Increased susceptibility to oxidative damage as a cost of accelerated somatic growth in zebra finches

Most animals do not grow at their maximal rate. This might appear puzzling because the early attainment of a large body size incurs several selective benefits, such as reduced risk of predation and earlier reproductive output. Several hypotheses have been suggested to explain this paradox. Among them, the cost due to high levels of oxidative stress, as the consequence of sustained metabolic activity during growth, has been put forward. In this study, we wished to assess this cost in captive zebra finches (Taeniopygia guttata). In order to manipulate access to food and consequently early growth, hatchlings were randomly assigned to reduced or enlarged broods. Even though nestlings raised in enlarged broods were smaller when 20-days-old compared to nestlings raised in reduced broods, they grew faster during the following 20-day period. When 60-days-old, we measured the resistance of red blood cells against a free radical attack and correlated these values to nestling growth rate. In agreement with the prediction, we found that red blood cell resistance to free radicals was negatively correlated with growth rate, nestlings that grew faster being those with the weakest capacity to resist a free radical attack. These results support the hypothesis that oxidative damage might constrain growth rate. Furthermore, the fact that the relationship was only established during the period of accelerated growth after the initial delay also suggests that compensatory growth can negatively affect the individual resistance to oxidative damage.