7.2: Primate Brain
- Page ID
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Mammals have relatively large brains for their body size (chapter 6). When scientists take body size into account, however, by calculating the ratio of brain size to body mass for each species (termed relative brain size) some species have larger brains than would be expected from their body sizes. But strikingly, primates have much larger brains for their body size than most other mammals (Figure below).
This finding intrigued researchers because brain tissue is known to be metabolically costly. Basal metabolic rate (BMR) is the rate of energy expenditure by an organism at rest at a non-stressful temperature. Scientists have also calculated the cerebral (brain) metabolic rate and shown that whereas other mammals use 2-6% of their BMR on brain maintenance, most primates use 9-14%. The most advanced primates, humans, use a staggering 20%. If the primate brain has evolved to be large in spite of these costs, the increases in brain tissue must confer important advantages.
So why can’t primates speak the way humans do? Part of the answer is that we are bipedal, which has changed the way our voice box sits in our throat. Another reason is the shape and size of primate brains. Humans have much larger brains, with larger frontal lobes and areas associated with speech and communication (Broca’s area, for example). Many great ape species have been able to learn human communication in the form of American Sign Language (ASL). Washoe, Dar, Louslis, Mocha, and Tatu (chimpanzees) and Koko (a gorilla) were all fluent in ASL. Washoe even taught her adopted son, Loulis, how to sign without any help from humans! Kanzi (a bonobo) understands spoken English and can us a lexigram computer program to communicate with humans. (Etheridge-Criswell, 2018).