Some people appear to have boundless energy, while others favour a more lethargic lifestyle. ‘Within a given species, there is often a high inter-individual variation in resting metabolic rate’, says Frédéric Angelier, from the Centre d'Etudes Biologiques de Chizé, France, yet it wasn't clear how this individuality comes about. Knowing that good and bad experiences from early in life can have a long-term influence on heath and growth, Sophie Dupont, also from the Centre d'Etudes Biologiques, wondered how stress experienced during youth might affect the metabolism of adult house sparrows (Passer domesticus). But, instead of giving young chicks a fright, Jacquelyn Grace, from Texas A&M University, USA, fed four doses of the hormone corticosterone – produced by animals when they find themselves in a tight corner – to fledgling sparrows. Then, Dupont followed the youngsters through to adulthood, measuring their oxygen consumption and CO2 production at the age of 2 years when they were warm (25°C) and chilly (5°C), to calculate their metabolic rates and find out how much harder their bodies have to work to keep their temperature up when it is cold. In addition, she recorded how much mass the birds lost overnight while going without food.
Not surprisingly, the chilly sparrows burned through more oxygen than the warm sparrows to keep their temperature up. However, when Dupont, Grace, Olivier Lourdais, François Brischoux and Angelier compared the metabolic rates of the sparrows that had been fed a stressful diet when young with the metabolic rates of sparrows that had not consumed corticosterone, it was clear that the birds that had been stressed as youngsters had much lower metabolic rates than the birds that had grown up without a stressful experience. And the stressed birds coped better overnight, losing less mass than the birds whose fledgling-hood had been plain sailing.
It seems that having a stressful start in life reprograms the youngsters’ metabolism to produce more-efficient adults; ‘it could orient the phenotype towards an energy-saving strategy’, says Dupont. However, the team warns that the birds may be less nimble and more vulnerable to the cold, placing them at additional risk. They say, ‘Further studies focusing on fitness metrics (survival and reproduction) are now required to fully assess the fitness costs and benefits of this corticosterone-mediated effect of developmental conditions on metabolism’.
