Brain size (or its proxy, encephalization quotient) usually varies allometrically with mass -- larger creatures need larger brains to control their larger bodies, apparently.

Dinosaurs are popularly known as small-brained. While this certainly doesn't hold in general -- theropods and especially birds were particularly intelligent -- but sauropods and thyreophorans (ankylosaurs and stegosaurs) seem to have preposterously small brains. How were they able to control their large bodies with such small brains? Are there modern animals with similar proportional brain size, and if so does that give any clues as to how these dinosaurs functioned?

  • $\begingroup$ I would like to know why a small brain cannot control a big body, if that is the case. A small brain would be so much less metabolically expensive. You'd think that there would be a strong selection pressure to have the smallest brain that could do the job. So a lion should have the same size brain as a house cat, and so on. $\endgroup$ Nov 4 at 3:38

I don't think there is a concrete answer, because we don't really know the minimum size of a vertebrate brain in relation to body size. Many vertebrates, including many fish, get along fine with tiny brains in large bodies.

For many decades, scientists speculated for this very reason of small brain size, that dinosaurs seemed to have an extra brain, or ganglion, at the hip. Especially sauropods and thyreophorans appear to have had a widening of the spinal cord cavity around the sacral area suggesting this possibility. This pelvic ganglion or "hip brain" was then presumed to be responsible for running the hind legs and tails. However, this idea has not gained any further evidence or traction in the scientific community and is generally dismissed.

  • $\begingroup$ I too had read this theory (bulged portions in spinal chord) in certain science magazine; which concluded dinosaurs were dependent on much more on quick reflex than thought-activities. $\endgroup$ Oct 1 '16 at 12:33
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    $\begingroup$ Interesting theory. Can you please add some references to your answer? $\endgroup$ Oct 1 '16 at 15:16
  • $\begingroup$ FWIW I've heard the plausible conjecture that the lower limit of brain mass is roughly the mass of the spinal cord (i.e., the brain + cord is at least roughly twice the cord). $\endgroup$
    – Charles
    Oct 1 '16 at 22:31
  • $\begingroup$ A good modern point of comparison in modern animals is the koala. The giant pandas are likely headed in the same direction, too. It seems that the lack of a need to process complex sensory information to acquire food or avoid predators, the nutritional inefficiency of the diet, and the fact that the number of systems isn't increasing, just their size, is also relevant. Do we have coprolites from these dinos to see how well digested their food was? $\endgroup$
    – Sean Lake
    Oct 19 '16 at 16:51
  • $\begingroup$ Well, as I said, there is not really any scientific traction for the "pelvic ganglion" theory anymore, so there is not much documentation to find other than some speculative papers. Dinosaurs are now generally presumed to be simply small-brained vertebrates for the most part. $\endgroup$ Oct 29 '16 at 6:29

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