I have read, and read, and read documents on this subject but still have no conclusions.

Everything I have read explains why mammals don't have a nucleus (to make more room for the haemoglobin and allow for more oxygen to bind as well as making it easier for the RBC to fit through capillaries) but the reports don't explain why birds, reptiles and fish may still have nucleus in a red blood cell.

Is there really any other function a nucleus has within a red blood cell apart from being the 'brain of the cell?

It's doing my head in, all I've gained from this research is the understanding that birds have a far more advanced respiratory system and their capillaries are bigger than mammals, this still doesn't explain the need for a nucleus, it's actually suggesting they don't need one!

Any enlightenment will be great,



  • $\begingroup$ +1 vote. I was also wondering this last time though I never bothered to research $\endgroup$
    – user35897
    Sep 17, 2017 at 15:01
  • 1
    $\begingroup$ Does there have to be an explanation? I personally would doubt the fairy story about mammalian red cells making room for haemoglobin. Typical post hoc justification. They just evolved differently. $\endgroup$
    – David
    Sep 17, 2017 at 19:14
  • $\begingroup$ Many good points already made above. I'd only add enucleate red cells confer a solid evolutionary advantage for many mammals. Apex land predators, eg cheetahs, probably couldn't sprint if they had the lesser capacity of nucleated red cells. Also allows some animals to occupy higher altitude niches - eg mountain goats etc $\endgroup$
    Oct 14, 2019 at 19:40
  • $\begingroup$ Small point: Camelids have enucleated erythrocytes, albeit elliptical ones. See vetclinpathimages.com/2018/03/27/normal-camelid-erythrocytes $\endgroup$ Jul 7, 2022 at 9:58

2 Answers 2


It's difficult for vertebrates to aquire/evolve enucleated cells, that's why they are very rare previous to the mammalian adaptation. Salamanders have evolved enucleated cells. Research suggests that it may be due to a salamanders big genome and big cell nucleus, compared to the salamanders small body size.

Mammals may have acquired the trait because they evolved from small animals with miniaturized capillaries and blood cells and large genomes, and the advantage was kept afterwards. Large genomes inhibit flow and flex of nucleated blood cells.


The smallest bird genomes are found in the most miniturized clades of birds at 1 Gb. Mammals of a similar size can have carry 2-3 Gb of chromosomes.

Lizards have lower energy than mammals.

Fish are most active in highly oxygenated water, i.e. salmon, which allows them to be very energetic, water is a more efficient breathing medium than air, hence the small size of gills. fish and humans both contain 6-7 percent blood by volume.

Birds have smaller haemoglobin including the nucleus, and have a different kind of capillary size and structure with advantages and disadvantages, and lighter hollow bones with less marrow. Birds didn't radiate to new shapes as well as mammals did.

Enucleate cells are fairly rare in the plant and animal world, so they perhaps have a hidden expense. Maybe it's not just a factor of shedding the nucleus, and in fact there is a cost of adapting the primary circulation system's immunity against paramecia and pathogens without a nucleus. various viruses can take advantage of enucleate cells.

Perhaps that is the reason why birds did not evolve backwards into dinosaurs. some factor made their genome comparatively small. It could be the lack of enucleated bloodcells that forced them to shed genes as an alternative. Perhaps that's why they lost the dinosaur genes which held code for quadrupeds, arms, jaws, horns and teeth and the things that birds don't have.

  • $\begingroup$ Thanks for all your answers, it's pretty much summed up what I had in mind! $\endgroup$
    – user36536
    Sep 18, 2017 at 16:21
  • $\begingroup$ May I ask what are your resources for your answer please? It's helped me a lot! $\endgroup$
    – user36536
    Sep 18, 2017 at 16:58
  • $\begingroup$ Thanks Steph, I mostly use google scholar for specific references, some of it is difficult to research, skeletal shape advantages and immunity and physiological adaptions of enucleosis is less easy to find specifics on. $\endgroup$ Sep 19, 2017 at 4:00

enucleation is just one of multiple solutions to the same problem, and each has their own costs.

When compared side by side bird blood carries the same amount hemoglobin as mammalian blood. birds maximize oxygen content my evolving smaller hemoglobin molecules, mammals evolved smaller hemoglobin carrying cells.

Each solution has its own cost, bird erythrocytes still have mitochondria which reduces oxidative stress on the cells while meaning each cell is larger and thus capillaries need to be larger. But oddly, even though bird blood cells age slower (accrue oxidative stress slower), birds have a higher turnover of blood cells. Since enucleation limits the speed at which mammalian blood cells can be produced they cannot replace blood cells faster even though they are wearing out faster.

Once a group starts evolving down one of these strategies and starts evolving means to counter or at least mitigate the costs there is no benefit and quite a high cost in switching to a different method, much like both groups respiratory system they get locked in evolutionarily.

Only in mammals and birds is the demand for oxygen high enough for the costs to be worth it. They need to maximize blood oxygen carrying capacity and take the benefit outweighs the costs. Other groups don't evolve these specializations because for them the cost is not worth the benefit there blood oxygen capacity is more than adequate. Its the same reason they don't evolve the advanced respiratory systems of mammals and birds.






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