Re: Elephants vs mice (fwd)

[fhf <maay100@bgumail.bgu.ac.il>]

	May I add my 2 bits to the elephant/mice/LNT threads, initiated by
Dr. Cohen's question why elephants aren't more likely to get more cancer
than mice because they are so much bigger and have a lot more DNA.

        Dr. Cohen's question is very fundamental as it has to do with
evolution and the natural lifetimes of individuals within species. The 
problem concerns the manifestation of cancer in different animals living and
evolving under the same conditions of radiation exposure from natural 
sources.

	Cancer is a rather rare disease in young people and we are much more 
likely to get the condition later in life. I submit the hypothesis that 
the general time and likelihood of onset of cancer is 'programmed' 
genetically as part of a general biological mechanism which has the 
effect of limiting the lifetime of members of a species.

        It seems to me evident that all species have their natural
lifetimes. That of man is three score and 10, perhaps a little greater for
woman, but mother nature polishes us off when we aren't much more use for
survival of the tribe. Evolutionary selection has led to this
situation. Teleologically speaking, if we died too soon, we wouldn't 
be able to educate the young ones, and I suppose if we persisted too 
long, there would be too many around of no value for survival of the 
group.

        The same goes for mice and elephants. Both have evolved to have
natural life times consistent with their reproductive rate and their
environment leading to optimum survival of their species over eons of time.
These natural life times are without doubt programmed into the genetic
structure. Elephants could not survive if they only lived a couple of
years. And mice living to a 100 years, if they could evade the house cats
and other predators, would rapidly  overrun their food supply.

        How does nature get rid of us? Evidently, by the diseases we are
prone to get in old age, which prominently include cancer. Of course, most
animals in the wild perish from other causes before they get old enough
to get cancers.

      When mice, humans and elephants live long enough to develop 
malignant cancers, they develop them when the genetic programming of the
animal withdraws the naturally efficient protection. 

       So the question is not why elephants don't get more cancers than
mice, but rather, what are the exquisitely sensitive and effective
biological mechanisms which prevent species of the animal kingdom from
developing cancers until a suitable later period in their natural lives, in
spite of the continued presence of the natural levels of radiation and
the many other natural mutagens and carcinogens?

        The LNT theory must take these biological mechanisms into account.

                                Michael Quastel MD PhD
				Head, Nuclear Medicine
				Soroka Medical Center,
				POB 151, Beer Sheva, Israel 84101
				Fax: 972-7-6400765
				Email: maay100@bgumail.bgu.ac.il
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>> Author:  Bernard L Cohen <blc+@pitt.edu> at -MailLink
>> Date:    6/19/97 11:07 AM
>>
>>
>>         The basis for linear-no threshold theory is that a single gamma
>> ray striking a single DNA molecule in the nucleus of a single cell can
>> cause a cancer. Since an elephant or other large animal has many
>> thousands of times more such DNA molecules than a mouse, what
>> explanations have been offered for why elephants don't get a lot more
>> cancers than mice from radiation exposure? Why aren't human adults more
>> sensitive than small children? Why is radiation dose defined as energy
>> deposited per unit organ mass, rather than just total energy deposited
>> which determines the number of such hits?
>>
>> Bernard L. Cohen
>> Physics Dept.
>> University of Pittsburgh
>> Pittsburgh, PA 15260
>> Tel: (412)624-9245
>> Fax: (412)624-9163
>> e-mail: blc+@pitt.edu