[not in handout, see intranet]

  Number of neurons in the nervous system

1,000,000,000,000    gifHomo sapiens (maybe 1014)
350,000,000,000    gifChimpanzee
100,000,000,000    gifRhesus monkey
500,000,000    gifMouse
300,000,000    gifOctopus
50,000,000    gifStickleback (small fish)
850,000    gifHoney bee
250,000    gifFruitfly
20,000    gifSea slug (Aplysia)
381    gifThread worm male
302    gifThread worm (hermaphrodite)

*NB* for numbers over 20,000 these numbers are usually speculative estimates. The figure of 1012 or 1 billion for the human brain is an old one, but it serves for a rough comparison with the chimpanzee and rhesus monkey.

In general, number of neurons is correlated with brain weight or volume, but not exactly, as larger animals have larger neurons.

An internet site listing some brain weights is at http://faculty.washington.edu/chudler/facts.html.