Cerebral blood flow rates in recent great apes are greater than in <i>Australopithecus</i> species that had equal or larger brains

Abstract

Brain metabolic rate (MR) is linked mainly to the cost of synaptic activity, so may be a better correlate of cognitive ability than brain size alone. Among primates, the sizes of arterial foramina in recent and fossil skulls can be used to evaluate brain blood flow rate, which is proportional to brain MR. We use this approach to calculate flow rate in the internal carotid arteries <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>(</mml:mo><mml:msub><mml:mrow><mml:mover><mml:mi>Q</mml:mi><mml:mo>˙</mml:mo></mml:mover></mml:mrow><mml:mrow><mml:mrow><mml:mi>ICA</mml:mi></mml:mrow></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:math>, which supply most of the primate cerebrum. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mover><mml:mi>Q</mml:mi><mml:mo>˙</mml:mo></mml:mover></mml:mrow><mml:mrow><mml:mrow><mml:mi>ICA</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:math> is up to two times higher in recent gorillas, chimpanzees and orangutans compared with 3-million-year-old australopithecine human relatives, which had equal or larger brains. The scaling relationships between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mover><mml:mi>Q</mml:mi><mml:mo>˙</mml:mo></mml:mover></mml:mrow><mml:mrow><mml:mrow><mml:mi>ICA</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:math> and brain volume (<i>V</i>_br) show exponents of 1.03 across 44 species of living haplorhine primates and 1.41 across 12 species of fossil hominins. Thus, the evolutionary trajectory for brain perfusion is much steeper among ancestral hominins than would be predicted from living primates. Between 4.4-million-year-old <i>Ardipithecus</i> and <i>Homo sapiens</i>, <i>V</i>_br increased 4.7-fold, but <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mover><mml:mi>Q</mml:mi><mml:mo>˙</mml:mo></mml:mover></mml:mrow><mml:mrow><mml:mrow><mml:mi>ICA</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:math> increased 9.3-fold, indicating an approximate doubling of metabolic intensity of brain tissue. By contrast, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mover><mml:mi>Q</mml:mi><mml:mo>˙</mml:mo></mml:mover></mml:mrow><mml:mrow><mml:mrow><mml:mi>ICA</mml:mi></mml:mrow></mml:mrow></mml:msub></mml:math> is proportional to <i>V</i>_br among haplorhine primates, suggesting a constant volume-specific brain MR.