Large vs. tiny vascular channels in tetrapods

Huttenlocker and Farmer 2016
have tied red blood cell (RBC) size to vascular diameter in the bones of fossilized tetrapods in part to determine when greater aerobic capacity evolved… something we actually already know based on stance, extant homologs, etc. Their conclusions support tradition and are broadly applicable (Fig. 1). This appears to be a lot of work with a large resulting dataset.

The Huttenlocker and Farmer abstract reports
“We find that several fossilizable aspects of bone microstructure, including the sizes of vascular and lacunar (cellular) spaces, provide useful indicators of RBC size in tetrapods.”

The Huttenlocker and Farmer graphic
show that turtles are about like amphibians in vascular and RBC size. (Fig. 1). It also indicates that, compared to extant mammals, cynodonts in the earliest Triassic sometimes had smaller vascular channels and RBCs, sometimes had similar RBCs and sometimes had larger RBCs. However, the chart also indicates that the pigeon has larger RBCs than a duck. Alligators and crocodiles are different from one another and two congeneric lizards are different from one another in RBC size, but not by much.

Results:
The shrew and only one of the rabbits appear to have the smallest RBCs, based on the figure 1 graphic. Based on the figure 2 graphic, the pigeon has more of the smallest vascular diameters.

A few questions arise:

1. Bone cross-sections in some warm-blooded mammals are similar to some cold-blooded reptiles if the largest channels are not considered. Does this mean anything?
2. White blood cells are often much larger than red blood cells. How does that work in the smaller channels?
3. How does ontogeny, especially degree of development at birth, affect RBC size and metabolism? Perhaps that’s the next step in this study.
4. How does body size alone correlate with RBC size?
5. How does relative level of activity alone correlate with RBC size? Perhaps adding sloths and hummingbirds would be appropriate. Perhaps sedentary vs, athletic humans, too.
6. Do active (warm-blooded) fish and sea turtles have smaller RBC sizes than passive fish and desert turtles?

Interesting study.

References
Huttenlocker AK and Farmer CG 2016. Bone microvaculature tracks red blood cells zip diminution in Triassic mammal and dinosaur forerunners. Current Biology online here.