Re: Thirst and Evolution (was Re: Evolution, "adaptation")

Len Piotrowski (
Wed, 4 Sep 1996 21:13:16 GMT

In article <506llk$> (Bryant) writes:

An interesting phenomenon. I won't pretend to be an expert but let me make
some comments, especially with regard to particular neuropeptides and the
"sensation of thirst." To set the stage, since there seems to be a problem in
this discussion over behavior and traits, I will assume that a neuropeptide
related to "thirst" truly and physically exists and is a product of genes
subject to the "forces of evolution."

In what you've described so far, neuropeptides are not identified as products
of sensation. That is, receptors or sense organs (intrinsic or internalized I
suppose) do not produce them and they are not "sensed" by the neural net
(nervous system/brain) as normal perceptions are. However, they appear to
stimulate or retard properties of the neural net to act in certain ways,
hopefully to diminish the sensed deprivation.

If this is true, the sensed perception of thirstiness should be independent of
the production of these neorpeptides. Control mechanisms monitoring the level
of thirst against an intrinsic optimum level should be responsible for the
production of neuropeptides, which subsequently have an impact on behavior
devoted to relieving thirst proportional to their abundance. However, the
"sensation of thirst" is not, by this model, caused by the production of
neuropeptides. At best they appear to accelerate control mechanisms designed
to bring the state of thirstiness back to intrinsic control levels.

Some questions arise to mind: what is exactly meant by "thirst" and the
"sensation of thirst;" what is meant by 'mental (and physiological)
state that is tied to "physical phenotype"'; and what is meant by "a product
of selection favoring water acquisition and retention."

I cannot respond entirely to this last statement simply because I am not
familiar with the details of the organic system in which neuropeptides are
found. If they perform to enhance water acquisition and retention, then indeed
they would be presumably favored by selection in certain situations.

The perception and sensation of thirst, on the other hand, appears to me to be
a function of a primary sensory receptor. I don't understand the role of this
thirst neuorpeptide under this configuration. This is due largely to my lack
of understanding of how this thirst neuorpeptide is an indicator of the so
called 'mental state' which I hope means the physiological state (intrinsic
state in my conception) of an organism's degree of thirstiness versus a
control product to stimulate water acquisition and retention. In other words,
this particular neuropeptide is not representative of a sensation of thirst,
but a means to enhancing or accelerating thirst quenching behaviors.

In as much as these neuropeptides are part of the control mechanism versus the
sensory receptors or intrinsic control values, they are possibly subject to
reorganization of the properties of the control system. In other words,
despite what is dictated by the genes, behavior can be changed which directly
affects the production of these particular neuropeptides, whether there is a
sensation of thirst or not, perhaps to the point of self-sacrifice.

Whether or not this is an acceptable characterization or even particularly
plausible to adaptationists is a question. It is a rather rough cut analysis
of organic physical processes from a non-expert relating a behavioral point of
view that retains a non-adaptationist perspective. However, this might all
just be baloney.