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Re: sauropods: homotherm,heterotherm or gigantotherm?
> Well. If a sauropod was endothermic, it could gather food much more
> quickly than an ectothermic counterpart could. More important, it
> would have much more efficient digestion (recall that in general a ten
> degree difference in temperature doubles the speed of all chemical
> reactions). Would these factors outweigh the increased food
> requirements of the endothermic version? I don't know, and neither
> does anyone else, as no-one's ever published any numbers on this.
I think this is an oversimplification - being an ectotherm does *not*
mean having lower body-core temperature. This is the distinction
between endotherm/ectotherm on the one hand and
heterotherm/homoiotherm on the other.
A lizard usually has the same
core temperature as a mammal (or even higher) as long as it is
active. For a mass homoiotherm (if this is truly possible, which I do
not know), the body temperature should be more or less the same as for
an endothermic homoiotherm.
It can be supposed (and AFAIK this is one of the points of Greg Paul
explained in DA, also beautifully explained in Chris Lavers book) that
being endothermic allows to sustain high activity over a longer time
(i.e., it increases the endurance because it allows for high levels of
*aerobic* activity), but AFAIK, the connection is not proven.
(BTW, if anyone could send me a copy of the Terramegathermy paper, I
would be very grateful. )
If the connection holds, the argument would probably go like this:
1. Large animals have to use food with a small nourishing value
2. Thus they need proportionally more food than small animals
3. Thus they need to be actively feeding over a large amount of time
4. Thus they have to have high endurance
5. Thus they have to be endothermic.
OTOH, the food requirements for an ectotherm are rather small. A few
years ago, I made the following calculations which are more or less
along the line of your question above and are similar to the
calculations in the complete dinosaur on Trex lawyer consumption
(Warning: Some calculations ahead, if you don't like number juggling,
just skip to the part marked ******)
I tried to estimate the food requirements of big vs. small animals.
Taking the equation for the metabolic rate found in The Complete
Dinosaur, valid for mammals:
metabolic rate = 3.75 * M**0.75
I get a specific metabolic rate (i.e. rate/mass)
sp. m.r. = 3.75 * M**(-0.25)
This is in Watt/kg.
So with this, the spec. matabolic rates of animals of different sizes
and the total energy requirements are:
1 kg: 3.75 W/kg 3.75 W
100 kg: 1.18 W/kg 118 W
10000 kg: 0.375W/kg 3750 W
Now Watt is not a handy unit, so I convert everything to kcal/day,
something everyone who ever has seen a diet plan can relate to.
Fortunately, you just multiply by a factor of 20 (not very exact, but
these are estimate anyway). So, how much food do you need to get this
energy? As it is always said that being big gives you the advantage of
being able to use food with low nutritious value, I assume food with a
value of 100kcal/kg. This is more or less what tomatoes have for a human -
you will not get fat on this for sure!
****** So our three animals need per day:
1kg: 750 g
100kg: 23.6 kg
10000kg: 750 kg
These numbers look reasonable, as they would mean that an elephant would
have to consume a few hundred kg of low-value food per day.
As it is not practical to have more than perhaps a few percent of your own
mass in your guts (would make you VERY slow), this shows that it is not
possible for a small mammal to live on this kind of low-level food,
whereas a big one can easily afford to do so.
If we increase the food value, the numbers below change accordingly,
but note that cellulose break-down by bacteria may need a few days, so
the amount of food to be in the guts would still be prohibitively
low. (According to McNeill Alexanders book on optima it is possible to
get about 75% of the food value out of plants with aid of
bacteria. This would mean about 3000kcal/kg cellulose, which is
probably something like 1000kcal/kg plant matter).
Note that all this are very rough estimates because my numbers for the
food value are only guesswork and of course simply extrapolating the
scaling equation to very large numbers is problematic.
Now what if dinosaurs had reptile metabolic
rates? In this case, every figure above has to be divided roughly by a
factor of 10, so even the small 1kg-reptile can easily exploit the
low-level food source. And the turtle I kept as a child was indeed happy
with a few leaves of lettuce, a tomatoe or a carrot, and it did not have
to eat huge amounts of those. The 10-ton herbivore would need about
75kg per day (or less if the food value is higher) which seems not
very much to me. So I am not sure whether this argument alone explains
why large herbivores are always endotherms.
So I think both options are possible and the large herbivore can
sustain its food requirements either as ectotherm or endotherm.
Priv.-Doz. Dr. Martin BÃker
Institut fÃr Werkstoffe
Langer Kamp 8