David Gremillet, Amelie Lescroel, Grant Ballard, Katie M. Dugger, Melanie Massaro, Elizabeth L. Porzig, David G. Ainley
Some animals are just fitter than others, with fitness defined as the ability to survive, reproduce and contribute to the gene pool of the next generation. Fitness is determined by genetics and the environment. Strong links between fitness and energy expenditure also seem likely, in animals as in humans: hyper-active individuals may out-compete less energetic ones, but those who really push it too far may also experience reduced fitness. Very few studies have tested the link between energetics and fitness in animals under natural conditions, mainly because measuring energy expenditure in free-ranging wildlife is extremely challenging. Yet, just as your smartphone now informs you about your daily energy expenditure, wild animals can also be fitted with miniature electronic trackers of their energetics. These can record accelerations 20 times per second in three dimensions, and the sum of all accelerations is a proxy for energy expenditure. We attached such 18 gram electronic tags to 115 Adélie penguins during their breeding season on Ross Island, Antarctica. The birds were going out to sea for half a day to six days before returning to the breeding colony to feed their chicks, and we estimated their daily energy expenditure during that period. We also knew the breeding performance of each bird over multiple seasons, as this colony has been studied extensively for the past 18 years. According to our predictions, we found that birds with intermediate levels of energy expenditure tended to achieve highest breeding performance. They therefore showed higher fitness than birds with very low, or very high levels of energy expenditure. Birds in better body condition also spent less energy. Our study underlines the great potential of 3D acceleration recordings as an aid for rating the fitness of wild animals and assessing their capacity to deal with environmental change.