IGF-1 receptor inhibitor OSI-906 reduces growth in nestlings of a wild passerine

Young animals need to grow to a large body size fast to maximise their survival prospects until sexual maturity. However, body size varies substantially in wild populations, and neither the selection pressures maintaining this variation, nor the regulatory mechanisms are well understood. IGF-1 administration has been shown to accelerate growth, but this does not necessarily imply that natural variation in growth rate is IGF-1 dependent. To test the latter we administered OSI-906 to pied flycatcher Ficedula hypoleuca nestlings, which has an inhibitory effect on IGF-1 receptor activity. We performed the experiment in two breeding seasons to test the prediction that blocking the IGF-1 receptor downregulates growth. As predicted, OSI-906 treated nestlings had lower body mass and reached a smaller structural size than siblings receiving a vehicle only, with the mass difference being most profound at the age preceding the highest body mass growth rate. The IGF-1 receptor inhibition effect on growth varied with age and year of study, and we discuss possible explanations. The OSI-906 administration results indicate that natural variation in growth rate is regulated by IGF-1, and constitutes a novel tool to study causes and consequences of growth variation, but details of the underlying mechanism still need to be resolved.