Scientific American - September 2018

September 2018, ScientificAmerican.com 63

Cat "¹ ́‘žŠ ́ ́ym

ȟ ̈¹ïĀ›D ̈y

Human $D`DÕùy  ̈yțD ́ï

Cerebral

cortex

yàyUàD ̈`¹àïyā

Cerebellum

yàyUy ̈ ̈ù®

CerCerebeebellallarr

neuron count

unknown

CerCerebeebellallarr

neuron count

unknown

 ̈yțD ́ï "¹ ́‘žŠ ́ ́ymȟ ̈¹ïĀ›D ̈y Human ›Ÿ®ÈD ́Ćyy $D`DÕùy Cat

Circle area shows

number of neurons

ÀUŸ ̈ ̈Ÿ¹ ́

ÀĈUŸ ̈ ̈Ÿ¹ ́

‹ĈUŸ ̈ ̈Ÿ¹ ́

0 12345601234567891011

Chimpanzee Human

Dà ̈ĂŸ ́†D ́`Ă "DïyŸ ́†D ́`Ă ùÿy ́Ÿ ̈y

021 34

0

10

20

30

40

50

60

70

80

90

100

Macaque

‘yÊĂyDàåË

0yà`y ́﹆mù ̈ï

àDŸ ́<¹ ̈ù®y

0ùUyàïĂ

D`›m¹ïàyÈàyåy ́ïå 5ày ́m ̈Ÿ ́y

¹ ́yåùU¦y`ïŸ ́DåïùmĂ

September 2018, ScientificAmerican.com 63

BRAIN VS.

BODY SIZE

Humans have a large brain

compared with its expected

dimensions for their body mass.

The encephalization quotient (EQ), as

it is known, is 1 if the brain/body mass

ratio meets expectations. Humans have

D ́1¹†é~è1冹à ̈¹ ́‘žŠ ́ ́ymȟ ̈¹ï

whales are 2–3; elephants are 1–2;

macaques are 2; and

cats are 1.

NEURON
NUMBER
A much scrutinized measure
of brainpower has to do with the
number of an animal’s neurons—and
where they are located. Humans have
more neurons in the cerebral cor tex,
16 billion, than almost all other
mammals, although the long-
Š ́ ́ymȟ ̈¹ïĀ›D ̈y
has more.

PAC K I NG

IN THE BRAIN

CELLS

In humans, the cerebral cortex

makes up 82 percent of the brain’s

mass but contains only 19 percent

of the total neurons, whereas the

cerebellum holds 80 percent

or so of the neurons but only

occupies 10 percent

of its mass.