Stegosaurus, with its distinctive plates and spiked tail, is one of the most recognizable dinosaurs from the Jurassic Period. For over a century, this massive herbivore has been defined by two biological concepts: the surprisingly small size of its actual brain and the notion that it possessed a secondary brain located near its hips. Modern paleontological research has clarified the anatomy of this ancient animal, replacing old assumptions with a detailed understanding of its nervous system and ecological success.
The True Size of the Stegosaurus Brain
The idea that Stegosaurus had a brain the size of a walnut is an exaggeration, though the core concept of a disproportionately small brain is accurate. Paleontologists in the 1880s, such as Othniel Charles Marsh, first determined the size of the brain by creating an endocast, a mold of the dinosaur’s cranial cavity. This technique, now supplemented by advanced Computed Tomography (CT) scans, confirmed the brain was exceptionally small relative to its immense body mass.
For an animal that could weigh up to five metric tons and reach nine meters in length, the brain itself weighed only around 70 to 80 grams. This small organ was closer in size to a dog’s brain or a lime, representing one of the lowest brain-to-body ratios among all known dinosaurs. Scientists quantify this disparity using the Encephalization Quotient (EQ), which compares an animal’s actual brain size to the expected size for its body weight.
Stegosaurus registers an exceptionally low EQ, often estimated around 0.2, placing it near the bottom of the scale among dinosaurs. This low score suggests that very little brain mass was available for complex cognitive functions beyond basic survival requirements. The small size led early researchers to believe the animal could not function efficiently without additional neurological assistance, contributing to the “second brain” myth.
What Was the “Second Brain” Really For?
The “second brain” myth originated with the discovery of a large enlargement in the neural canal, located in the sacral (hip) region of the vertebrae. This cavity, which houses the spinal cord, was substantially larger than the cranial brain, leading Marsh to characterize it as a “posterior brain case.” It was theorized that this enlargement served as a secondary nervous center to coordinate the movements of the massive hind limbs and the powerful spiked tail, known as a thagomizer.
This hypothesis is not supported by modern anatomy and has been largely dismissed. The sacral space almost certainly did not contain a brain, which is defined by a dense concentration of neurons used for complex thought. If it were a brain, it would require the specialized tissues found in the skull, but there is no evidence for this.
The scientific consensus now points to this enlargement housing a structure called the glycogen body. This organ, also present in modern birds and some reptiles, is a mass of tissue that stores glycogen, a form of energy readily available to supply the nervous system. This large energy store was likely necessary to power the substantial muscles of the hind legs and tail, which required an immediate source of fuel.
The cavity may have also contained a dense concentration of nerves acting as a neural relay station. This nerve bundle could have managed simple reflexes and basic motor control for the posterior body, allowing for rapid, localized reactions without signals traveling to and from the small cranial brain. The function, therefore, was mechanical and metabolic, not cognitive.
Rethinking Stegosaurus Intelligence
While Stegosaurus possessed a small brain, its long tenure on Earth suggests its cognitive abilities were perfectly suited for its environment. Survival for millions of years is a testament to successful adaptation, irrespective of low EQ scores. Modern paleontology has shifted focus away from raw brain volume as the sole measure of an animal’s behavioral success.
Scientists now analyze factors such as brain organization and inferred complex behaviors to understand how Stegosaurus thrived. The bony plates along its back, once thought to be purely defensive, are now widely believed to have served as structures for display, species recognition, or thermoregulation. These uses suggest a level of visual processing and social interaction.
The animal’s nervous system was functional and efficient for its lifestyle as a low-browsing herbivore. It required a brain capable of managing basic sensory input, such as detecting predators and finding food, and controlling the defensive swing of its spiked tail. The small brain size reflects that Stegosaurus did not need the complex problem-solving abilities or advanced social structures seen in dinosaurs with higher EQ values.