In response to:

Our 86 Billion Neurons: She Showed It from the November 24, 2016 issue

To the Editors:

I would like to provide some perspective on the numerology of neurons as expressed by Steven Mithen in his review of Suzana Herculano-Houzel’s book The Human Advantage [NYR, November 24].

First—unlike physics, in which constants such as the speed of light can be determined with an accuracy of one in a billion, biological systems are characterized by a high degree of variability. Thus, brain size and number of neurons in most species vary by a factor of two. Thus, the 86 billion nerve cells counted by Herculano-Houzel’s method is not a universal hallmark of Homo sapiens but an average of the brains of four elderly Brazilian men.

Second—the adult male brain is about 150 grams heavier than the female one. For the neocortex, responsible for perception, memory, language, and reasoning, this disparity translates to about 23 billion neurons for men versus 19 billion for women; yet there is no difference in their average IQ.

Third—the relationship between intelligence and number of neurons is weak within and across species. Thus, the neocortex of the long-finned pilot whale contains an estimated 37 billion neurons, twice as many as the human neocortex. Consider honeybees for an even less mammalian-centric point of view. They recognize faces, communicate the location and quality of food sources to their sisters via the waggle dance, and navigate complex mazes with the help of cues stored in short-term memory. Yet they do this with fewer than one million neurons. Are we really 86,000 times smarter?

Christof Koch
President and Chief Scientific Officer
Allen Institute for Brain Science
Department of Biology
University of Washington
Seattle, Washington