Re: Lizard engines and rat engines


"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:
>
> >Perplexed in Peoria wrote:
> >
> >> "dkomo" <dkomo871@xxxxxxxxxxx> wrote in message news:damprt$1f33$1@xxxxxxxxxxxxxxxxxxxxxx
> >>
> >>>Perplexed in Peoria wrote:
> >>>
> >>>
> >>>>"dkomo" <dkomo871@xxxxxxxxxxx> wrote in message news:daghdr$23qu$1@xxxxxxxxxxxxxxxxxxxxxx
> >>>>[snip]
> >>>>
> >>>>
> >>>>>Given the severe energy demands of warm blooded animals, it is a wonder
> >>>>>that mammals and birds (who are also warm blooded) ever evolved from
> >>>>>their last common ancestor, which was a cold blooded reptile of some
> >>>>>kind. And this full-blown warm bloodedness must have developed
> >>>>>independently in the evolutionary lines leading to each.
> >>>>
> >>>>
> >>>>Perhaps the ancestors of mammals and birds were cold blooded beasts
> >>>>in a hot climate. That is, they were actually "warm blooded", though
> >>>>they didn't have to work at staying warm. The innovations (permitting
> >>>>body temp to stay high even though the environment is cool) arose
> >>>>as these beasts sought to extend their range into more temperate
> >>>>climates.
> >>>>
> >>>>
> >>>
> >>>I'll be writing a post shortly describing two leading theories about the
> >>>evolution of homeothermy. There is a problem with any theory that
> >>>assumes warm bloodedness was directly selected for by evolution. The
> >>>problem is that at the initial stages of moving from cold bloodedness to
> >>>warm bloodedness, the energy costs greatly exceed any incremental benefits.
> >>
> >>
> >> ISTM that there are three aspects to homeothermy that must be explained:
> >> 1. An upper setpoint, such that if body temperature rises beyond it,
> >> the organism takes action to lower the temperature.
> >> 2. A lower setpoint, such that if body temperature falls below it,
> >> the organism takes action to raise the temperature.
> >> 3. The fact that the two setpoints are so close together.
> >>
> >> G. C. Williams, in "Domains, Levels, and Challenges" suggests a fourth
> >> issue. Why, he asks, is there not more variation between organisms in
> >> the setpoint? Mammals all have a setpoint near 37C and birds have a
> >> setpoint just a couple degrees higher. But one might expect that each
> >> species would have a different setpoint, depending on its environment,
> >> needs for speed and/or cognitive function, and evolutionary history.
> >>
> >
> >It depends on just what you mean by "near 37C", but in general there is
> >quite a bit of variation in the setpoints among mammals. Quoting from
> >Lavers' _Why Elephants Have Big Ears_:
> >
> >"The group of animals known as monotremes, comprising echidnas and the
> >duck-billed platypus, keep their bodies between 30 degrees C and 32
> >degrees C, marsupials such as kangaroos between 34 degrees C and 36
> >degrees C, placental mammals and ratite birds (ostriches, rheas, kiwis,
> >and emus) between 36 degrees c and 39 degrees C, and passerines
> >(songbirds) between 40 degrees C and 42 degrees C. The preferred body
> >temperature of most active lizards, snakes, and large flying insects is
> >typically in the mid to high 30s too."
> >
> >> However, this strikes me as no mystery at all. Note how many enzymes
> >> are "tuned" for optimality near the setpoint. The location of the
> >> setpoint can't evolve any more than the genetic code can evolve. Too
> >> many things already depend upon it staying the same.
> >>
> >>
> >
> >I would guess that with mammals, there's quite a bit more at stake than
> >the optimal running of particular enzymes. Each animal's entire
> >physiology is tuned for its particular temperature, and the animal will
> >die if it strays by more than a few degrees from this temperature in
> >either direction. Mammals are high maintenance animals in this sense.
> >I'm referring to body temperature here rather than ambient temperature.
> > As we know, mammals have a vast repertoire of tricks to keep
> >themselves alive and functioning in extreme climates.
> >
> >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.

