Mammalian red blood cells do not have a nucleus, which is different from birds, amphibians, reptiles, and fish (see image above). Many textbooks report that the absence of a nucleus provides room for more hemoglobin within the cells, which is important to fuel the relatively high metabolic rates of mammals. Hemoglobin is an important oxygen-binding molecule that allows the red blood cells to transport oxygen to tissues throughout the body where it is used for metabolism. The lack of a nucleus also allows mammalian red blood cells to squeeze through tiny capillaries in the body.
Like mammals, birds have high metabolic rates yet their red blood cells are nucleated and contain organelles. A recent article published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology explored variations in birds and mammals that may explain their different strategies for maintaining high metabolic rates with or without a nucleus, respectively.
By comparing 181 avian and 194 mammalian species, the new study determined there is actually no difference in the mean concentration of hemoglobin in red blood cells between birds and mammals and both had higher hemoglobin concentrations than ectotherms. Interestingly, birds appear to compensate for the space the nucleus takes up by having hemoglobin present inside the nucleus as well. Having high hemoglobin levels in red blood cells is consistent with the higher metabolic rates and oxygen demands of endotherms as compared to ectotherms.
Another hypothesis for the loss of the mammalian nucleus and organelles is that it reduced cell volume and at the same time increased the surface area to volume ratio of mammalian red blood cells, which would help oxygen move into the cells more easily. Although the current study found that the mean cell volume of birds is higher than mammals, when they accounted for the phylogeny of birds, this difference was no longer significant. In addition, birds have several adaptations that ensure they can transport gases efficiently to support their high metabolism without the need to shrink their red blood cells. These adaptations include more capillaries in flight muscles, heart and brain compared to mammals in addition to having a thinner barrier between the blood and gas allowing for faster oxygen uptake and delivery.
The elimination of organelles such as the mitochondria and endoplasmic reticulum from mammalian red blood cells is thought to offer the added benefit of warding off oxidative stress. However, human red blood cells are capable of producing a lot of oxidative stress even without the mitochondria. Here too, birds seem to have solved this problem as they have less oxidative damage in their red blood cells even though the cells produce more free radicals than mammals.
The findings from this study are interesting as they point to the evolution of quite different strategies to support high metabolic rates in birds and mammals as compared to ectotherms.
Yap KN, Zhang Y. Revisiting the question of nucleated versus enucleated erythrocytes in birds and mammals. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. In Press. https://doi.org/10.1152/ajpregu.00276.2020