From: swf@tools3teradata.com (Stan Friesen)
Newsgroups: sci.bio
Subject: Re: Brainsize - the great brain race
Date: 6 Jan 93 18:45:00 GMT
Distribution: world
Organization: NCR Teradata Database Business Unit

In article <C0Dt77.Cnn@newcastle.ac.uk>, w.p.coyne@newcastle.ac.uk writes:
|> --------------------------------------------------------
|>    Why did mammals evolve large brains 250 Ma?
|> Reptiles of 300 Ma, dinosaurs and modern reptiles all have the same 
|> brain size.

Well, right off. you start out with something that is not true.
Birds have substantially larger brains than reptiles - in fact their brains
are almost as large as those of mammals. (Though the enlarged portion consists
of the optic tecta and the corpus striatum, rather than the cerebral cortex
as in mammals).

Dinosaurs vary quite a bit in brain size, with the sauropods having brains
*smaller* than typical reptiles, and advanced theropods having brains nearly as
large as those of birds.

|>  Mammals evolved 200Ma and they had brains 4 to 5 times 
|> bigger than the reptiles.  Most of the increase was due to the 
|> cerebral cortex.
|>                   WHY?
|>        {my comment - the ancestors of mammals lost out in competition
|>                   with ancient reptiles and so were pushed to the 
|>                   marginal niches.  This forced rapid evolution 
|>                      ie adapt or die }

Probably true, as far as it goes.
But other factors must be involved, since theropod dinosaurs also underwent
a substantial increase in brain size.

|> Reptiles are mainly visual animals and much of the processing of this
|> data is done at the periphery of their nervous systems - the retina.

Ditto for birds and dinosaurs.  Which is, naturally, why it was the optic tecta
that enlarged in that lineage.  (Though I suspect that the auditory tecta might
also have enlarged).

|> Mammals were small nocturnal creatures, so forced to depend on
|> smell and hearing.  To extract spacial and temporal data from sounds and 
|> smell required processing of data at a higher level and this was done 
|> inside the brain.

Good point, it was the olfactory cortex that enlarged first, after all.

|> They suggest the increase in brain size  was due to 3 reasons, 
|> the first 2 by Jerison and a 
|> third by John Allman, though the third is disputed by some.
|> 
|> resaon 1 - Mammals had to handle different types of information
|> reason 2 - Having a centralised processing system meant data from sight
|>            sound, touch and smell could be integrated to create a more
|>            detailed mental model of the world

Though the upper diencephalon (the optic and auditory tecta) could also perform
this same integration function (though with less input from smell).

|> reason 3 - Endothermy and the cortex evolved together.  Endothermy means 
|>            early mammals could occupy certain niches better than reptiles
|>            but they required much more energy (5 to 10 times more than a 
|>            similar sized reptile).

This sounds nice - only problem is that birds *are* endothermic, and dinosaurs
almost certainly were (new evidence found last year has pretty well clinched the
case for this).

Of course that might have something to do with the fact that both birds and
theropod dinosaurs had enlarged brains also.

|>    Why did mammals evolve even larger brains 65 Ma.?
|> Another increase in mammal brain size occured starting 65Ma.  Within
|> about 30 million years average mammal brains increased to become
|>  4 to 5 times larger,
|> with the biggest increases occuring with the evolution of ungulates,
|> carnivores and primates.  Insectivores and marsupials did not change
|> much.
|> 
|> Jerison argues:-
|>   With the exinction of the dinosaurs the mammals were able to move
|> into the niches they had occupied.  But mammals were night not day
|> animals so they had to evolve improved vision.  They could possibly 
|> have returned
|> to the reptile-like peripheral processing system their ancestors had 
|> had but instead they adopted a system where the vision processing was
|> done in the forebrain (the thalamus and cortex).  This revamped vision
|> was then integrated with the smell and sound systems.

Nice idea.  Certainly mammals underwent a very rapid explosive radiation about
the same time as the extinction of the dinosaurs.

|> Allman's arguements include:-
|>   A larger brain provided "behavioural buffers" - more efficient 
|> resource use required better mental models which required a bigger
|> brain.

I hesitate to put is quite so absolutely.  I would rather say something like:
one mechanism for providing more efficient resource use involves producing
better mental models via a larger, more complex brain.

Other approaches are certainly possible - insects are often far more efficient
at resource use than any mammal, ans they are notably lacking in complex mental
models.

|>    Why did primates evolve brains large compared to other mammals?
|> 
|> Monkeys have brains 2 to 3 times larger than the mammal average.
|> The article mentions that many explanations have been put forward
|> but there does not yet seem to be a concensus.  Among some of the 
|> reason are:-
|>  1 Allman's behavioural buffer
|>  2 All species have the biggest brains thay can afford and primates
|>    have such large brains because they are able to maintain a sufficiently
|>    rich diet.
|>  3 Social intelligence - species in larger groups have bigger cortices

I think of these three, the third is most likely.  But I would add a fourth
possibility:
    4 Adaption to changed life style - in particular arboreal life, with its
      requirement for complex 3-D coordination.
      [Note, this is similar to the reason postulated for the second burst
       of brain size increase in mammals, due to the switch to diurnal life].

|> ---------------------------------------------------------------------
|>    Why did humans evolve such large brains compared to primates?
|> 
|> Humans (and dolphins ) have brains 6 times bigger than the mammal average.
|> the main reason human brains are bigger than chimps is that humans have
|> a massive cortex - 70% to 80% of the total size.
|> 
|>   Hominids evolved 7.5 to 5 Ma but brain size remained in the ape range
|>  until about 2.5 Ma.  It has been found that 2.5Ma the following began
|>     - the rapid increase in brain size
|>     - the earliest found stone tools
|>     - dramatic global cooling and the beginning of a cycle of Ice Ages
|>       followed by interglaocials which has continued till today.
|> 
|>   2.5 Ma two types of hominids evolved the small toothed large brained 
|> Homo and the large toothed small brained Australopithicines.

Well, I would probably invoke reason #3 from above here.
[Tool use being, in part, a form of complex social behavior, since it requires
learning from others to do it well].

-- 
sarima@teradata.com			(formerly tdatirv!sarima)
  or
Stanley.Friesen@ElSegundoCA.ncr.com
Newsgroups: sci.bio