This paper, published recently in the journal PLoS ONE, tells of how some researchers in Belgium took a cast from the inside of a dinosaur skull, to learn some things about its brain.
Amurosaurus was a hadrosaur – a duck-billed dinosaur – that was alive in the late Cretaceous period approximately 70 million years ago. It lived in East Asia, where today the Amur river divides Russia and China. We know from the fossil remains that it was a large herbivorous dinosaur, with a crest on its head. It probably weighed between 2 and 4.8 tons. But today, when we can even figure out what colour some dinosaurs might be, what can we say about their most elusive aspect – their behaviour?
After taking a silicon rubber cast of the brain shape, the researchers calculated the encephalization quotient (EQ) for Amurosaurus. This is used to estimate intelligence among living and extinct animals.
It’s calculated by comparing the size of the brain to the size of the body. However, the basic brain-to-body size ratio doesn’t give very useful results for small and large animals because small animals tend to have larger brains (compared to body size) than big animals. Perhaps it takes the same amount of brain matter to co-ordinate functions such as breathing, regardless of the size of the animal. If we tried to predict intelligence using the basic ratio, mice and humans would have roughly the same result, small birds appear to be geniuses, and elephants really lose out because of their giant bulk.
The EQ corrects for this. It says, with what we know about animals, what brain size would we predict for an animal of this size? An EQ of 1 means the animal has exactly the brain size that would be expected. An EQ lower than 1 means a relatively small brain, but if the animal turns out to have an EQ bigger than 1, it has a larger brain than you’d expect, and it might be more intelligent or have other specialised brain functions.
EQ scales vary, so you can use them to compare all reptiles, or all mammals, or all primates. For instance, a primate such as a DeBrazza’s monkey might have a high EQ when compared to all mammals, but they’re pretty unremarkable when you put them in the context of other primates.
A 2012 paper comparing EQs across mammalian species concluded that dolphins have an EQ of around 4, whereas Lagomorphs (the order containing rabbits) aren’t that bright and end up with an average of 0.77. Us humans, always outliers when it comes to brains, have an EQ of 5.72.
Of course, it isn’t perfect. EQ has been criticised for not taking into account things like different brain shapes (particularly folding of the cortex) that are known to influence intelligence. But it’s often all we have to go on in fossilised animals.
Back to Amurosaurus. Depending on how you estimate the hadrosaur’s weight, Amurosaurus had an reptilian EQ of 2.3 – 3.8. This is not bad for a plant-eating dinosaur. Most of the big sauropods and ceratopsians we know and love have much lower EQs (Diplodocus gets 0.53-0.69; Triceratops 0.7). The therapods such as Tyrannosaurus and Allosaurus had bigger brains, as you might expect from an animal that has to execute a hunting strategy in order to eat.
So what did the hadrosaurs do with their larger-than-average brains? If you look at the different brain regions, the Amurosaurus brain has a relatively large cerebrum.
From the Belgian paper: “The presence of an enlarged brain and cerebrum relative to body size is usually equated with increased behavioral complexity in vertebrates.”
The authors suggest that Amurosaurus was a social animal, living in large groups. Social interactions such as recognising members of your herd, maintaining a hierarchy and mate selection mean that those who live in groups usually need larger brains to deal with all this. They also tend to display more of the characteristics we think of as “intelligence”. Fossil Amurosaurus (and other hadrosaurs) have been discovered in large groups, adding weight to this theory.
So, using a cast of the inside of a skull, and the EQ calculation, we can make inferences about the lifestyle of an animal that lived 70 million years ago. Amurosaurus won’t win Mastermind, and is not even particularly unusual among dinosaurs, but I like to think of them grazing in the Cretaceous swamp, sorting out the pecking order just like chickens do today.
Links and papers:
- Dinosaur encephalization quotients. Evans, D.C. 2005. New evidence on brain−endocranial cavity relationships in ornithischian dinosaurs. Acta Palaeontologica Polonica 50 (3): 617–622
- 2012 paper on mammal encephalization across taxa; explains the formula for EQ and has interesting discussion. Paywall.
- The evolution of the social brain – social system correlated with encephalization.Susanne Shultz, R.I.M Dunbar 2004. The evolution of the social brain: anthropoid primates contrast with other vertebrates. Proc Biol Sci.; 274(1624): 2429–2436.
- Jerison is credited with coming up with the idea of the encephalization quotient – here’s a paper from 1985. Jerison HJ. 1985. Animal intelligence as encephalization [and discussion]. Philos Trans R Soc Lond B Biol Sci. 1985 Feb 13;308(1135):21-35.
- Description of Amurosaurus. Godefroit, P., Bolotsky, Y.L., and Van Itterbeeck, J. 2004. The lambeosaurine dinosaur Amurosaurus riabinini from the Maastrichtian of Far Eastern Russia. Acta Palaeontologica Polonica 49 (4): 585–618