Glucose tolerance predicts survival in old zebra finches.

The capacity to deal with external and internal challenges is thought to affect fitness, and the age-linked impairment of this capacity defines the ageing process. Using a recently developed intra-peritoneal glucose tolerance test (GTT) in zebra finches, we tested for a link between the capacity to regulate glucose levels and survival. We also investigated for the effects of ambient factors, age, sex, and manipulated developmental and adult conditions (i.e. natal brood size and foraging cost, in a full factorial design) on glucose tolerance. Glucose tolerance was quantified using the incremental 'area under the curve' (AUC), with lower values indicating higher tolerance. Glucose tolerance predicted survival probability in old birds, above the median age, with individuals with higher glucose tolerance showing better survival than individuals with low or intermediate glucose tolerance. In young birds there was no association between glucose tolerance and survival. Experimentally induced adverse developmental conditions did not affect glucose tolerance, but low ambient temperature at sampling and hard foraging conditions during adulthood induced a fast return to baseline levels (i.e. high glucose tolerance). These findings can be interpreted as an efficient return to baseline glucose levels when energy requirements are high, with glucose presumably being used for energy metabolism or storage. Glucose tolerance was independent of sex. Our main finding that old birds with higher glucose tolerance had better survival supports the hypothesis that the capacity to efficiently cope with a physiological challenge predicts lifespan, at least in old birds.