Re: Big is beautiful - bioenergetics again (long)

["David Marjanovic" <david.marjanovic@gmx.at>]

> 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.

This has been suggested before (I can only recall the Padian & Currie
Dinosaur Encyclopedia), but AFAIK you're the first one to actually do the
maths!

> Now here comes the first surprise: 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.
>
> So does this show that being big is energetically favourable only for an
> animal with high metabolic rate? I never heard this argument for dinos
> being hot-blooded before. Am I totally in the left field here?

This sounds very plausible. To me it also sounds totally new.

> After this calculation I tried to understand WHY the metabolic rate scales
> as it does. It was pointed out to me that this has probably to do with
> energy costs of locomotion, something I now also read in the book of
> McGowan.  The standard argument is as follows:
> [...]
> So our slow-going mouse will have
> to react VERY quickly whenever the ground is uneven, even if it would like
> to be slow. (If you ever hiked in the mountain, you have perhaps noticed
> that sometimes it is indeed easier to go more quickly on uneven ground, so
> as not to have to make quick adjustments during slow moving).

Yes; though another effect is playing in here IMHO: When you try to walk
slowly up a muddy slope, you will never arrive on the top. When you run it
up, your chances are much higher because the mud isn't given the time to
slide away under your weight. (A very weak form of what the basilisc lizards
use to run on water IMHO.) Similar for going downhill -- if you run down
you'll probably arrive on your feet, if you try to slowly walk down you'll
arrive on your back.

> So I argue
> that for efficient locomotion your stride frequency must be not much
> slower than the reaction time you need. Now this argument I also never saw
before
> - is it again something only a stupid physicist can cook up, with no
> relation to the real world?

I haven't ever seen this either, but it does sound like the real world...

> If not, it has another interesting aspect: It has been argued that a T rex
> cannot have run because it would severly injure itself on falling.
> However, as it is so big, it would have a lot of time to catch itself,
> when stumbling. So perhaps, the T rex simply did not fall, because for it
> the reaction time was quite large.

Sounds plausible too. Giraffes also simply don't fall when they gallop at 50
km/h; they wouldn't survive it either.

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I don't want to bore anyone by repeating all the condolences, which I
somehow feel I should. I hope that I don't offend anyone by not doing so.