Necks and Brains

["Lloyd Gabel" <LPGABEL@msn.com>]

I would like to make some comments concerning the recent discussions on
long necks and brain size.  I should mention that I am not a Paleontologist,
but rather a New Jersey pharmaceutical industry pharmacologist/physiologist
with a long standing and ardent interest in ordovician invertebrates and a
preoccupation for reading the various www paleo materials, digests and club
discussions.

I can not understand the attractiveness of the hypothesis that brain size
would undergo an adaptive decrease in mass simply in order to decrease
the demand for cerebral blood flow.  In most species, the proportion of total
corporeal blood flow which reaches the brain is relatively large and constant,
and tends to remain adequate even in situations where the body intake of 
oxygen becomes limited.  Nerve tissue has a very limited capacity to endure
or survive hypoxic situations, while other tissues, such as muscle, can switch
from an aerobic to an essentially anaerobic metabolic mode for generating
energy when warranted.  Brain size and neck blood pressure are not the only 
determinants of brain blood flow, and the minute to minute variability of the
cerebral blood diameters play a very significant role in the regulation of the 
flow.  The diving reflex in aquatic mammals is a good example of selective 
blood flow redistribution and shunting to the nervous system during hypoxic 
periods by vasoconstriction and dilation.  We humans reflexly shunt blood 
towards the brain when we suddenly stand up from a sitting or prone position 
by an involuntary, temporary closing down of vessels in the periphery to 
prevent gravitational pooling of blood in the periphery which could in turn 
cause decreased cardiac output, cerebral anoxia and a resultant dizziness or 
fainting.

James Norton's comments on tracheal dead space and airway resistance in long 
necks are well taken, and in many respects the changes in respiratory 
mechanics as neck length increased could imply an even greater adaptive 
maneuver than that required for keeping adequate brain blood flow.  It would 
be nice to know the mean tracheal diameter of the beasts under consideration 
in order to even begin to extraplolate, at least hypothetically, what sort of 
respiratory tidal volume would be necessary to maintain a normal blood oxygen 
content in the face of such tremendous dead space.

Lastly, Mickey Rowe mentioned that some birds shed song generating parts of 
their brain and then regrow them later in order to temporarily decrease their 
total weight and thereby improve flying efficiency.  Unless the brain case 
decreases in volume at the same time, the shed brain tissue has to be replaced 
with something of equal volume such as other types of cells or fluid, which 
would not seem to represent a great savings in overall weight.  An argument 
can be made that the loss in certain types of cells within a constant overall 
brain mass, along with the adaptive processes for specific cell loss and later 
regeneration, would seem to require more evolutionary development than simply 
having the bird remain non-communicative while in flight.  Besides, in terms 
of energy conservation, how much influence could a couple of brain cell 
groups, either lost or kept quiescent, have on a bird's overall energy needs 
for flight ?

Also, thanks for the constant pleasure of reading your material.

Lloyd Gabel    LPGABEL@MSN.COM