Brains, intelligence, and how they don't correlate

["David Marjanovic" <david.marjanovic@gmx.at>]

Summary of

Gerhard Roth: Kleine Gehirne -- große Gehirne. Evolutionäre Aspekte und
funktionelle Konsequenzen [Small brains -- big brains. Evolutionary aspects
and functional consequences], Naturwissenschaftliche Rundschau 52, p. 213 --
219, 6/1999

(no abstract, no summary, no nothing in English...)

This is actually a paper on why the human brain is not exceptional, just in
the upper range for mammals, but during this the paper clarifies several old
misconceptions.

- The oldest one: Humans don't have the biggest brains. Well, most or all of
you will already know that the biggest brain is that of the sperm whale and
weighs 7 to 9 kg. Among humans brain weight varies between 1000 and 2000 kg
and is totally uncorrelated with intelligence (Anatole France had a
1-kg-brain, I just wonder who weighed that). Average for recent humans is
1.3 kg (for neandertals it was higher, something like 1.45 kg).

- Brain size is determined by body size, if you rise body weight by 1, brain
weight rises by ~ 0.7 (nobody knows why 0.7). That's called negative brain
allometry -- "Small animals thus have absolutely seen small brains, but
relatively to their body weight big ones, while very big animals have
relatively to their bodies very small brains. Shrews have brains that make
up up to 10 % of their body weight, while in the blue whale, the biggest
living animal, the brain constitutes just 0.01 % of body mass. [...] Man
lies in the top group regarding his relative brain weight respectively
volume of 2 % of body volume, but not at all lonely at the top, as is often
claimed. Man shares this pole position with some very small monkeys, bats,
shrews and birds that all have an equally large and sometimes very much
larger brain relatively to body size/mass. This is no wonder, because very
small animals have, as mentioned, throughout a relatively bigger brain than
large animals."

- What _is_ unusual about the human brain "is the fact that it is, regarding
human absolute body size, extraordinary big. It is possible to calculate
whether and how far the relative brain weight of a mammal lies above or
below the average. If the relations in a cat, which represents an average
value, is arbitrarily set at 1, then humans have a relative brain size that
is 7 to 8 times larger than the mammalian average. This does not make humans
unique, however, because some dolphins have a brain that is still 5 to 6
times larger than the average." A figure shows that shrews, some mice, rats,
squirrels, cats, dogs, horses and African elephants lie on the average line.
Bats, pigs and blue and sperm whales, among others, are below it. "We could
still be rather proud of our brain, if we would have an answer to the
question what having a relatively large brain means in the first place."
After all, shrews have not yet made it into the news because of particular
intelligence.

- "Another common argument says that not absolute or relative brain size
matters but the size of the cortex respectively the number of its
convolutions. We humans are indeed particularly proud of our big and highly
convoluted neocortex, and there is a widespread assumption that the size of
the human cortex as well as the number of its convolutions are unique. This,
too, is an error. As [cool] figures [...] show, whales, dolphins, and also
elephants have a much bigger neocortex with many more convolutions than our
brain.
        If one takes a closer look at the affair it becomes apparent that
cortex size is not, as many believe, determined by some extreme evolutionary
selection factor for intelligence, but simply by brain size: Big brains have
big cortices that vault themselves simply for architectonical reasons.
Therefore whales, dolphins, and elephants have a much bigger and more
convoluted cortex than we."
        It's just another allometry.

- "Big brains also means big front brains. This is so interesting because
the front brain, the prefrontal cortex, is commonly regarded as the seat of
intelligence, personality and action planning. But the human front brain is
just as large in relation to body weight respectively total brain size as it
should be among primates."
        A few months ago there was a paper in Nature about this simple fact.
It was intended to be rather sensational... heh heh =8-) .

- What _should_ be important for intelligence is the _absolute_ number of
nerve cells in the brain -- having more nerve cells means the potential for
much larger and much more complicated networks. The nerve cell density in
dolphin brains is surprisingly small -- about 1/4 of what's normal --, and
their cortex is very thin and simple. In total dolphins have as many nerve
cells in their cortex as a chimp -- which fits their intelligence well.
"There remains the question after the capability of the 4-kg-brain of
elephants. The few existing data show that elephants have a relatively thick
cortex. Even though the cell density seems to be a bit lower than that of
humans, an elephant should have as many neurons in its gigantic cortex as a
human, namely between 100 billions and a trillion[*]." Apart from their
proverbial memory not much exceptional is known about elephant intelligence.

*There seems to be some confusion about large numbers between British and
American English -- anyway, this means 10^11 and 10^12.

- What about the number of synapses per neuron? Varies between 100 and
100,000 in one and the same brain. Small brains may have more synapses per
nerve cell than large ones.

About the increase in brain size in human evolution -- "This strong
phylogenetic brain increase [sic] in humans that affected nearly all parts
of the brain probably results from a genetic accident and cannot so far be
unambiguously connected to outer selectionary pressures, even though there
are extended speculations about this. A long discussed hypothesis is that
man is a paedomorphic chimpanzee. Young chimpanzees are indeed much more
similar to humans than adult animals. Among other things they have a
relatively much bigger brain and are much more able to learn than their
adult conspecifics."

Unlike me, *Velociraptor* doesn't seem particularly childish, so human-like
intelligence at least can probably be excluded :-)

Well, I guess wolves, ground-living birds of prey etc. are probably the best
analogs for dromaeosaur intelligence, rather than chimpanzees, but that
doesn't really help us, let alone movie makers, I think.