metabolic oddities (was erect crocodile problems)

["Ralph Miller III" <gbabcock@best.com>]

Dave <cadams@hh.gpz.org> wrote:
> The only reason a terrestrial vertebrate can take advantage of the energy
> efficiency of ectothermy is because it can flop down on its belly most of
> the time.  Greg Paul has said previously on this mailing list that a
fully
> erect posture probably forces elevated aerobic exercise capacity.  I
would
> take out the probably.  All vertebrates with fully erect postures, large
> and small, have high performance cardiovascular systems, high
surface-area
> respiratory systems, and physiological core temperature regulation.

There may be one small family of reptiles that would take issue with the
above: the Chameleontidae (chameleons), which are characterized by a fully
erect posture, which enables them to walk slowly along small branches in
pursuit of insect prey.  (For a mammalian counterexample, read on).  Of
course, this "feat" of erect posture must be much easier for a lightweight,
slow-moving lizard to accomplish without the benefit of an elevated aerobic
exercise capacity than it would be for, say, _Brachiosaurus_! 

> Selection for energy efficiency might indeed be expected to push towards
> ectothermy.  The problem is that endothermy is such a huge physiological,
> molecular, and morphological commitment  that it is very difficult to
> reverse.  There is no evidence that this has ever happened on this
planet,
> despite a few million years of opportunity.  Selection for energy
> efficiency in hummingbirds and bats is so strong that it has resulted in
> the evolution of torpor states.  But NOT ectothermy.> 

Permit me to introduce you to another small and easily overlooked oddity:
the naked mole rat of Africa.  According to an article in the August 1992
edition of _Scientific American_, _Naked mole rats_ by Paul W. Sherman,
Jennifer U. M. Jarvis, and Stanton H. Braude:

"Unlike any other mammal, a naked mole rat's body temperature fluctuates
with ambient temperature.  Rochelle Buffenstein and Shlomo Yahav of the
University of Witwatersrand in South Africa found that as a result of high
heat exchange, small body size and low rates of metabolic heat production,
naked mole rats are poikilothermic, or cold-blooded.  In nature, the
animals live in a relatively thermostable environment: temperatures in the
deep tunnels, which are about 50 centimeters underground, remain close to
30 degrees Celsius year-round.  Naked mole rats regulate their body
temperature behaviorally by basking in warm soil near the surface or by
huddling during cold snaps."

Further discussion appeared in _Not just another pretty face_ by Elizabeth
Pennisi in the March 1986 issue of _Discover_:

"The mole rat is also one of the few mammals (and the only rodent) that has
lost its fur.  This rids it of hiding places for parasites -- no small
consideration for creatures that live in close quarters.  Nakedness also
facilitates the transfer of heat to and from the body.  The 80 degree to 85
degree (Fahrenheit) temperature and high humidity in mole rat burrows make
it unnecessary to conserve body heat as other mammals must.  To get warm,
the mole rat can huddle with nestmates or go to tunnels near the sun-baked
surface; to cool off, it can descend to deeper burrows.  With a metabolic
rate less than half that of other rodents, it lives six times or more as
long as those its size.  Nineteen of Jarvis' original colony are at least
13 years old."

Under "Breakthroughs," the Mach 1992 _Discover_ reports:

"...mole-rats have fewer than 100 hairs a piece..." and

"...when biologists Rochelle Buffenstein and Shlomo Yahav of the University
of  Witwatersrand in South Africa exposed mole-rats to temperatures ranging
from 53 to 98 degrees (Fahrenheit), the animal's body temperatures always
stayed just one degree warmer than the air.  At 53 degrees the mole-rats
froze up in temporary rigor mortis.

"As Buffenstein points out, however, mole-rats never see 53 degrees.  In
the warm climate of equatorial Africa, the temperature in their burrows,
which they never leave, is usually about 85 degrees.  If it does get
colder, they can always huddle together for warmth.  So for mole-rats,
Buffenstein speculates, warm-bloodedness is probably not worth the
metabolic effort -- especially since they would just have to eat more, and
the tubers they grub for, at considerable expense of energy, are sparse and
not very nutritious."

So here we have a case of tiny, active, social creatures which walk with
their legs upright, but which are poikilotherms (which rest on their
bellies) nonetheless, in spite of what their "mammalness" might suggest. 
And they have solved one of the problems that comes with a hairy or
feathery integument: parasites.  The solution: hair loss.  One might regard
it as telling that the only known "ectothermic mammal" is tiny yet
virtually _hairless_.

So I propose that the insulated theropod, _Sinosauropteryx_, along with
numerous other insulated coelurosaurs, Mesozoic birds, mammals, and
pterosaurs, may have indeed been troubled by parasites (as are extant
mammals and birds), but in my opinion it would be rash to conclude that
these aforementioned extinct, insulated creatures were all ectotherms. ;0)

-- Ralph Miller III     gbabcock@best.com

You can see naked mole rats using their massive jaw muscles (accounting for
25% of total body muscle mass) to chew through their acrylic tube tunnel
enclosures at the Brookfield Zoo in the Chicago area.  They can reportedly
chew through concrete as well.