Glucose regulation is a repeatable trait affected by successive handling in zebra finches

The capacity to adequately respond to (physiological) perturbations is a fundamental aspect of physiology, and may affect health and thereby Darwinian fitness. However, little is known of the degree of individual variation in this capacity in non-model organisms. The glucose tolerance test evaluates the individual's ability to regulate circulating glucose levels, and is a widely used tool in medicine and biomedical research, because glucose regulation is thought to play a role in the ageing process, among other reasons. Here, we developed an application of the intraperitoneal glucose tolerance test (IP-GTT) to be used in small birds, to test whether individuals can be characterized by their regulation of glucose levels and the effect of successive handling on such regulation. Since the IP-injection (intraperitoneal glucose injection), repeated handling and blood sampling may trigger a stress response, which involves a rise in glucose levels, we also evaluated the effects of handling protocols on glucose response. Blood glucose levels decreased immediately following an IP-injection, either vehicle or glucose loaded, and increased with successive blood sampling. Blood glucose levels peaked, on average, at 20 min post-injection (PI) and had not yet returned back to initial levels at 120 min PI. Glucose measurements taken during the IP-GTT were integrated to estimate magnitude of changes in glucose levels over time using the incremental area under the curve (AUC) up to 40 min PI. Glucose levels integrated in the AUC were significantly repeatable within individuals over months (r = 50%; 95% CI 30-79%), showing that the ability to regulate glucose differs consistently between individuals.