The question of which dinosaur had the biggest brain is indirect because brain tissue does not survive fossilization. Paleontologists estimate brain size and shape by examining the endocranial cavity, the space left inside the skull. Determining the most intelligent dinosaur is not simply a matter of measuring the largest cavity volume, however. Researchers must use indirect measures and comparisons with living animals to understand the cognitive capacity of these extinct giants.
Absolute Brain Size: A Misleading Metric
If the question refers to the largest physical brain mass, the answer points toward immense dinosaurs like the long-necked sauropods (Brachiosaurus or Argentinosaurus) or large theropods like Tyrannosaurus Rex. These animals possessed the largest overall skulls, which naturally housed the biggest brain cavities. For instance, a Tyrannosaurus Rex brain cast suggests a mass of approximately 350 grams, larger than the brains of many modern reptiles.
The challenge with absolute size measurement is the sheer scale of the dinosaur’s body. A brain of several hundred grams is minuscule for an animal that weighed up to 70 tons. The massive body required a large portion of the nervous system simply to manage basic functions like locomotion and organ control. Therefore, a literal interpretation of “biggest brain” is scientifically irrelevant for assessing intelligence.
Measuring Intelligence: The Encephalization Quotient
To address the relationship between brain size and body mass, scientists use the Encephalization Quotient (EQ). The EQ measures relative brain size, calculated as the ratio between an animal’s actual brain mass and the brain mass expected for its body weight. A value of 1.0 means the brain size is exactly what is predicted for an average animal of that size. A score above 1.0 indicates a larger-than-expected brain.
Paleontologists determine the EQ components—brain size and body size—from fossil evidence. Brain volume is estimated by creating an endocast, a mold or digital reconstruction of the brain cavity inside the skull. Body mass is typically estimated by scaling up from the size of limb bones or using other skeletal measurements. The EQ provides a standardized way to compare the cognitive potential of extinct species with modern animals, such as crocodiles or birds.
The Dinosaurs with the Highest Relative Brain Size
When EQ is applied, the dinosaurs with the largest brains relative to their body size are found among the maniraptoriform theropods, the group that includes the ancestors of modern birds. The highest EQ value among non-avian dinosaurs is consistently attributed to the troodontids, a family of small, bird-like predators. Species once grouped under the name Troodon are estimated to have had an EQ as high as 5.8 in some early studies.
Other small, predatory dinosaurs like dromaeosaurids, often called “raptors,” also exhibited relatively high EQ scores. These small theropods had EQ values significantly higher than those of large, herbivorous sauropods and armored dinosaurs, which often scored well below 1.0. Although high for a dinosaur, these EQ values remained below that of modern humans (around 7.5) and slightly below the range of some modern birds. The high EQ suggests these smaller, active predators possessed the cognitive capacity for more complex behaviors than their larger, slower relatives.
Interpreting Dinosaur Brain Structure and Behavior
Beyond simple size, the shape and structure of the endocast offer specific clues about a dinosaur’s sensory and motor abilities. The brain cavity’s proportions reveal which brain regions were most developed, indicating sensory dominance. For example, large olfactory bulbs in the endocasts of Tyrannosaurus Rex and some hadrosaurs suggest that the sense of smell was highly important, likely used for hunting or locating food.
The cerebellum, which controls coordination and movement, and the optic lobes, responsible for processing visual information, also vary in size and shape across different groups. Endocasts of many active theropods show relatively well-developed cerebral and optic regions, suggesting keen eyesight and advanced motor control necessary for a predatory lifestyle. Analyzing the inner ear structures, which are closely associated with the braincase, can provide insight into a dinosaur’s hearing range and the angle at which it held its head. These structural details provide a richer picture of dinosaur behavior than relative brain size alone.