It is hard work for animals to obtain food from the environment. They must move quickly to capture their prey or travel long distances to find suitable plants to eat. It is also a lot of work for animals to digest the food they obtain and then allocate food nutrients to maintaining their own health. The total amount of work an animal does per day is termed its daily energy expenditure. Scientists are interested in measuring the daily energy expenditure of animals in their natural environments. To do this, researchers use the fact that animals breathe in oxygen (O2) and breathe out carbon dioxide (CO2) and water (H2O). Similar to the last time that you ran up a couple of flights of stairs, when animals work hard, they breathe in more O2 and breathe out more CO2 and H2O. The technique uses special non-toxic water that is injected into animals. It estimates daily energy expenditure by measuring the amount of CO2 animals breathe out over a multi-day period. Regular water is composed of a ratio of two hydrogen atoms (H) to one oxygen atom (O) to make H2O. In the special water used in this procedure, the ratio between H and O is the same, but a proportion of the O and H atoms are naturally labelled O* and H*. For the technique, the special water is injected into the animal. The O* and H* molecules naturally mix with the other O and H atoms in the body water. A blood sample is collected to determine the initial concentration of O* and H*. Animals are released after the initial blood sample and behave normally for about two days. As they are busy working over this time interval they breathe out O* atoms in both CO*2 and H2O*, whereas the H* atoms are only breathed out through H*2O. The animal is then recaptured and another blood sample is collected. Scientists are able to quantify the amount of CO2 expelled, and thus get a measure of how hard the animal worked by comparing the ratio of O* and H* in a final blood sample to that in the initial blood sample. If the animals worked hard and expelled a lot of CO2 there will be a greater difference in the concentration of O* and H* between the initial and final blood sample. The amount of CO2 expelled can then be converted into an estimate of the daily energy expenditure of that animal.

-QF