Sauropods are plant eating, long-necked dinosaurs, a group that includes the largest land animals ever to walk the earth. As much as sauropods permeate pop culture, their most striking feature, their long necks, remain mysterious. Why did nature select for such a long neck? Many assume that, like giraffes, the longer their neck, the higher food they could reach. However, this assumption is riddled with holes.
First, the blood pressure required to service a brain that high above the heart would push the limits of biology, perhaps requiring multiple hearts working in series or hardened veins to siphon blood up the neck. For comparison, healthy humans have a systolic blood pressure of about 120 torr. Giraffes: 180 torr, much higher to pump blood up a nearly two-meter neck. Sauropods, on the other hand, have to push blood through a vertical neck that is often over ten meters long, which would require blood pressure of approximately 600 torr! That presents a serious physics challenge that would require multiple hearts working in series or hardened veins, like plumbing pipes, to siphon blood up the neck. No evidence for such adaptations can be found in fossils or living animals.
Second, the torso of most sauropods is angled downward, with shorter front legs than back legs. Some scientists proposed that this simply shifts body weight toward the rear to facilitate rearing up on their hind limbs. However, we can rule out this unwieldy idea. The front limbs do not contain the micro-fractures that would be consistent with regularly returning to all fours.
Third, the sauropod neck does not appear to be very flexible. Computer modeling of its neck vertebra demonstrate that Diplodocus, if it strained to the limits of its skeleton, could not even raise its neck above parallel with the ground! These were clearly animals that browsed on low-lying plants. So, what were they reaching for?
Imagine what it’s like to be an animal that size. Weighing over a dozen tons, the stability of the ground they are standing on is a chief concern. Falling or getting stuck in the mud could be deadly. However, a long neck is an excellent way to get the head close to water or over treacherous ground while keeping the massive body on stable, dry land. Such a neck would have no need for flexibility, height, or ludicrous blood pressure. Perhaps long necks are simply an emergent property of being a land animal that large.
So that’s it, right? As compelling as this answer might seem, scientists don’t judge hypotheses by how satisfying they are. Ideas in science have power to the extent that they can be falsified and proven wrong. It is exceptionally difficult to falsify the idea that long necks evolved to span Paleozoic mud. There may be better solutions that no one has thought of yet. We must therefore tolerate ambiguity and accept only data-driven conclusions, because natural selection is far more creative than we.