Re: Lizard engines and rat engines
- From: William Morse <wdmorse@xxxxxxxxxxxx>
- Date: Thu, 4 Aug 2005 01:29:04 -0400 (EDT)
"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in
news:davmfp$1edk$1@xxxxxxxxxxxxxxxxxxx:
>
> "r norman" <rsn_@xxxxxxxxxxxx> wrote in message
> news:dat0cp$ee5$1@xxxxxxxxxxxxxxxxxxxxxx
>> On Sun, 10 Jul 2005 20:28:08 -0400 (EDT), dkomo
>> <dkomo871@xxxxxxxxxxx> wrote:
>> >Reptiles, on the other hand, can continue to operate across a wide
>> >body temperature range, albeit at reduced efficiency. Someone wrote
>> >in another post that reptiles can do this because they have
>> >different sets of enzymes that kick in at different temperatures to
>> >maintain the animal's metabolism.
>> Nicely put, dkomo. You'll be a comparative physiology, soon, at
>> that rate!
>> To answer GC Williams' point (Perplexed's 4th): why are all the set
>> points so similar? Like so many questions in biology, you can't
>> answer definitively because it is difficult (impossible?) to collect
>> the data. However there are several issues that are probably
>> involved. One reason is evolutionary. The set point may have
>> evolved rather early in mammalian evolution: monotremes have one,
>> marsupials (and some non-shrew insectivores and edentates) a bit
>> higher, pretty much all the placentals the highest.
That dog won't hunt. The set points for birds and mammals are
extraordinarily close, given that they evolved homeothermy separately. And
then we have honeybees, who keep their hives at a temperature of 34C by a
variety of heating and cooling mechanisms. Sorry, but contingency only
takes us so far. There is quite obviously a desirable temperature for
terrestrial animals.
> Thanks, r_norman, and dkomo too, for very thoughtful postings.
> The data suggest the following hypothesis to me: The common ancestor
> of monotremes, marsupials, and placentals had a more permissive system
> of temperature regulation. It allowed body temperature to range from
> say 30C to 38 C. The enzymes were not closely "tuned" to a particular
> temperature in this range, or else there were a variety of isozymes.
>
> Since then, the monotremes have evolved to lower the maximal setpoint,
> the placentals have evolved to raise the minimal setpoint, and the
> marsupials and larger placental insectivores have evolved to move
> both setpoints toward a spot in the middle. In the course of doing
> so, some isozymes have been lost and some formerly temperature-range-
> tolerant enzymes have become less tolerant. A certain amount of
> evolutionary flexibility has been lost. It would be very difficult
> now for a placental species to expand the range of tolerance toward
> the ancestral state. It would be very difficult for a monotreme to
> raise its setpoint(s) toward the placental 37C.
Sloths (placentals) have a varying temperature that can be as low as 24C.
And of course hibernators lower their body temperature and then raise it
again.
It would seem instead that in general a higher maximal setpoint is better
from a competitive standpoint. This makes sense based on the relationship
between rate of reaction and temperature, with the problem being how to
preserve protein and DNA stability when internal temperatures get above
about 40 C. And temperatures above that are obviously a problem, as witness
the many cooling mechanisms that have been evolved by animals living on the
African savannah. They could have just raised their setpoint, or allowed
their temperature to rise on occasion, but few of them do.
One question I do have is why haven't marsupials been able to raise the
maximal setpoint. I have on occasion argued that the marsupials were
outcompeted by placentals when the two have come in contact because of the
marsupial's lower setpoint, but I have seen the argument that marsupilas
reigned in Australia due to dry conditions. This might favor a lower
setpoint.
> Evolving the machinery of homeothermy is something of a one-way
> journey. You can't go home again and start over.
I disagree with this - as note my counterexamples above. But I will admit
that there is considerable inertia in the system, so that changing a
temperature set-point is very much more difficult than changing tooth shape
- it will involve a whole host of changes in physiology.
Yours,
Bill Morse
.
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