Re[2]: ERECT LEGS, WALKING SPEEDS & METABOLICS (was naked mo

[ccp@okway.okstate.edu]

     GS Paul wrote:
>...erect legs first appeared in very small protodinosaurs, and many dinosaurs 
were small. Giant dinosaurs merely inherited erect legs - why people keep 
saying that dinosaurs had erect legs BECAUSE they were so big escapes moi.< 

response:
Well, vous draw attention to an important distinction here. What I wrote was 
that "erect posture and columnar legs are a likely prerequisite for really 
large body mass...large body size "forces" erect posture no matter how fast 
the thing moves or can move, and regardless of its metabolic rate." My point 
therefore was that dinosaurs could be so big BECAUSE they had erect legs, and 
I was trying to point out that hypothetically tachymetabolic sauropods and 
hypothetical bradymetabolic sauropods both would have had to have erect 
posture as a function of size alone, so posture (considered alone) in large 
sauropods can't be a crucial test of their metabolic status. Note that this 
posture per se argument can be separated from the argument from locomotor 
speeds.  

Paul:
>Being gigantic in 1 G is very hard to do. If anything giant size itself may 
force high AEC on land.<

response:
This statement of "fact" is exactly what I objected to in opening this can o 
worms. Why? Why should giant size "force" high aerobic exercise capacity? (And 
"high" compared to what?) Again, this (non-)argument should be viewed as 
separate from the locomotor speed argument. 

Paul:
>More specifically, the mass specific cost of locomotion decreases with size. 
Problem is, so does mass specific AEC. The exact slopes for both are iffy, but 
they both decrease in parallel. What it all means in the end is that a giant 
reptile still has such a pathetic AEC that it cannot sustain a mammal-like 
walking speed. A 50 tonne tachyaerobic {sic; presumably "bradyaerobic" is 
meant here} animal has an AEC of only 8000 kcal/h. Since it costs over 4000 
kcal for such an animal to walk a kilometer, it can aerobically sustain a 
walking speed of only 2 km/h. But sauropods regularly walked 3-5 km/h, the 
same high speed observed in elephants. Conclusion. Sauropod energetics were 
probably more like those of elephants than reptiles.< 

response:
     First, in my opinion, the use of mass-specific units confuses things a 
     lot more than it clarifies things. Animals don't live mass-specific 
     lives, they just live; an elephant doesn't move a gram at a time, it 
     just moves. In fact, in ecologically relevant whole-animal units, both 
     aerobic exercise capacity (aerobic scope for exercise, = maximal 
     aerobic metabolic rate minus resting metabolic rate, kcal/h) and the 
     cost of transport _increase_ with body mass among extant animals. 
     Obviously, it costs a large animal more energy to move at a given 
     speed than a small animal. Since the cost of transport (kcal/km) is 
     approximately independent of speed within the aerobically supported 
     range, it always costs a large animal more energy to move a kilometer 
     than a small animal regardless of speed.
        But the question is: tachyaerobic or bradyaerobic? I don't know the 
     source for your calculated AEC of 8000 kcal/h, but stipulating it for 
     the sake of argument: Scaling up from extant reptiles (dangerous, but 
     we have no choice), the predicted standard metabolic rate of a 
     50-tonne "physiological reptile" at a body temperature of 35C would be 
     about 1700 kcal/h (+/- maybe 15% depending on which allometric 
     prediction equation you use)...therefore your AEC implies a factorial 
     aerobic scope of only 5-6, which is definitely the low end of the 
     observed range. Give that beast a reasonable factorial scope of 10 and 
     suddenly it's twice as fast.
        But really all this wanking around with allometric extrapolations 
     is a waste of time because a) long extrapolations are very sensitive 
     to small differences in the assumed scaling exponent and b) the 
     confidence intervals of the prediction from an equation extrapolated 
     out that far will be incredibly wide. In other words, given the 
     observed variation among extant reptiles, there really is _no way_ to 
     confidently predict the resting _or_ the maximal metabolic rate of a 
     50-tonne specimen. We simply have no suitable comparison. 
        Stipulated that big sauropods had erect posture and could walk 
     indefinitely at 5 km/h. I still see no compelling reason for me to 
     have to grant them an avian or mammalian level of metabolic rate. I do 
     see lots of compelling reasons (heat dissipation, energy intake 
     requirements) to NOT grant them such a metabolic level.