Re: Why do some animals have nucleated red blood cells, while we don't

Why birds, amphibians and other animals have nucleated red blood cells
may best be answered by a zoologist. I think it may have to do with which
animals make cells intravascularly (in the blood stream) as opposed to
extravascularly (in the bone marrow or other hematopoietic tissue).

All cells need a nucleus for replication and maturation.  Red blood cells
also have a nucleus during their stages of development.  Since this occurs
in the bone marrow we do not normally see these nucleated red blood cells 
in the circulation.  As the red blood cell matures and is ready to leave
the bone marrow, it extrudes (spits out) its nucleus.

The reason for anucleated red blood cells in humans is best explained by
understanding its function.  Red blood cells contain hemoglobin which 
carries oxygen to every cell in the body.  In humans some of the smallest
blood vessels (capillaries) can be so narrow that a nucleated red blood
cell would have a difficult time passing through.  Even an anucleate red 
blood cell is larger (8um) than capillaries (2-3um).  The red blood cell's 
shape (a biconcave disc) can best accomplish this feat.

More information about human red blood cells can be obtained from any
Hematology textbook; found in hospital libraries, medical school
book stores and colleges offering clinical laboratory courses.