and correcting for body temperature accounts for some of what’s left over. Together, the two variables explain more than three-quarters of the variation in hatching times for fish, insect, amphibian, and bird eggs. If a salmon egg could somehow be expanded to the same size as a hen’s egg, and then incubated at the same temperature, they would hatch after approximately the same length of time. The two would also take approximately the same amount of time to reach maturity.
There is more to growth and development than energy. A builder with a generator but no bricks would build nothing. Likewise, organisms need materials as well as calories: A body won’t work properly without a balanced diet and what it can do depends on what chemicals it gets. Just as they vary in size and temperature, organisms also vary in their chemical composition, and this variation can explain another tranche of their biological differences.
After carbon the two most important chemical elements found in living things are nitrogen, which is a large component of proteins, and phosphorus, which is found in ATP, the energy molecule, and also in the molecules that carry genetic information, DNA, and its close cousin RNA. In many organisms, such as plants, it seems to be the availability of these elements, rather than solar energy, that limits growth. It is these elements that chemical and organic fertilizers add to soils. Bone meal, for example, is rich in phosphorus.
Different species contain similar proportions of protein and so nitrogen. The amount of phosphorus is much more variable. Fast-growing species have more phosphorus because RNA is part of the cell’s protein-making machinery, so fast-growing cells need more of it. RNA is also used in the process that turns the information in the genome’s DNA into protein, giving hard-working cells an extra demand for phosphorus. Animals with high growth rates therefore also contain a lot of phosphorus. And when the metabolic ecologists compared relative phosphorous content to growth rate, they found that this quantity explains some of the variation in developmental times not accounted for by body size and temperature.
Chemistry, then, is a third factor, besides body size and temperature, that controls metabolic rate. To explain metabolic rate and how it sets life’s other rates, we must consider materials as well as energy and