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eaten words and food for thought
What can I say to such super responses, especially yours, Ralph? Seldom
have eaten words tasted so good.
You say the mole rats have a metabolic rate "less than half" that of other
rodents. At a body size of <1 kg, an ectotherm would be expected to have a
resting metabolic rate <20% that of a comparably sized endotherm. I wonder
where the mole rats fall. Also, you say that their body temperature
approximates ambient. But what about their metabolic rate? Does it
decrease steadily with decreasing ambient temperature?
On the subject of chameleons I must disagree. The oft-repeated
"observation" that chameleons have erect postures is something of an
exaggeration. Chameleon humeri deflect considerably from vertical,
typically they hold them angled both laterally and caudally. Their legs
sweep outward while walking in typical lizard fashion and their trunks make
typical lizard machinations. They do not routinely stand with their legs
directly under them, and it hardly needs to be said that chameleons do not
walk at the speeds of endotherms. Chameleons, both terrestrial and
arboreal, spend most of their time off their feet.
As to the postures of crocodilians, again their humeri and femura are held
at a considerable angle to vertical and their legs sweep outward while
walking. Their trunks bend from side to side. Certainly they can cover
considerable distances over the course of a day, but anyone who has spent
any time with crocodilians will tell you that they simply do not walk at
the speeds of endotherms.
The green iguana study referred to was Moberly, 1968, The metabolic
responses of the common iguana, Iguana iguana, to activity under restraint,
Comp. Biochem. Physiol. 27. I only used this example because it was a
thorough study, you may use any lizard you like. A 50-kg human can walk
indefinitely at a speed of 7 kph. Try to get a 50-kg Komodo dragon to do
this. It is "limited...by many details of its muscular, respiratory, and
cardiovascular physiology." Exactly. The fact is, sprawling posture
animals walk at sprawling posture animal speeds and erect posture animals
walk at erect posture animal speeds. Of course there is variation within
these groups, but surprisingly little. I am puzzled by the occurence of
these two statements in the same post:
"Walking speeds are a function of stride length and stride rate, and I see
no necessary causal mechanism to link either with erect posture."
"I'd guess that a sauropod poking along as slow as it could continually
walk would kick a sprinting iguana's ass."
If posture has no bearing on walking speed, why would a sauropod have a
minimum speed faster than a sprinting iguana? Can't it walk as slow as it
Why don't birds walk at lizard speeds? I don't mean egrets stalking
crawfish in the water. I mean WALKING, getting from point A to point B.
After all, most of them are quite capable of getting around without
walking. So why do they insist on walking at "endothermic" speeds? For
the same reason that humans do not walk at 0.1-0.5 kph. Go ahead, try it.
Walk at 2 meters per minute. Yes, you can do it. But it feels very
unnatural. Your body is not designed for that speed.
In an endotherm, the added energy consumption of standing is relatively
small because most of its energy budget goes into thermoregulation. I was
not arguing that as an endotherm a sauropod must devote a much greater
portion of its energy budget to muscular contraction because it cannot sit
down. I was arguing that if it were an ECTOTHERM, spending all day on its
feet would require it to consume much more energy. This defeats the
greatest advantage of ectothermy.
As for monotremes, Greg has long since pointed out that the lack of a fully
erect posture does not require ectothermy. This does not imply the
reverse. I will be very interested indeed to look more closely at how mole
rats stand and locomote.
In order to establish causation, we have to conduct a controlled
experiment. If anyone has living dinosaurs to experiment on, I would
appreciate hearing about it. Meanwhile, the study of dinosaurs will
involve well-informed speculation, the analysis of correlations, and even
intuition. Those who believe that other fields of science are devoid of
this are deluding themselves. Everything from molecular biology to
astrophysics involves interpretation, indirect evidence, and assumptions
based on experience and intuition. And incidentally, the oft-repeated
mantra that correlation does not equal causation is misleading. ANY
correlation implies a causal link. It does not imply that A directly
causes B. It may be a "spurious" correlation, as they say. Perhaps
unmeasured factor C causes both A and B. Or perhaps A causes C causes D
causes B. Or something more complex. But every correlation implies
All living ectotherms on this planet fall along a fairly narrow regression
line of metabolic rate vs. body size. Why? Why should an insect and a
lizard the same size have similar metabolic rates? Not one ectotherm has a
resting metabolic rate approaching that of an endotherm of similar size.
Why not? Similarly, birds and mammals, two groups that have no common
ancestor less than 250 million years old, both have metabolic rates well
above ectothermic and their regression lines are virtually
indistinguishable. Why should a house mouse and a warbler have similar
metabolic rates? One is forced to wonder whether there is something
universal going on. That's why I'm very interested in the mole rats.
Obviously there has to be a period of intermediacy. Are we catching mole
rats in the process of evolving ectothermy?