Becoming a parent can be the most rewarding yet arduous experience. Incubating king penguins have to tough it out on sub-Antarctic beaches, taking it in turns to venture off in search of food while waiting for the egg to hatch. But at what cost? How much energy do the stay-at-home parents consume? René Groscolas and his colleagues from the Institut Pluridisciplinaire Hubert Curien, France, and the Université Laval, Canada, explain that scientists usually estimate energy expenditure by simply measuring a bird's heart rate. However, the team suspect that the act of measuring birds' oxygen consumption as a function of their heart rate could have flustered the animals and sent their heart rates rocketing, causing scientists to underestimate energy consumption rates (p. 153).

The team decided to remeasure incubating king penguins' energy consumption rates without agitating the birds. Knowing that king penguins lose weight while incubating their individual eggs, the team recorded the incubating birds' heart rates, measured their weight loss and converted the weight loss into energy consumed. Plotting the birds heart rates against their energy consumption, Groscolas and his colleagues found that the birds were using as much as 26% more energy than had been estimated previously. ‘This result suggests that stress induces a disproportionate increase of heart rate versus oxygen consumption and that the use of energy expenditure/heart rate relationships obtained in stressed birds could lead to underestimated energy consumption values,’ say Groscolas and his colleagues.

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Heart rate as a predictor of energy expenditure in undisturbed fasting and incubating penguins.
J. Exp. Biol.