King penguins (Aptenodytes patagonicus). Image credit: Fabrice Bertile (CC BY 4.0)
Smaller animals often live shorter lives and use energy at a faster rate than their larger, longer-lived counterparts. This is partly related to differences in their resting metabolic rate, which is the energy expended to maintain basic bodily functions over a given time. For example, mice have high metabolic rates and short lifespans, whereas elephants live much longer and have lower metabolic rates per gram of body mass. However, many birds – despite having high metabolic rates – can live far longer than mammals of a similar size.
Birds also have the highest blood glucose levels of any vertebrate group. Through a process known as glycation, glucose and other sugars can attach themselves to molecules such as proteins. The resulting glycated proteins are thought to have negative effects on the body, which can contribute to the ageing process. Therefore, the amount of glycated protein in the blood is often used as a marker for harmful blood glucose levels in humans, which can be indicative of diseases such as diabetes. However, it remained unclear how birds resist the negative impacts of high blood glucose levels and glycation.
To investigate, Moreno-Borrallo et al. used measurements from 88 different bird species to explore how glucose levels and glycation of a protein called albumin are related to diet, lifespan and other variables. As expected, species with higher blood glucose levels had higher levels of albumin glycation. However, species with very high glucose levels showed relatively low glycation, suggesting these birds can resist the negative effects of high blood glucose.
Surprisingly, the analysis showed that species with higher glucose levels also tended to live longer, although this increase in lifespan eventually levelled off. This is contrary to the idea that species with higher metabolic activity have evolved shorter lifespans. Moreno-Borrallo et al. also showed that glucose levels decrease with body mass but are not related to any other traits. Glycation, on the other hand, is impacted by diet, with land (but not aquatic) carnivores showing higher levels than omnivores.
These analyses systematically explore how glucose and glycation levels relate to traits such as lifespan and diet across a wide range of bird species. The results will be valuable to evolutionary biologists and may also have implications for human health, particularly in understanding how glycation can be resisted during ageing. Future research should also focus on identifying which diets may help protect against glycation.
