The Stegosaurus, with its distinctive plates and spiked tail, remains one of the most recognizable dinosaurs. Its brain size, however, often sparks curiosity and has led to misconceptions about its cognitive abilities.
The Actual Brain Size
Despite its substantial body, the Stegosaurus possessed a remarkably small brain. Estimates suggest its brain was roughly the size of a lime or a dog’s brain, not quite as diminutive as the commonly cited walnut comparison. This organ weighed approximately 80 grams (2.8 ounces). For an animal that could reach up to 9 meters (30 feet) in length and weigh over 4.5 metric tons (5 short tons), this brain size represents an exceptionally low brain-to-body mass ratio. A human’s brain-to-body ratio is considerably larger, approximately 500 times greater than that of a Stegosaurus.
The “Second Brain” Concept
For many years, a popular theory suggested that Stegosaurus had a “second brain” located in its pelvic region. This idea emerged from observations by early paleontologists, who noted a significant enlargement in the spinal canal near the dinosaur’s hips. Scientists initially hypothesized this enlargement might house additional nervous tissue, allowing the creature to better control its massive hind limbs and tail. However, modern scientific understanding has largely debunked the notion of a true “second brain.” This enlarged space in the spinal column is now understood to be an area for nerve bundles, or potentially a glycogen body.
How Stegosaurus Functioned
The relatively small brain of Stegosaurus, coupled with the absence of a true “second brain,” indicates it likely did not possess complex intelligence. Nevertheless, its neural processing system was sufficient for its survival in the Late Jurassic period. Its primary defenses were its considerable size, the large armored plates along its back, and the formidable thagomizer—four sharp spikes on its tail used for protection against predators like Allosaurus. Stegosaurus was a herbivore, feeding on low-lying vegetation, a dietary habit that did not demand extensive cognitive abilities. Its relatively slow movements, a consequence of its anatomy with shorter forelimbs compared to its hindlimbs, also aligned with a less neurologically demanding lifestyle.
Studying Ancient Brains
Paleontologists estimate the brain size of extinct animals such as Stegosaurus by examining endocasts. These are natural or artificial molds created from the internal cavity of fossilized skulls. By analyzing these endocasts, researchers can gain insights into the general shape and volume of the brain. While endocasts provide valuable information about the physical dimensions of the brain, they do not directly reveal details about neural complexity, the number of neurons, or the animal’s overall intelligence. Modern techniques, including CT scans, further enhance the ability to study the internal structures of fossilized remains, offering a more detailed, non-destructive view of ancient brain cavities.