Ambient temperature affects the metabolic rate of animals. When it is cold a homeothermic animal has to burn stored energy reserves, first glycogen and fat and then (when these are exhausted) protein, in order to generate heat and maintain its body temperature. The scaling of surface area (the site of heat loss to the atmosphere) to body volume (where the heat is generated) means that small homeothermic animals such as song birds can lose up to 15% of their body fat in one cold night. To burn this fat the bird must increase its metabolic rate, which increases its O2 consumption. Imagine that we conduct an experiment in which we place small birds inside metabolic chambers overnight and vary the air temperature. The hypothesised causal process is shown in Figure 5.1.

Unfortunately, we can't directly measure any of these three variables; they are unmeasured, or latent, and so I have enclosed them in circles following the conventions of path diagrams. If we measure the air temperature using a thermometer then we aren't directly measuring temperature – the average kinetic energy of the molecules in the air. Instead we are measuring the height of a column of mercury enclosed in a hollow glass tube. In fact, we can't even measure the actual height of the mercury exactly, since our observed height will include some measurement error. Nor can we directly measure metabolic rate. Typically, one measures the rate of gas exchange (O2 decrease or CO2 increase) between the air entering and leaving the metabolic chamber.