A human needs a large hole in the head – to be smart

The intelligence of animals can be estimated by the size of the holes in the skull which the arteries pass through, according to novel research by biologists at the University of Adelaide.

The scientists found that the connection between intelligence and hole size results from brain activity being related to brain metabolic rate – and this can be estimated from the size of the arteries that supply the brain with blood.

“Arteries continually adjust their diameter to match the amount of blood that an organ needs by sensing the velocity next to the vessel wall. If it is too fast, then the artery grows larger, too slow and the artery shrinks,” project leader Professor Emeritus Roger Seymour said. “If an artery passes through a bone, then simply measuring the size of the hole can indicate the blood flow rate and in turn the metabolic rate of the organ inside.”

Seymour, and a former honours student, Sophie Angove, measured the ‘carotid foramina’ which allow passage of the internal carotid arteries servicing the brain in primates and marsupials and found large differences.

He said it was not known how humans evolved to this state because direct measurements of brain metabolic rate had not been made in living monkeys and apes. But the research found it was possible to estimate brain metabolic rate from the size of the arteries that supply the brain with blood.

In a paper published online in the latest issue of the Journal of Experimental Biology, the scientists say the evolution of primates involved increasing body size, brain size, and presumably cognitive ability.

“Cognition is related to neural activity, metabolic rate and blood flow rate to the cerebral cortex. These parameters are difficult to quantify in living animals [but] this study shows that it is possible to determine the rate of cortical brain perfusion from the size of the internal carotid artery foramina in skulls of certain mammals, including... primates and marsupials,” the report states.

Calculating blood flow rate

The scientists calculated the combined blood flow rate in both internal carotid arteries as a proxy of brain metabolism for 34 species of primates and compared this with the same analysis for 19 species of marsupials.

“During the course of primate evolution, body size increased from small, tree-dwelling animals, through larger monkeys and finally to the largest apes and humans,” Seymour said.

“Our analysis showed that on one hand, brain size increased with body size similarly in the two groups. On the other hand, blood flow rate in relation to brain size was very different, with the relative blood flow rate increasing much faster in primates than in marsupials.”

The significant result was that blood flow rate and presumably brain metabolic rate increased with brain volume much faster than expected for mammals in general, Seymour said. By the time of the great apes, blood flow was about 280% higher than expected.

“The difference between primates and other mammals lies not in the size of the brain, but in its relative metabolic rate. High metabolic rate correlates with the evolution of greater cognitive ability and complex social behaviour among primates.”

Seymour said the human brain contained nearly 100 billion nerve cells with connections measured in the trillions. Each cell and connection used a minute amount of energy but, added together, the whole brain used about 20% of a person’s resting metabolic rate.

Primate brain evolution

In their report in the journal, the researchers say the” trajectory of primate brain evolution” appears to have been determined before the great apes arrived.

“To facilitate the increasing brain size, an increase in energy intake or decrease in energy expenditure on other organ development may have occurred. [Among the apes], including human and chimpanzee, a dietary change from herbivorous to omnivorous resulted in consumption of more energy-dense foods.

“This shift may have not only increased energy intake without the need to increase foraging time, but also decreased the energy needed for digestive organs. The invention of cooking, which essentially softens and pre-digests food outside the digestive cavity, may have further reduced energy requirements of digestion, allowing for a reduction in the size of the gastrointestinal tract.”

The report says that although humans have a large brain in relation to their body mass, cerebral blood flow or perfusion to the brain appears to be simply another step in primate evolution “and is unexceptional”.

“Whether primates differ from mammals in general is not known, because we are unable to gauge brain perfusion in other groups with the technique of this study.”

Seymour told University World News that he and Angove had tried to estimate blood flow in the brains of birds to get a measure of their intelligence. But the foramen – the hole at the base of the skull in animals – was hidden in the bird skull. To measure this would have had to be done with micro-CT scanning and was not possible at this time “but we may ultimately do it”.