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.

Evolving the machinery of homeothermy is something of a one-way journey.
You can't go home again and start over.

> Another reason is physiological.
> You definitely do NOT want to have a set point below ambient. It is
> extremely difficult (costly in energy and in water supply) to create
> an air conditioning system biologically. It is very easy to create a
> furnace. And you do not want your set point to be too far above
> ambient for several reasons: the higher the set point, the more
> expensive it is to stoke the furnace and the higher the set point the
> faster proteins tend to denature. However, your set point must be a
> bit above the highest expected temperature -- it does take some
> temperature gradient between your body and the environment to lose
> heat effectively. So you end up regulating at something above the
> maximum expected environmental temperature. That turns out to be in
> the high 30's or low 40's in a wide range of climate zones.
>
> Perplexed's first three points are all part of the same story. You
> keep one preferred temperature, not two. But there are distinctly
> several different temperature ranges to consider which involve very
> different physiological mechanisms.
>
> Above an "upper critical temperature", you crank up the cooling
> system trying to evaporate water to keep cool. Humans (and a few
> other animals) sweat. Many animals pant (saliva and body fluids that
> vaporize in the lungs). Some use urine. There are often tricks
> involving countercurrent exchange -- panting animals often inhale
> through the nose and exhale through the mouth. The cool nasal
> passages may have special circulation to the brain to keep it cool.
> Countercurrent heat exchangers between the head and body allow the
> brain to stay cooler than the rest of the body.
>
> Within a certain temperature range, the "thermal neutral zone" the
> metabolic rate is minimal and animals use purely physical means to
> regulate. They adjust their peltage and plumage to maximize or
> minimize insulation, adjust their posture to increase or decrease
> exposed surface area, adjust blood flow to preferentially channel hot
> blood towards or away from special surface areas of exchange. The
> rabbit and elephant ears are prime examples of such areas but the skin
> of the extremities is another major one. Something you all should
> know is that this neutral zone can be fairly broad and encompasses the
> range, for example, at which we humans feel quite "comfortable". In
> this zone, there is no special penalty for regulation -- metabolic
> rate is independent of external temperature. Of course there is a
> "setup charge" to be a regulator in the first place -- the normally
> high temperature produces a normally high basal metabolic rate which
> does have cost.)
>
> Below a "lower critical temperature" the animal has done everything
> possible to minimize heat loss so the only thing left to do is to
> increase heat production by increasing metabolic rate. Again there
> are many mechanisms available. One is with muscle activity --
> walking, running, just being active in general. Another is with
> non-productive muscle activity -- shivering. That uses ATP (producing
> heat) without doing any useful work. A third is "non-shivering
> thermogenesis", a special metabolic pathway that uncouples the
> cytochrome chain in the mitochondria so that metabolism doesn't
> produce any ATP at all -- all the free energy of the chemical
> reactions then ends up as heat.
>
> Each of these mechanisms turns on and off at different times, over
> different temperature ranges, and in different amounts in different
> animals. The only thing that is important is that, as it gets colder
> and colder, you activate more and more heat conserving and heat
> producing processes and that, as it gets warmer and warmer, you
> activate more and more heat releasing and heat dissipating processes.
> The exact mix and set points for each process is quite irrelevant.
> Put all the equations in one big pot and they all equal negative
> feedback regulation and a single "setpoint".

I agree that there are a variety of setpoints for a variety of responses,
and that taken together in any modern homeotherm they constitute a single
negative feedback system with a single setpoint. However, I am claiming
that the ancestral condition involved two setpoints, and a range of
tolerated temperature in between.

> Incidentally, dkomo, you won't be surprised to hear that reptiles,
> amphibians, insects, fish, and the whole gamut of non-regulators have
> a whole series of tricks up their sleeves (assuming they wear shirts)
> to compensate for low metabolic rates at low temperatures. Changing
> enzymes (using isozymes) is but one of these. But that is another
> posting for another day, if anybody is interested.


.

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