Suzana Herculano-Houzel

Some have more neurons, others have fewer: Where to draw the line?


I just received an email from somebody who was concerned about how my research on numbers of neurons in cats and dogs (and so many other species…) has been portrayed in the media. Her points: so many reports of “sorry, cats, you’re dumb” might encourage people who mistreat and abuse their pets; along the same lines, claims that humans “stand out” cognitively in having the most neurons in the cerebral cortex might encourage, and perhaps even justify, thoughts of human abuse over all other creatures.

I had received messages expressing a similar concern before: where does one draw the line below which “cruelty is acceptable”? If cortical neurons are those that endow brains with flexible and complex cognition, and the more the better (in terms of flexibility and complexity of information processing), how many are enough for self-awareness, for suffering, for caring? When is the number of cortical neurons “large enough” to warrant personhood and legal rights? To warrant not being eaten? Some ask this in search of legitimacy, comforted by the knowledge that no other species has as many neurons in the cerebral cortex as we do* (apparently). Some have political motivations, and want to have this or that species granted personhood status, too; others are just looking for an easy way out of the animal condition of not doing photosynthesis: we, animals, have to eat other life forms.

The main message I always try to get across from my work is that we humans are just another animal. We are a scaled-up primate, the animal with the biggest primate brain and thus the largest number of cortical neurons, which I believe is the simplest basis for all our cognitive feats (once we educate those neurons by feeding them knowledge through education) – but a primate, nonetheless. Differences between humans and other animals are a matter of quantity, not quality; in keeping with this continuum, I think there is neuroanatomical reason to work with the hypothesis that ALL vertebrates have some level of consciousness. We’re not the only ones.

So no, I don’t set a bar anywhere. In terms of who is deserving of respect, I don’t even set a bar at who has ANY neurons.

With the awareness that we are animals, and therefore belong in the same Nature as all others, comes the realization that, as animals, we need to eat. We don’t have the genes for photosynthesis, for grabbing matter our of thin air. That means that we have to ingest other forms of life. Yes, LIFE; not just animals. A raw carrot is chewed alive. Fruits are alive and breathing as we pop them into our mouths (something that I love to remind vegans who start proselytising). I say this in all seriousness: my point is that we should be aware of our place in nature, and be respectful of ALL others, even as we kill them in order to sustain ourselves or chew them still alive, as we do with plants.

What people choose to do with the knowledge we can offer them is something that, as much as we disagree with them, they alone can decide (the alternative turns very rapidly into a totalitarian state, which some would argue is what you get with ANY type of law, but that is a whole other discussion). We can always keep offering more information, of course, in the hope that they will someday choose to act differently. This applies to journalists and pet owners alike – and to parents, voters and so many other people we will disagree with during one’s life.

So here is my answer to those who ask what can possibly be gained from examining how many neurons different species have, human or not, and what difference does that make in terms of cognition: I believe it helps humans to know that we are JUST ANOTHER ANIMAL. Some animals may be more cognitively capable, others less (cats vs dogs, gorillas vs humans), but NONE is less deserving of respect because of that. I do noy believe, or argue, that “intelligence” or “number of neurons”, or anything that can be put a number on, could/should be used as criterion for “what creature is it ok to NOT be respectful to”.

I don’t and won’t draw a line anywhere. I’m respectful of the bugs that I would gladly move back outdoors rather than kill. I’m respectful of the fish that I eat. I’m respectful of the beautiful cows and pigs that some day end up on my plate (again, because of my animal nature), and of the gorgeous vegetables that I roast alive so that I can live another day. 

Yes, the continuum seems to put humans at the top in terms of cognitive capability. Yes, some people will choose to state that that position warrants/justifies whatever-we-want. I don’t subscribe to that. Unfortunately, I can’t make every journalist or reader understand that. But then again, it is the fact that some disagree, and some disagree enough to write to me, that brings people together and fosters respectful discussion and exchanges of opinions and ideas. As long as that is the case, I’m happy.

Originally published on December 28th, 2017

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