only way the plant that produced the seed can reproduce successfully is by producing more seeds than can be eaten.
On the basis of your reading of LoM, can you think of other hazards associated with specialising as a seed eater?
Few plants produce a continuous supply of seeds and in some temperate areas and desert regions the supply is seasonal.
The seasonal availability of seeds and the intermittent gluts associated with this seasonality were described at the beginning of LoM (p. 64, in particular). If you need reminding of some of the implications of a diet rich in seeds, reread Chapter 3 from the beginning to the end of the description of the marmot's behaviour on p. 67.
The TV programme shows different rodents engaged in types of behaviour that get around the difficulty of using a source of food that is seasonal. Recall two of these behaviours.
(1) Storing or hoarding food. (This habit was also described in the section of LoM that you may have just reread.)
(2) Hibernating, having 'fattened up' during the season of plenty. This strategy is also described in LoM, for example in marmots [p. 67], which adopt a very different overwintering strategy to that of the much more continuously active beavers [p. 72].
As DA points out at the end of Chapter 2 [pp. 59-60], there can be costs associated with adopting a specialised diet. He described how the anteater's ability to eat ants and termites goes hand-in-hand with an inability to eat the wide range of invertebrates available as food to the non-specialist insectivores. In the sixth course in this series, S182_6 Studying mammals: the opportunists, we will return to examine this problem when we investigate the dietary choice of pandas. Despite the drawbacks of a specialist diet, we find that many rodent species do specialise in eating seeds, and the benefits of this habit are high because seeds are very nutritious.
What is meant by 'nutritious'?
A nutritious food source is rich in energy. As DA puts it: 'seeds are particularly worth stealing for each is packed with … energy-rich food' [p. 61]. (The italics are mine.)
Bear in mind that nutritious food sources would also contain minerals and vitamins, in addition to energy-rich components. But in this respect, seeds are less beneficial; they have low amounts of the important minerals calcium and iron, for example, which are needed by rodents for the manufacture of bones and teeth, and blood, respectively.
Study Table 1 in the box above. Which nutrient is the most energy-rich?
Fat, with an energy content of 38 kJ g−1.
So does this measurement mean that seeds have a high fat content? Well some of them do (see Table 2 below) but, of course, the story is rather more complicated. There is the digestibility of the various dietary components to be considered, as well as the energy expended in obtaining the food. Gathering seeds from grasses and other types of plant may not be very arduous, but it is time-consuming; the TV programme 'Plant predators' contains some delightful sequences of pika rushing to and fro with swatches of grass, quite literally making hay while the sun shines. But grinding seeds such as grain is another matter, for it requires not only strength and stamina but also specialised equipment. When we look into the jaws of rodents, we generally find teeth that are highly specialised tools.
Table 2 Composition of some seeds (expressed as percentage by mass). These values are approximate and they ignore the other constituents (e.g. minerals and vitamins) present in comparatively small amounts.