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follow-up on sauropods



I have seen Farlow's remarks on sauropod energetics in The Dinosauria.    I
agree that the overheating problem is there but it is by no means fatal to
an endothermic hypothesis.  Sauropods have a very high surface area/volume
ratio, much higher than most large mammals.  Additionally, it is not
difficult to imagine vascularized neck flaps and any number of other
heat-dissipation adaptations.  

Since metabolic rate scales with a larger mass exponent in reptiles as
opposed to birds and mammals, it has been suggested that with very large
animals, metabolic rates will converge and the distinction between
endothermy and ectothermy may be meaningless.  The problem with this idea
is that the regression lines for reptiles and birds converge at >11,000
tons!  At <100 tons, the lines are still well-separated.  The regression
lines for reptiles and mammals converge at an even higher value.  

Let me put it this way.  Suppose you are a 40-ton Brachiosaurus brancai.
Can you lay down?  No.  Can you sit down?  No.  24 hours a day (or is it 23
in the Jurassic?) you are on your feet.  24 hours a day you have to keep
your 5-ton tail and 10-ton neck elevated, and your 20-ton trunk relatively
straight.  And by the way, you also have to have a sufficiently energetic
cardiovascular system to push blood 20 feet or so upward to reach your
head, a feat so difficult that some paleontologists have questioned whether
any cardiovascular system could sustain such pressures!  Think of the
energy that must be consumed and the internal heat generated if you do
nothing but stand still all day.  Now suppose you are a 40-ton whale shark.
 You do not have to hold your body erect.  You do not have to push blood 20
feet up to your head.  Both of these animals have tremendous thermal
inertia.  Both have lower metabolic rates than small animals.  But the
dinosaur has a high-performance cardiovascular system and a powerhouse
metabolic engine.  The fish is a big slow-moving blimp (no offense, I
actually think they're cool).  You ain't gonna see it breaching, leaping
over poles at Sea World, or doing any of the high-performance aerobic
exertions cetaceans do. 

With regard to growth rates, my use of the term fast was vague.  All birds
have what I would call "fast" growth.  They reach a large percentage of
their adult size in a matter of mere months, often weeks.  This may be
contrasted with reptiles, which generally take a year or more to reach
adult size.  Some salamanders grow quite rapidly, reaching adult size in
only a few months, but only in the larval stage.  They generally do this by
capitalizing on the abundant zooplankters and other small animals in
temporary ponds, so it is doubtful that any terrestrial ectothermic
vertebrate can do this.  Like all ectotherms, they can adjust their growth
rate dramatically according to food availability.  Birds and mammals are
far more limited in this respect.  If unable to acquire enough food to
sustain their high growth rates, they simply die.

In this context, I consider all dinosaur development to be fast and
relatively inflexible, similar to that of birds.  The large size of many
dinosaurs makes it very likely that they took longer than most birds to
reach maturity.  This is confirmed by the dinosaur bone studies I have
seen.  But dinosaurs clearly show the endothermic pattern of growth.
Although some birds and mammals are capable of living 50 years or more,
they reach adult size in far shorter times.  Humans are quite exceptional
among endotherms in the long time they take to reach adult size; the vast
majority of endotherms, large and small, take <5 years to do this.  I
suspect that generally dinosaurs took <10 years, and I consider estimates
of >20 years to be very off the mark.  The problem of finding enough of the
proper food to sustain such rapid growth does not seem to stop ostrich
babies from multiplying their mass 50-fold in a matter of months.  (I wrote
this before I read Greg's post.  Great minds think alike.  [Dave's
Greatness Relation:  great = everyone who agrees with me])  Admittedly the
size differential between adults and neonates is much greater in sauropods,
but this merely means that they will take somewhat longer to reach adult
size.    

With regard to energy investment, it might be argued that elephants and
other large mammals manage to carry embryos for long periods, yet they are
endothermic herbivores that have high rates of intake.  But notice that
elephants and other large mammalian herbivores have only a single large
offspring.  This may be contrasted with large mammalian carnivores which
generally have 3 or more relatively small babies.  Large herbivores need
their babies to be big and as precocial as possible.  Get on your feet,
kiddo, and let's move.  So why couldn't sauropods do this?  Because
eventually a size is reached that puts an endothermic herbivore in a real
bind.  It becomes an issue of whether even eating 24 hours a day, it can
sustain itself.  It has to keep moving as it defoliates the young growth in
a given area.  Energy efficiency is of prime importance.  This is why I
much prefer Greg Paul's relatively gracile restoration of Brachiosaurus
brancai to others I have seen.  Large amounts of fat are out of the
question in my view, as is viviparity.  

The reason I lean toward precocial babies for sauropods is that I have a
hard time envisioning the adults restricting themselves to the vicinity of
the nest for months.  Neonates particularly would presumably need new
shoots, and the adults would have had to range farther and farther afield
as the babies grew.  At the same time they would have to keep themselves
fed.  This would be very difficult and I think precocial babies that could
keep up with the adults to some extent are much more likely.  Birds that
have altricial young generally have carnivorous, not herbivorous babies.
It is interesting and rather surprising to me that Maiasaura peeblesorum
seems to have altricial young.  But this animal is considerably smaller
than many sauropods and here one parent could conceivably forage while the
other tended the nest.  I don't see this as feasible for larger sauropods.

With the strategies clearly evidenced in some dinosaurs and the obvious
constraints, I am forced to wonder whether dinosaurs fed their babies some
sort of processed food, the way some birds and all mammals do.  Such an
adaptation solves many problems of parental care, and I note that mammals,
having hit upon this strategy, have never abandoned it, despite a wide
range of morphologies and habitats.  I will have to think deeply and come
up with some good ways of testing this hypothesis.

Best regards,

Dave