There is a strong association in mammalian species between metabolic rate, size, and life span. When pulling in bird species to compare, however, it is observed that they tend to have higher metabolic rates and longer life spans at a given size. So the question here is what exactly is going on in bird metabolism that allows for this more heated operation of cellular metabolism, necessary to meet the demands of flight, without the consequences to life span observed in mammalian species. The open access paper here is illustrative of research in this part of the comparative biology of aging field. Is there anything in this ongoing work on metabolism and aging that might one day lead to methods of extending mammalian life? Perhaps, perhaps not. Altering the operation of metabolism is a poor second best to repairing the damage that causes aging, but one never knows what might emerge from fundamental research at the end of the day.
Mitonuclear communication is at the heart of metabolic regulation, especially in fundamental processes such as cellular respiration. All endothermic organisms have evolved high metabolic rates for increased heat production. However, birds and mammals evolved endothermy independently of each other, and demonstrate some stark differences. Birds live significantly longer lives compared with mammals of similar body size, despite having higher metabolic rates, body temperatures, and blood glucose concentration.
The underlying physiological mechanisms that explain differences between mammals and birds are varied, and include differences at tissue- and cell-levels. For both of these groups, mass-specific basal metabolic rate (BMR) decreases with body size and body size accounts for much of the variation in BMR, however, much variation among species still remains to be explained. Because BMR is defined fundamentally as the sum of tissue metabolic rates, it follows that variation in BMR may relate to the relative size of central organs.
Alternatively, cellular machinery of the tissues of birds and mammals may differ. Metabolic intensity of tissues is thought to vary because of differences in numbers of mitochondria within cells, concentrations of metabolic enzymes, activity or quantity of the membrane sodium-potassium ATPase pump, and the number of double bonds in fatty acids of cell membranes. Because of differences in whole-organism metabolic rate, we may also expect differences within the rates of cellular processes, including oxidative stress.
Oxidative stress is a balance, inherent to all aerobic organisms, between the potential damage that could be accrued by reactive oxygen species (ROS) and the resources cells have to thwart that damage through the antioxidant system. This process has gained momentum in the ecological physiology literature because it has been implicated in determining rates of aging. Here, we sought to quantify parts of the oxidative stress system in a diverse group of birds and mammals. Our question was two-fold: does oxidative stress (a product of aerobic respiration and thus BMR) scale with body mass in these two groups? And are there differences in oxidative stress between birds and mammals?
Our first finding is that cellular metabolism and every parameter that we measured to quantify oxidative stress in birds and mammals does not scale with body mass. This implies that differences at the cellular level might make small contributions to scaling at the organ level, pointing to the fact that scaling of metabolism may reside in higher levels of organization. An obvious explanation may be that organ sizes between similarly-sized birds and mammals may be disproportionally larger in birds compared with mammals, leading to higher BMR.
Secondly, birds showed significantly lower basal cellular oxygen consumption, lipid oxidative damage, and lower activities of catalase. These results together imply several possible physiological mechanisms, none of which are mutually exclusive: (i) birds may have cells with significantly fewer mitochondria or with mitochondria that are more uncoupled; (ii) birds may be less burdened by ROS production compared with mammals; or (iii) birds may have membranes with lower membrane polyunsaturation compared with mammals.