Energetics and posture (long; was:erect crocodile problems)

[ccp@okway.okstate.edu]

{My comments in brackets}
______________________________ Reply Separator _________________________________
Subject: erect crocodile problems 
Author:  <cadams@hh.gpz.org> at SMTP
Date:    6/8/98 3:13 PM
     
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.

TonyC asks: Has this been measured then? I would very much like to see this 
data if you have a reference...Again, the only data I've seen put the 
difference between erect posture and lying down (biped) at about 4% 
increase O2 consumption (I think).

{The best measurements are for those most cooperative of research subjects, 
humans, in which metabolic rate while standing up is about 15% higher than 
lying down, with a lot of variation due to small differences in posture, 
alertness, etc. Direct measurements of other animals are few and far 
between; for sheep it varies from 11-22% or so (again, depending on the 
level of alertness when lying and when standing). A study of horses found 
_no_ measurable difference between standing and lying; horses are of course 
famous for sleeping while standing, and they have powerful suspensory 
ligaments that apparently support their weight with negligible expenditure 
of energy. [this info from Blaxter's 1989 book _Energy Metabolism in 
Animals and Man_]
        A more common technique is to measure the metabolic cost of walking 
and running at different speeds; the graph of metabolic rate vs. speed is 
typically approximately linear over the aerobically supported range, and 
when you extrapolate the line back to a speed of zero the intercept is 
typically considerably increased over measured resting levels. This 
increase is usually interpreted as a "postural cost," i.e. the cost of 
holding the body in a locomotory posture without actually locomoting, and 
is usually on the order of 50-100% higher than measured resting levels, for 
lizards, toads, and insects as well as mammals. The postural cost is 
relatively higher in smaller animals [Blaxter p. 160]}
     
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.  

{"The only reason"? "forces"? This is purely conjectural opinion. A modern 
ectothermic poikilotherm could stand up all day long at a metabolic rate 
twice resting and still not even come close to the minimal, basal 
metabolic rate of an endothermic homeotherm of the same body size. And how 
does erect posture "force" elevated aerobic exercise capacity? It seems 
more likely to me that both are correlates of selection for higher levels 
of locomotor performance, without being causally related to each other. 
The vastly higher metabolic rates of endothermic homeotherms are not 
directly due to posture, but to thermoregulation.}

All vertebrates with fully erect postures, large and small, have high 
performance cardiovascular systems, high surface-area respiratory systems, 
and physiological core temperature regulation.

{Correlation does not equal causation. Really, the postural explanation 
for the evolution of endothermy (tracable at least to Heath 1968) is not 
generally regarded as viable any more; see Benton 1979 Evolution 
33:983-997 re dinosaurs, and Hayes and Garland 1995 Evolution 49:836-847 
for a brief review of major hypotheses for the evolution of endothermy in 
general.}
     
     RalphM: 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... 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_! 
     
     {Except that among extant mammals the relative incremental increase in 
     metabolic rate due to posture is _lower_ in larger animals. Also 
     consider the monotremes, perfectly good endothermic homeotherms with 
     sprawling posture.}
     
     
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. 

{Well, it depends on the snake. Habitually arboreal snakes have a number of 
cardiovascular adaptations that prevent blood pooling, including remarkably 
high blood pressure (recent work by Lillywhite). But of course snakes are 
much smaller than sauropods. I don't pretend to have solved the riddle of 
how sauropods got blood to their brains, or back up from their feet for 
that matter, but as far as I can tell the same problems remain even if we 
make the thing magically tachymetabolic. Clearly the brain was perfused, by 
virtue of very high arterial pressures, arterial valves, auxiliary hearts, 
or whatever, but, arguably, "endothermy" is neither necessary nor 
sufficient to solve the problem by itself.}
     
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.
     
     {This assumes that the "need" for endothermic metabolism is directly 
     related to postural support. It just ain't. The benefits of endothermy 
     lie elsewhere, probably first in increased aerobic capacity and 
     therfore endurance, and then later in themoregulation.}
     
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.

{Not sure I follow...big cats lie around but have never evolved 
ectothermy? Yeah, but that's because their endothermy functions to support 
their high aerobic capacity and their thermoregulatory strategy, not their 
erect posture. And how does erect posture "force" high walking speeds? 
Walking speeds are a function of stride length and stride rate, and I see 
no _necessary_ causal mechanism to link either with erect posture. 
Although (maybe because) I'm relatively ignorant on the subject, I always 
take trackway-estimated walking speeds with a grain of salt; they are 
likely on the high side. As for the straw iguana:}

TonyC: I think comparisons with green iguanas reveal little - they are 
tree- climbing specialists, not walking specialists.. Also, logically, 
from A (mammal physiology) not equal to B (iguana physiology), and C (dino 
phys) not equal to B, you can't conclude C=A.

{Hear, hear; not to mention the vast difference in body size!!! I'd guess 
that a sauropod poking along as slow as it could continually walk would 
kick a sprinting iguana's ass. Besides which, even a better-chosen example 
(a big varanid, say) is limited in its maximal aerobic speed by many 
details of its muscular, respiratory, and cardiovascular physiology. 
Posture just doesn't enter into it. "Carrier's constraint" 
notwithstanding.}
     
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 {I assume this should 
read "lower"} 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.

{All this is true, and in fact a pretty good summary. The "semantic morass" 
is indeed in some ways unfortunate, but it is absolutely necessary. The 
dichotomies ectothermic/endothermic, homeothermic/poikilothermic, 
thermoregulator/thermoconformer, and tachymetabolic/bradymetabolic MEAN 
DIFFERENT THINGS. _Most_ extant birds and mammals are endothermic 
homeotherms (therefore tachymetabolic thermoregulators). _Most_ other 
extant terrestrial tetrapods are ectothermic poikilotherms (therefore 
bradymetabolic). But there are innumerable exceptions among extant animals 
(see below) and, of relevance to dinosaurs, there's excellent reason to 
believe that things were not always thus. Today's dichotomies were likely 
yesteryear's gemisch. These issues are far from "clear-cut;" that's what 
makes them so interesting! By insisting on metabolic thermoregulation at 
low ambient temperatures, you are using the single term "endotherm" in a 
sense that greatly expands its true and very useful meaning.}
     
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. 

{no argument there}

 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.
     
     RalphM: Permit me to introduce you to another small and easily 
     overlooked oddity: the naked mole rat of Africa.  {documentary 
     quotations snipped} 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_.
     
     {There is also the Namib Desert golden mole, which also has small 
     size, high thermal conductance, and low basal metabolic rate. Although 
     capable of limited metabolic thermoregulation at lowish temperatures, 
     in the field these animals evidently elect not to use those 
     capabilities: like NMRs they are mammals that have reverted to 
     poikilothermy if not outright ectothermy. (J.Arid Environments 
     18:221-237, 1990)
        As for bats and hummingbirds (and a variety of other small 
     mammals), yes, torpor is in a way a result of selection for energy 
     efficiency (in response to the problems posed by small body size in 
     endothermic homeotherms), but 1) they retain the considerable benefits 
     of endothermic homeothermy while active, so ectothermy is very 
     unlikely to re-evolve (i.e. the fitness benefits of endothermy 
     outweigh the efficiency of ectothermy) and 2) while torpid, they do in 
     fact abandon strict endothermic homeothermy, approaching an 
     intermediate state that has much in common with ectothermic 
     poikilothermy. So in a way they _have_ reevolved a part-time 
     ectothermy.
     
Best regards,
     
Dave    
     
     {OK,
     CC Peterson}