Knowing how much energy an animal uses is essential for ecologists to understand how ecosystems function. One technique that physiologists use to measure energy expenditure in the field is the doubly labelled water (DLW) method. By injecting water enriched with heavy oxygen (18O) and deuterium (2H) into an animal, physiologists can measure the change in body water 18O:16O (lost as CO2 and H2 O) and 2H: 1H (lost only as H2O) ratios to calculate the amount of carbon dioxide produced and energy consumed by an animal.
Jannik Schultner from the Norwegian Polar Institute and University of Tromsø explains that there are two widely used methods for taking DLW measurements: one where the birds are released immediately after the injection and a second where the birds are held for an hour after injection to allow the heavy water to become evenly distributed throughout the bird's body fluids before a blood sample is taken to establish the initial 18O:16O and 2H:1H ratios. But the metabolic measurements are only reliable if the animals continue behaving normally during the course of the experiment. Schultner explains that Jorg Welcker had noticed that kittiwakes that had been detained after their injection spent more time away from the nest after release than birds that had been released straight away. Joining Geir Gabrielsen's lab, Schultner, Erling Nordøy and their colleagues decided to investigate what effect the two DLW methods had on kittiwakes' behaviour and energy expenditure to find out how reliable DLW metabolic measurements are ().
Travelling to Svalbard, Norway, the team tagged and weighed pairs of nesting birds and injected them with DLW. Releasing some birds immediately after the injection, the team held others for an hour before taking the initial blood sample and releasing the birds. Then the team monitored the injected birds, recording how often they returned to their nests and collected additional blood samples 24 h and 72 h after the injection. Returning to the lab, Schultner sent the blood samples to John Speakman's lab at the University of Aberdeen to measure the 18O:16O and 2H:1H ratios and estimate the birds' metabolic rates.
Comparing the behaviour of the detained and undetained birds, the team were surprised to see that the two groups behaved differently. The detained birds stayed away from their nests for about 4 h, while the undetained birds returned within about 10 min. And, when the team recaptured the birds and took later blood samples, the detained birds stayed away from their nests longer than the undetained birds. Finally, when the team compared the metabolic rate estimates obtained from the 18O:16O and 2H:1H ratio measurements, the detained birds' metabolic rates were 15% lower than those of the undetained birds.
So it seems that kittiwake's behaviour is more adversely affected by the second DLW method than had been previously thought, significantly reducing energy expenditure estimates. Schultner adds that the second method was perceived to be more accurate than the first – because researchers measure the initial 18O:16O and 2H:1H ratios rather than estimating them – but this may not be the case. ‘I guess when applying the DLW method it is important to weigh the advantages and disadvantages of the different protocols against each other, and then to observe closely what happens to the animals during the study to confirm that the data are as strong as assumed,’ says Schultner.
