Tuesday, September 10, 2013

How are brain mass (and neurons) distributed among humans and the major farmed land animals?

Summary: I estimate, for humans and the most common domestic land animal livestock populations, their absolute and relative contributions to the total brain mass of the combined category. Humans make up the vast majority of the aggregate of brain mass for these populations, followed by cows. Chickens account for less than 1% of the total neural tissue of humans and domestic land animal livestock. As a proportion of the total number of neurons across these populations, cows and chickens are closer, due to the relatively high neuron density of chickens and humans, while the human proportion is higher.

AnimalGlobal populationBrain mass (g)Total grams of nervous systemPercentage of human and farmed land animal brain mass.

This post does not address farmed marine animals, because data quality and conceptual issues are less clear than for land animals (the role  of wild-caught fish, etc), although I may address that at a later time. Population figures come from the UN Food and Agriculture Organization and are from 2011.

At the time of this posting the United States Census Bureau World Population Clock gave an estimate of the world human population at 7,110,521,293.

For brain mass, this table provides numbers for humans, cows, sheep, and pigs (where a range is given, I take the midpoint). For chickens, Rekhamper et al. (2002) provides brain masses for 8 breeds of chickens, ranging from 2.6 g to 4.4 g, and I use 4 g in the table.


Neuroscientist Suzana Herculano-Houzel has performed numerous studies to determine neuron count in various species, and argues that neuron count better tracks cognitive abilities than brain size. She provides an estimate of 85 billion neurons for a human brain, and 221 million neurons for the Red Junglefowl, a wild relative of domesticated chickens. She also studies scaling patterns of brain size and neuron count in various taxonomic groupings, and in her book estimates that a cow would have ~3 billion neurons.

A neuron-count with these estimates would have almost no effect on the relative proportion of chickens, but would reduce focus on cows to on a par with chickens.

AnimalPopulationSource of estimateNeuronsTotal NeuronsPercentage of neurons in human population
Human7,110,521,293.00Isotopic fractionator85,000,000,000.00604,394,309,905,000,000,000.00100.00%
Cow1,426,389,031.00Herculano-Houzel projection in The Human Advantage3,000,000,000.004,279,167,093,000,000,000.000.71%
Chicken20,708,002,000.00Red Junglefowl221,000,000.004,576,468,442,000,000,000.000.76%

Wild invertebrates, rather than humans or farmed animals, account for the vast majority of neurons on Earth.


Aron said...

Could you elaborate on the point of this post? Do you think that the potential for (positively or negatively) valuable experiences is proportional to brain mass (perhaps above some threshold)?

Brian Tomasik said...

Thanks, Carl.

How are the numbers affected if you weight by neuron count instead of mass? I would guess animals would go up a bit?

Even if we weight by something proportional to brain size, that something may be more efficient in smaller brains (which probably have a larger fraction of emotional processing, less cognitive overhead, lower network inefficiencies, etc.). On the other hand, humans are more likely to be "conscious" than animals, so these two effects may oppose each other to some degree.

Carl said...


Using neurons instead of brain mass would actually increase the prominence of humans relative to farmed land animals. In terms of neurons per gram of brain mass chickens are very close to humans, while the large ungulates have fewer neurons per unit brain weight. It looks like humans would make up 95%+ of the neuron count, as opposed to 90%+ of the brain mass.


I have discussed some reasons for interest elsewhere, and plan to discuss them more in the future. This post is mainly intended to provide a reference for those who are already interested.

Florent Berthet said...

Neat. Do you have similar number for wild animals?

Diana said...

Could you integrate the Encephalization Quotient? Is that similar to what Brian is asking?

Ben said...

This is very cool Carl. You may go down as one of the few people to present a serious challenge to veganism as an ethical priority.

My understanding is that a lot of brain mass is used for "unconscious" behaviors - the autonomic system being one example. What happens if you remove those neurons from your count? (Or even better, count only those parts of the brain involved in pain sensing.) I'm guessing this is what Diana is getting at with EQ.

SoerenMind said...

In an interesting recent TED talk it is said that brain weight is a linear function of neuron count in primates, but increases faster than linearly in rodents. Thus a rodent brain with 86 billion neurons (that's the average human brain) would way 36kg.

Since it just appeared on TED, it might be a recent finding so I thought I'd mention it here. It certainly matches what Carl says. I wonder what this looks like for small animals then.