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
  • $\begingroup$ 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

3 Answers 3


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! $\endgroup$
    – stephslr04
    Sep 18, 2017 at 16:21
  • $\begingroup$ May I ask what are your resources for your answer please? It's helped me a lot! $\endgroup$
    – stephslr04
    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

According to Quara

One, mammals are younger in natural history than birds. Before mammals, nucleated blood cells were the norm. Mammals evolved enucleated blood cells to use oxygen more efficiently.

Two, birds have a different respiratory system that lets them transport oxygen efficiently without needing enucleated blood cells. They have "flow-through" respiration that lets air move continuously through the body. Mammals have alveoli that are like a dead-end for moving air. 

According to

this website

Red blood cells (RBC) or erythrocytes are continually formed in the bone marrow. RBC originate from nucleated stem cells, which mature into nucleated erythroblasts, then differentiate into a-nuclear reticulocytes and finally into RBC. RBC are terminally differentiated cells (they cannot divide anymore) and are shed from the bone marrow into the blood circulation. They live approximately 120-180 days. In contrast to mammals, RBCs in birds, reptiles and other "lower" vertebrates have a nucleus. The a-nucleated erythrocyte, as it is seen in mammals, is considered more evolutionarily "advanced". (see www.ultranet.com/~jkimball/BiologyPages/V/Vertebrates.html). The lower vertebrates (e.g., birds) are considered earlier on the evolution ladder and have a different circulatory system (seewww.sciencenet.org,www.historyoftheuniverse.com/blood.html, http://library.thinkquest.org/3564/lessons/lesson3/lesson3.html). In addition to the differences in the circulatory system, mammals have smaller end-bloodvessels (capillaries of about 3 micron in diameter) than birds. In order to squeeze through these small blood capillaries, RBC which are about 10 micron in diameter, must be very flexible. The presence of a nucleus would prevent big nucleated RBC to squeeze through these small capillaries. Therefore, during the evolutionary development, nature has found that it was better to get rid of the nucleus and also other cell organelles (e.g., endoplasmic reticulum for protein synthesis) which were not needed for their actual function as oxygen carrier. Sue Thornquist (www.vet.orst.edu/clinpath/learning/vm736/avianhem.htm) also thinks that the absence of the nucleus in birds is based on evolutionary differences but she’s not sure whether this theory has been proved. As homeotherms evolved, they had increased oxygen demands due to different metabolic requirements. Birds appear to have adapted to increased oxygen demands by developing a "flow-through" respiratory system (interconnecting tubes for continuous flow, rather than blind-ended alveoli) that's more efficient than mammals'. Mammals may have diverged here and developed anucleated RBC's with increased oxygen carrying capacity to adapt to the increased oxygen demands

Hope this answer is good enough

  • 2
    $\begingroup$ I am afraid this answer is not good enough. It is fine to quote sources in support of an explanation, but we expect you to provide that explanation yourself. You need at least to summarize the gist of the arguments in the sources you quote, especially when they are so ridiculously long. $\endgroup$
    – David
    Sep 17, 2017 at 19:09
  • $\begingroup$ Thanks for all your answers, it's pretty much summed up what I had in mind! $\endgroup$
    – stephslr04
    Sep 18, 2017 at 16:21

Good discussion, very complex topic. I’m not sure we know/understand all variables and interactions at play in circulatory/respiratory physiology(not to mention aspects of the RBC involved). That said: Without a nucleus(and mitochondria), the red cell has less tendency to metabolize whatever O2 is available, as well as many of other well stated advantages of the enucleated state for increasing O2 availability at the cellular level. Camels, with nucleated RBCs, are an exception to the mammals enucleated RBC, implying the capacity for DNAs response to environmental pressures. Balancing advantages and disadvantages of fundamental physiological and cellular changes will direct the path of an organisms success (i.e. adaptive evolutionary experimentation with organisms reproductive success or failure being the final arbiter). The camel “giving up” the well established mammalian enucleated RBC (gaining adaptive physiological advantages) might be an apt example of the DNA biochemical feedback loop required to coordinate significant cellular changes within a species. Evolution is not a one way street, u-turns are available(which is why there’s no such thing as “junk” DNA, a story for another time). Keep cooking baby!

  • $\begingroup$ Welcome to Biology.SE! Answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. Also note that camels like other mammals have enucleated RBCs as discussed in this post ——— Please take the tour and then consult the help pages for additional advice on How to Answer effectively on this site and then edit your answer accordingly. Thanks! 😊 $\endgroup$
    – tyersome
    Jan 3, 2020 at 1:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.