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Re: RE: Extinction



Two things.  First, Neil Clark says:

] This assumes, of course, that dinosaur physiology did not control
] their temperature.  I prefer to use birds to compare physiology,
] being the main progenetic group of the dinosaurs.  And, as far as I
] know, birds don't rely on ESD (but I may be wrong?).

I can guarantee that at least not all birds rely on ESD.  I think that
none do, but in biology it's never good to bet too strongly an
absolutes like that...  So far as I know, all female birds are
heterogametic.  That is, in contrast to humans where gender is
determined by males having two different sex chromosomes, an X and a
Y, bird gender is determined by females having two different sex
chromosomes, a W and a Z.  Human sex chromosomes were named by their
appearance; I think the bird chromosomes were named as a half-humorous
way to distinguish them from the mammalian X/Y model.  There is a lot
of variability amongst animals that do not rely on ESD.  In frogs,
whether it is the male or female that is heterogametic depends on the
species.  Despite the ubiquity of two sexes and the reliance upon
steroid hormones as proximal mediators of differential developmental
patterns, the ultimate mediators of gender determination are highly
variable.  I would expect dinosaurs to show similar variability,
particularly since birds and crocodiles are so different in this
respect.

Second, Steve Grenard says:

> Studies are underway that have found substances known as
> cryoprtectants that are mobilized in some species when exposed to
> sub-zero temperatures, resulting in total freezing.

Now that we've all gotten used to controversy :-) I should mention
that it's not clear that any vertebrate can survive actual freezing.

> I myself have witnessed completely frozen baby water turtles thaw
> out months after being frozen with similar results.

I think that the painted turtle, _Chrysemys picta_ has received the
most study in this regard.  The claim has been made that during their
first year of life, it is normal for these animals to freeze solid in
the upper range of their territory.  However, recent work with both
painted turtles and snapping turtles suggest that the turtles will die
if ice crystals form within their cells.  If kept relatively dry, they
can last long periods at -2 to -4 degrees C, but ice crystals from
outside their bodies can "inoculate" them causing them to freeze
rapidly and die.  See for example:


 Author(s):      PACKARD GC; PACKARD MJ 
 Title:          DELAYED INOCULATIVE FREEZING IS FATAL TO HATCHLING PAINTED 
                   TURTLES (CHRYSEMYS-PICTA)
 
 Source:         CRYO-LETTERS V0014 N5 SEP-OCT 1993 pp. 273-284. 


> No amphibian or reptile can survive very hot temperatures for very
> long, however (e.g. varies with species with upper limits about
> 100-105 F for some).

At the other extreme, desert horned lizards will not eat if the
temperature is below 110 degrees F.

-- 
Mickey Rowe     (rowe@lepomis.psych.upenn.edu)