Encephalic Arterial Canals and Their Functional Significance

A fundamental question in evolutionary anthropology asks how the human brain evolved. Characterized as relatively large and energetically taxing, numerous hypotheses have been proposed to explain how the human brain has evolved to its current size through tradeoffs to improve fitness by increasing behavioral complexity while minimizing caloric costs. The comparative method has been a key approach to testing these hypotheses, but a major hinderance has been the lack of directly measured brain metabolic rates for many comparable species. This dissertation takes an anatomical approach to predict brain metabolic rates from osteological specimens, utilizing the following proposed relationships: 1) that the brain is supplied by arteries (encephalic arteries) which travel through bony canals, 2) the size of the canal reflects the size of the artery within, 3) the size of the artery is proportional to blood flow rate, and 4) encephalic blood flow rate is proportional to brain metabolic rate.

Radii of encephalic arterial canals of a growth series of humans (n= 305 individuals) and of adult mammals (n=329 species) were measured from cadaveric computed tomography scans and osteological specimens, and blood flow rates were predicted using anatomical and physiological equations previously published in the literature. The major goals of this dissertation were to better evaluate the use of encephalic arterial canals in the prediction of brain metabolism and to characterize how mammals vary in their brain size, encephalic blood flow rates, brain metabolism, and whole body metabolism to test hypotheses which have been proposed to explain human and primate brain evolution.

The first research chapter finds that the blood flow rates predicted from the sizes of the encephalic arterial canals tracks the changes in brain metabolism during human growth. The second research chapter finds that patterns of variation in predicted blood flow rate, brain size, and body size across primates and other mammals suggest that predicted blood flow rates are reflecting the metabolic substrate supply needs of the brain. Furthermore, evidence is presented that the relative metabolic rate of the brains of primates is lower than many mammals of comparable brain size. The third research chapter utilizes phylogenetically informed Bayesian methods to predict brain metabolic rates from predicted blood flow rates, and finds that humans devote a high (although not always the highest) proportion of their basal metabolic rate (BMR) and total energy expenditure (TEE) to brain metabolism, even compared to other primates. In turn, primates tend to have elevated predicted brain metabolic rates relative to their BMR and TEE compared to most other mammal groups.

Combined, the evidence presented within this dissertation suggests that 1) osteologically derived predictions of blood flow rates present a viable alternative to understanding patterns of variation in brain metabolic rates during human ontogeny and among mammalian samples, and 2) compared to many other mammals, humans and other primates have evolved physiological mechanisms to reduce the mass-specific metabolic rate of their relatively large, energy-hungry brains.