Brains in small animals

[Proteus]

Do small animals have smaller neurons, or is there less space in between them? There doesn't seem to be a proportionality in the size of the skull and intelligence. Parrots are very intelligent, for instance.

[bascule]

I believe there's a "normal" range for the ratio of body size versus brain volume. Smaller animals need smaller brains because they have smaller bodies.

 

That said, animals high brain volume-to-body size ratio are typically more intelligent. Humans and dolphins are two examples.

[Mokele]

Within mammals, brain size is pretty well determined by body size, though there are exceptions.

 

The nerves going to and from the limbs etc. *do* change size - smaller animals have smaller diameter neurons, though it's not a simple linear relationship. I saw a talk on this a few months ago, and the paper should be out vaguely soon-ish.

[Proteus]

Thank you! That was what I wanted to know. So some small animals with small brain volume yet are relatively intelligent compensate their low brain volume by decreasing the diameter of the neurons, not by increasing the mass density of the brain matter.

[GDG]

I'm not sure we can extrapolate from the peripheral nerve axon diameters to the CNS neuron sizes. Smaller animals probably have a smaller number of brain neurons. Intelligence may be more a matter of how they are connected.

[Mokele]

I'm not sure we can extrapolate from the peripheral nerve axon diameters to the CNS neuron sizes. Smaller animals probably have a smaller number of brain neurons. Intelligence may be more a matter of how they are connected.

 

I'm pretty sure we can't, but I don't know of any other specifically allometric brain data that's at or near the cellular level. I'm aware of lots of stuff about counting neurons in, say, the hippocampus of a bird that has to remember where stored food is vs one that doesn't, but you can't really get size stuff from that.

[bascule]

I popped open my copy of The Ancestor's Tale and found the section on the correspondence between brain mass and body mass in placental mammals.

 

Page 78 begins the section on this subject, and shows that placental mammals generally adhere to the following ratio:

 

3 * log(brain mass in mg) : 4 * log(body mass in kg)

 

The page includes a plot of various mammal species and how close they come to the 3/4 line. Humans and dolphins are definite outliers.

[jimmydasaint]

I was also wondering about this recently and came across this:

 

The neural density in the bee’s brain is about 10 times higher than that in a mammalian cerebral cortex, which most of us take to be the pinnacle of evolution on this planet.

http://www.scientificamerican.com/article.cfm?id=exploring-consciousness&page=2

 

This made me think about the relative neural density of insect/mammalian brains and led me to a new search.

 

Comparative neurobiologists have provided ample evidence that in vertebrates small animals have proportionally larger brains: in a double-logarithmic plot of brain weight versus body weight all data points conform quite closely to a straight line with a slope of less than one. Hence vertebrate brains scale allometrically, rather than isometrically, with body size. Here we extend the phylogenetic scope of such studies and the size range of the brains under investigation to the insects, especially ants. We show that the principle of (negative) allometry applies as well, but that ants have considerably smaller brains than any ant-sized vertebrate would have, and that this result holds even if the relatively higher exoskeleton weights of ants (as compared to endoskeleton weights of mammals) are taken into account. Finally, interspecific comparisons within one genus of ants, Cataglyphis, show that species exhibiting small colony sizes (of a few hundred individuals) have significantly smaller brains than species in which colonies are composed of several thousand individuals.

 

http://content.karger.com/produktedb/produkte.asp?typ=fulltext&file=BBE2007069003220

Edited October 30, 2009 by jimmydasaint

[bascule]

The neural density in the bee’s brain is about 10 times higher than that in a mammalian cerebral cortex, which most of us take to be the pinnacle of evolution on this planet.

 

That quote sucks, seriously.

 

The density of the human cerebellum is much higher than the neocortex too, even though you can live without a cerebellum but certainly can't live without a neocortex.

 

There's many reasons why the neocortex has a lower neuron density: Cortical neurons, especially pyramidal cells, are huge, some of the largest neurons known to nature. Cortical neurons are also densely interconnected. The amount of volume needed by cortical connections lowers the overall density of neurons.

 

The human neocortex is still contains the most complex neuron interconnection structure in all life systems, and cortical neurons are densely interconnected albeit not densely packed.

 

Bees are highly adaptive and sophisticated creatures with a bit fewer than one million neurons, which are interconnected in ways that are beyond our current understanding

 

Bees have less than one million neurons, whereas the human neocortex contains over one million neocortical columns, collections of cells that act like time-sensitive Bayesian classifiers.

 

I am going to go with the human neocortex is way more impressive than a bee's brain, despite the incredibly stupid parallels this article is trying to draw.

Edited October 30, 2009 by bascule

[jimmydasaint]

That quote sucks, seriously.

 

The density of the human cerebellum is much higher than the neocortex too, even though you can live without a cerebellum but certainly can't live without a neocortex.

 

There's many reasons why the neocortex has a lower neuron density: Cortical neurons, especially pyramidal cells, are huge, some of the largest neurons known to nature. Cortical neurons are also densely interconnected. The amount of volume needed by cortical connections lowers the overall density of neurons.

 

The human neocortex is still contains the most complex neuron interconnection structure in all life systems, and cortical neurons are densely interconnected albeit not densely packed.

 

bascule, buddy, you've missed the point entirely (to my surprise). The OP was discussing brains in small animals and an allusion was made to neural density and to body mass:brain size. I was making the point about my amazement with the overall neural density of a bee's brain. The article is about the possibility of bee consciousness but I quoted it as the only reasonable source from about 6 scholarly articles. The OP, and my intention was not to discuss neural density and the nature of consciousness in invertebrates.

 

 

 

Bees have less than one million neurons, whereas the human neocortex contains over one million neocortical columns, collections of cells that act like time-sensitive Bayesian classifiers.

 

I am going to go with the human neocortex is way more impressive than a bee's brain, despite the incredibly stupid parallels this article is trying to draw.

I am not knowledgeable enough to discuss the article, so I will agree with you.

 

Ah, the Bayesian classification system again. Do you have any scholarly references which show overwhelming evidence that Bayesian logic is solely used by humans please? Now that we are thoroughly off topic, back to the OP. Is there a correlation with brain size and body mass. You seem to indicate, and it seems to be backed up that there is a correlation. The OP referred to the unusual intelligence of vertebrates.

[Mr Skeptic]

In any case, I think that the synapse density would matter more than the neural density.

[bascule]

bascule, buddy, you've missed the point entirely (to my surprise). The OP was discussing brains in small animals and an allusion was made to neural density and to body mass:brain size.

 

Whoops, my bad. But that article was rather poorly written.

 

Ah, the Bayesian classification system again. Do you have any scholarly references which show overwhelming evidence that Bayesian logic is solely used by humans please?

 

Two things:

 

1) Neocortical columns aren't actually Bayesian classifiers. Bayesian classifiers are the most popular analogue to what neocortical columns are actually doing. However neocortical columns work fundamentally differently, in that they have a sense of time. The time at which symbols are processed is just as important as the symbols themselves.

 

2) Neocortical columns aren't exclusive to humans. They can be found in any animal with a neocortex (i.e. mammals) Humans just have a lot more of them, and the human version is more complex than the version found in, say, rats.