erect crocodile problems

["Colette H. Adams" <cadams@hh.gpz.org>]

The man asked to be convinced, so here goes.

When a physiologist measures the resting metabolic rate of a crocodile,
(s)he does not make it stand up to do so.  Standing up is not considered a
resting position for a reptile.  A crocodile on its feet consumes
considerably more energy than a resting crocodile.  Although it can
maintain such a position for a long time at warmer temperatures, it soundly
defeats the greatest advantage of ectothermy in the process, namely energy
efficiency.

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.

The problem becomes even worse if we give the crocodile an enormously long
neck.  Its low-performance cardiovascular system will be incapable of
pumping blood to its head.  It is striking and rather hilarious to extract
blood from the tail of a snake or lizard and see what a difference it makes
when you elevate the heart. 

Sophisticated stay mechanisms, columnar limb structure, and other
adaptations of large mammals have never enabled them to escape the need for
endothermic metabolisms.  There is only so much energy savings to be had
without producing such a rigid structure that the animal can barely move.

Yet in the final analysis, it does not matter how many hours a day you
spend on your feet.  Big cats lay around most of their lives, yet have
never evolved ectothermy.  This is because a fully erect posture forces a
walking speed beyond that which can be sustained by ectothermy.  Abundant
trackway evidence tells us that dinosaurs routinely walked at speeds of
5-10 kph.  Yet a green iguana will become exhausted within 17 min at a
speed of <0.5 kph.

The semantic morass that has been created in the field of bioenergetics in
my view serves only to confuse scientists and laymen alike, and cloud what
is really a reasonably clear-cut issue.  Many ectotherms have relatively
high thermal optima, manage to keep their body temperature within fairly
narrow limits during active periods, and have higher metabolic rates than a
given endotherm.  None of this should distract us from the fact that an
ectotherm's metabolic rate decreases with decreasing temperature, while an
endotherm is just the opposite.  None of this should distract us from the
fact that we can plot the resting metabolic rates of terrestrial
vertebrates against their body sizes and produce two very distinct clusters
of data, which do not overlap.  Birds and mammals form one cluster,
ectotherms another.  
 
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.

Best regards,

Dave