I recently took a class all about dinosaurs, and one topic was whether dinosaurs were cold-blooded, which has traditionally been the conventional view, or in fact warm-blooded. The evidence, it turns out, is mixed, and just learning about the different types of evidence is really interesting. One of the pieces of evidence listed stood out to me in particular: dinosaur blood pressure. I thought I would do a little research on this topic to find out more.
Basically, a warm-blooded (endothermic) metabolism requires high blood pressure to rapidly circulate the blood throughout the body. Endotherms have higher blood pressure than cold-blooded animals (ectotherms). So did dinosaurs have high or low blood pressure?
At first glance, this doesn’t seem particularly helpful. If we don’t know whether dinosaurs were cold or warm blooded, how would we know their blood pressure levels either? Fascinatingly, it is (somewhat) possible to answer this question!
Since one of the most critical functions of the heart is to pump blood to the brain, we can estimate dinosaur blood pressure based on the vertical distance between the dinosaur’s heart and brain. And we do have lots of fossils where we can measure this distance directly. Considering the extremely long necks of certain gigantic sauropods, this distance could be enormous. If we extrapolate blood pressures in dinosaurs from current animals, the pressures in sauropods would be so high that it is questionable how this was even possible without something rupturing or exploding.
However, the picture is further complicated by a separate but related debate about dinosaur posture. What did sauropods do with their enormously long necks? Did they keep them relatively low to the ground and horizontal when walking and feeding, or did they hold them much higher (closer to vertical) in order, for example, to reach higher vegetation on trees? The answer to this makes a huge difference when estimating blood pressure because it takes a much higher pressure to move blood up a vertical neck, against gravity, than sideways when the neck is kept low. The answer to this is we don’t know for sure, but there seems to be a lot of evidence in favor of the more vertical neck/ posture (from the structure of the bones in the neck), which leaves open the question of how exactly their bodies could pump blood with such high pressure to the brain.
One possibility is a gigantic heart to pump with more power, literally weighing several tons. This would probably be very inefficient, and the problem of such explosive high blood pressure would remain. Another alternative some have proposed are multiple smaller hearts in the neck to assist the primary heart. This isn’t physically impossible, but there doesn’t seem to be any evidence for this either. One other possibility would be using some sort of valves or muscle contractions in the neck, like living giraffes, to transport blood to the brain (although a sauropod neck is much, much longer than a giraffe’s neck).
Paleontologists like Robert Bakker in the 1970s and more recently Michael Habib argue something along these lines. Bakker suggested sauropods use neck muscle contractions to pump blood up the neck. Habib agrees and points to large neck bones called cervical ribs that are actually a very flexible form of bone, and can act as springs. This could have allowed a sauropod with a very long neck to move the blood without an impossibly high blood pressure.
This idea makes sense to me. These sauropod necks that were 30 or 40 feet long must have had extremely powerful muscles. From an evolutionary standpoint, why not co-opt these muscles into more than one function, and use them to assist the heart with pumping blood? Of course like many hypotheses and arguments with dinosaurs, this idea is also not without criticism.
There are other interesting related questions involving blood pressure that would never have occurred to me. For example, no matter what the mechanism to pump blood with such high force to the brain (whether a gigantic heart or assistance from neck muscles), when a sauropod did lower its head to the ground (e.g. to drink water), how did it prevent a huge rush of blood to its head? Modern day giraffes have evolved a web of little arteries that solves this problem, but it’s not clear whether dinosaurs had something similar.
There is a lot of speculation and unanswered questions about dinosaurs’ circulatory system. Unlike the skeleton, it didn’t fossilize and there’s no direct evidence of things like the size of the heart or the structure of arteries. But it’s an interesting area with vigorous debate because these were real problems that had to be addressed, and sauropods managed to address them one way or another!
How Long-Necked Dinosaurs Pumped Blood to Their Brains
The Evidence for Endothermy in Dinosaurs
Of Barosaurus and Blood Pressure
Bakker, Dinosaur feeding behaviour and the origin of flowering plants, Nature