Glycaemia and albumin glycation rates as fitness mediators in the wild: the case of a long-lived bird

Glucose is a vital metabolic component in the functioning of organisms, but it can also bind to biomolecules through non-enzymatic glycation reactions that result in loss of functions. While the effects of glycation on health have been well demonstrated in biomedical research, little is known about the effects of glycation in wild animals. Here, we studied how plasma glucose levels and albumin glycation rates vary with age and are related to fitness in a relatively hyperglycaemic long-lived bird, the Alpine swift (Tachymarptis melba). We measured plasma glucose and albumin glycation levels before and after reproduction in adult females of known age (2-14 years), showing that, while glucose levels increased in parallel with body mass, albumin glycation rates decreased within this period. Albumin glycation, but not glucose, varies with age, peaking at 5 years, consistently with other age-related parameters previously reported in this species. Interestingly, higher plasma glucose levels before reproduction were related to increased fledging success up to a certain threshold. In addition, in terms of dynamics, females gaining more mass lowered more their glycation levels, while those gaining less mass and lowering the more their glycation levels laid more eggs. Finally, higher body mass and plasma glucose levels after reproduction predicted a higher survival probability to the next season, whereas higher albumin glycation predicted lower survival, although in an age-dependent manner. Our study highlights adult plasma glucose and glycated albumin levels as new potential markers of ageing and fitness that should be further explored in this species.