Most migratory birds put other animal athletes to shame, completing journeys thousands of miles long fuelled by energy from their exceptional lipid metabolism. One of these champions is the ruff sandpiper Philomachus pugnax, a shorebird that flies a round trip of up to 30 000 km a year between wintering grounds in Africa and nesting grounds in northern Scandinavia. As Jean-Michel Weber from the University of Ottawa explains,researchers face a problem if they want to study the metabolism of flying birds, as it requires invasive measurements which are very difficult to do once a bird is airborne. So to find out more about a sandpiper's lipid metabolism, Weber and his colleague Eric Vaillancourt took a different approach and studied the birds during shivering, which raises the metabolism but makes it much easier to take measurements(p. 1161).

The team made two sets of measurements simultaneously to examine the birds'lipid metabolism. The first set involved using a respirometer to measure the total amount of oxygen the birds used and the amount of carbon dioxide they produced. By comparing the quantities of the two gases, the team could work out which fuel the animal was using: carbohydrate, protein or fat. They found that the birds were getting over 80% of their energy from fat when they were at rest. When they lowered the temperature from 22°C to 5°C for 2 h to induce shivering, they found that the birds' oxygen consumption and carbon dioxide production went up. However the ratio of the two gases stayed the same, showing that the birds were simply using the lipids faster to give them enough energy.

The second set of measurements were to find out the rate at which the birds were breaking down lipids. Most fats consist of a glycerol backbone with three fatty acids attached, so the body has to break up the molecules and free the fatty acids that can be used for energy. By measuring the rate that glycerol enters the blood stream, scientists can measure how quickly lipids are being broken down. To measure glycerol production, Vaillancourt carried out delicate operations on the birds, inserting two catheters into two different blood vessels in their necks. They used the first catheter to inject labelled glycerol into the blood stream. By comparing the amount of labelled to unlabelled glycerol in blood samples taken from the second catheter, the team could measure the rate of glycerol production.

The team were surprised to find that the rate of lipid breakdown in the birds when they were at 22°C matched the highest rate that had ever been measured in an animal. When they dropped the temperature to induce shivering,they found that the rate of glycerol production in the blood stream stayed the same in the normal and cold conditions, showing that shivering didn't boost the rate of lipid breakdown, just the rate at which the birds used lipids for energy, which they saw from their respirometer measurements. This is probably because the rate of lipid breakdown is so high that it doesn't need to increase under colder conditions, because more than enough fatty acids are being released. The birds' record-breaking lipid metabolism is probably essential to achieve their long migrations.

Vaillancourt, E. and Weber, J.-M. (
). Lipid mobilization of long-distance migrant birds in vivo: the high lipolytic rate of ruff sandpipers is not stimulated during shivering.
J. Exp. Biol.