The popular idea that some dinosaurs possessed a “second brain” in their hindquarters is a misconception. While certain large dinosaurs did have a spacious pelvic cavity, it did not house a brain.
The “Second Brain” Myth
The myth of dinosaurs having two brains originated in the 19th century with paleontologist Othniel Charles Marsh. He observed an enlarged neural canal in the pelvic region of dinosaurs like Stegosaurus and Camarasaurus. Marsh theorized this cavity might have contained a “posterior braincase” to help manage their immense size and complex hindquarter movements, given their relatively small cranial brains. This hypothesis, suggesting an auxiliary control center, gained widespread traction in popular culture. Despite paleontologists abandoning this idea due to lack of evidence, the “butt brain” concept persisted.
The Sacral Enlargement Explained
The enlarged space in the pelvic region of dinosaurs, once thought to be a “second brain,” is now understood as a sacral enlargement. This anatomical feature is a widened area of the spinal cord where nerve bundles are concentrated. Modern animals, particularly birds—the closest living relatives of dinosaurs—possess a similar structure called a glycogen body.
The glycogen body in birds is an oval structure located in the lumbosacral region of the spinal cord, composed of specialized cells rich in glycogen. While its precise function remains an area of ongoing research, scientists propose it may play roles in providing energy to the nervous system, aiding in balance, or coordinating complex movements of the hind limbs and tail. For dinosaurs like Stegosaurus, this enlargement likely facilitated the intricate coordination required for their massive hindquarters and powerful tail, perhaps even assisting in the swift movement of their thagomizer, or tail spikes, for defense.
Dinosaur Brains and Behavior
Dinosaur brains were, like all vertebrates, situated within their skulls. Many dinosaurs possessed brains that were relatively modest compared to their overall body mass. For example, Stegosaurus had a brain often compared in size to a lime, quite small for an animal weighing several tons. Sauropods, despite their colossal size, also had relatively small brains.
Understanding dinosaur intelligence relies on examining mineral infillings of their brain cavities, known as endocasts, and the shapes of these cavities. However, brain size alone does not definitively indicate intelligence, as the brain might not completely fill the skull cavity. Recent research suggests that the cognitive abilities of most non-avian dinosaurs were comparable to modern reptiles like crocodiles and lizards. Some smaller carnivorous dinosaurs, like Troodon, had proportionally larger brains for their body size, indicating a higher level of intelligence, possibly akin to modern flightless birds like cassowaries.