red blood cells

Maybe somebody knows the answer to this. Why do some animals, like birds, have red blood cell that are nucleated, and others, like mammals, do not?

According to EB:
mammalian red (blood) cells lack nuclei, an adaptation that makes them even more efficient in oxygen transport.

Although, IIRC, camels have nucleated RBCs...

quote:

mammalian red (blood) cells lack nuclei, an adaptation that makes them
even more efficient in oxygen transport.

I'm pretty sure birds blow away mammals when it comes to aerobic performance; the oxygen demands of flight must be quite enormous. Is this due to their "flow-thru" lung structure, or to nucleated RBCs? I've never heard anything regarding camels, but it would be interesting if true.

Dang i thought this was gonna be fun. I read nuCleated as nuDeated.Thought I'd found a new kink.

Another source implies the lack of a nucleus is need to keep the RBC flexible, so it can enter the capillaries.

Oh yeah, to nitpick: IIRC, the proper term for the un-nucleated variety is red blood corpuscle. The lack of a nuclei means it's not technically a cell.

Another source implies the lack of a nucleus is need to keep the RBC flexible, so it can
enter the capillaries.

But if birds are more aerobically efficient (and I'll take debate on this) --their nucleated RBCs are entering their capillaries just fine. My OP is WHY nucleated; I'm sorry if that wasn't clear. I think I've heard reptiles (and maybe amphibians, I don't know about them) also have nucleated RBCs. How come birds are so metabolically active/efficient and lizards not?

[John W. Kennedy]

Because evolution is not an engineer.

Lizards have a low metabolism because that's a good survival strategy -- you hardly have to eat at all, so most of the time you can stay concealed.

Birds have a high metabolism because that's a good survival strategy -- you can outrun things you're trying to eat, or that are trying to eat you.

Mammals have unnucleated RBC's because the mutation occurred in an early mammal or proto-mammal, and it worked well.

Birds don't because the mutation never happened in birds, or because the bone-marrow RBC-manufacturing process necessary for unnucleated RBC's carried too much of a weight penalty.

------------------
John W. Kennedy
"Compact is becoming contract; man only earns and pays."
-- Charles Williams

[mr john]

Every time I think of something and return there is John K.already posted it. Birds are closer to reptiles than mamals are, and still have the 'older' 'reptilian' cells.