Re: Fwd: Re: New Study, T rex Could've Been A Scavenger

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

> >> >[...] an elephant still needs big   
> >> >ears, despite having lower metabolic rates per mass than a mouse.   
> >>   
> >> ==============================================  
> >>   
> >> Ah, but as Spotila (1980) mentioned on this; is the fact that an  
> >> elephant's MR is only slightly higher than a reptile of equivalent 
> >> size 
>  
> >Is that a fact? I knew about e. g. sea cows, but elephants?  
> ================================================ 
>  
> According to Spotila: 
>  
> "...in order to maintain a constant body temp of 37 [degrees] C. ... A 
> large ectotherm (100 cm body diameter) would need a net heat production  
> of less than 0.1 cal cm^-2 min^-1... This is similar to the metabolic  
> heat production of elephants." 
 
_would need_. Is there an ectotherm with such heat production? This quote 
doesn't tell. 
 
> ________________________________________________ 
>  
> >> the result  
> >> of a requirement for this MR, or an evolutionary holdover (i.e.   
> >> automatic endothermy is hard to get rid of once evolved)?  
> >  
> >Then who says endothermy isn't plesiomorphic for dinosaurs?  
>  
> ================================== 
>  
> For one, I don't see any evidence to suggest it. Dinosaurs showed a 
> variety of adaptations for increased aerobic exercise, but these are  
> adaptations seen in other archosaurs, and other reptiles,before, during  
> and after the dino reign (including present day). 
 
There are quite interesting adaptations in basal crocodylomorphs, and 
maybe in rauisuchians, but apart from those... even *Pristichampsus* 
(apparently the only known terrestrial croc with known postcrania) looks 
like an ambusher, built for bursts of acceleration, not for endurance. 
Does not look like any basal theropod, basal sauropodomorph, basal 
ornithischian, or basal dinosauromorph; compared with *Pristichampsus* or 
any crocodyliform they are cursorial. 
 
Then you still haven't explained the heart problem, IIRC. Big animals, 
especially ones with long necks and tails, even more so when the neck is 
vertical, need huge hearts. And such a heart alone must have a metabolism 
and heat production that exceeds that of a sauropod-sized ectotherm. In 
addition, to feed this, big animals have to eat a lot, and hard-working 
internal organs produce even more heat. (Even just cutting proteins, 
polysaccharides etc. apart produces heat.) Our blood leaves the liver with 
a temperature of 41 °C. 
 
> _____________________________________ 
>   
> >> =========================================  
> >>   
> >> As I mentioned previously, endurance has a false correlation to  
> >> tachymetabolism. Yeah, if one compares all extant reptiles to all   
> >> extant mammals and birds, then one should get a noted correlation   
> >> between tachymetabolism and  
> >> increased endurance, but if one adds fish and insects into the mix  
> >  
> >Fish swim. Swimming is a lot cheaper than walking, as GSP quantified  
>  
> >(any movement of the body moves you through water; a leatherback  
> >turtle's metabolism could not sustain any reasonable walking speed). 
>  
> =========================================== 
>  
> I wonder if a whale's would?  
 
Of course. (Which may be why whales can sustain much higher speeds than 
leatherbacks.) 
 
> Either way, the "swimming is cheaper" argument only goes so far. Not all 
> fish are endurance champions. 
 
_Compared to ectothermic terrestrial animals_, most are. 
 
> >> As for brooding, I could bring up those pythons again.  
> >  
> >They are part of the argument that constant high metabolism is good for  
> >brooding. :-)  
>  
> ============================================= 
>  
> But, does it make a lot of evolutionary "sense" to evolve a constantly 
> high metabolism, for the "purpose" of keeping eggs warm during a  
> certain, limited, time of the year? 
 
The argument isn't my idea... OK, maybe if the brooding season lasts all 
year long (Mesozoic climate...), then it could be. Also, I didn't present 
the entire hypothesis (which is described in the Ostrom Symposium volume); 
high metabolism alone isn't enough, you also need adaptations to sustain 
that, i. e. to find enough to eat. 
 
> Wouldn't it just be cheaper to have a high 
> metabolism for those parts of the year? 
 
If the adaptation that increases your metabolism are the famous leaky cell 
membranes, then you can't switch it off again. 
 
> Side note: anyone know why skunk cabbage evolved endothermy? 
 
For evaporating its fragrance. 
 
> ______________________________________________ 
>  
> -----------------------------------------------  
> >> Is not gigantothermy anything more than just scaled up inertial  
> >> homeothermy?  
> >  
> >Sure. Which is why I mentioned it even though you didn't mention the 
> >word.  
> >Anyway, the real point of this is that gigantothermy doesn't give you  
> >chemical energy, while endothermy does, and for muscle contraction you  
> >need ATP.  
>  
> ================================================= 
>  
> Hold on a minute. Feel free to refresh my memory here, but I never 
> remember reading on endothermy giving any animal (or plant) chemical 
> energy. 
 
Slightly stupid wording on my part. But nevertheless, it works around a 
few corners. Tachymetaboly allows you to stay so active that you can find 
enough food to sustain tachymetaboly. It also means that you generate more 
ATP per time, which you can use for e. g. muscle work. 
        Gigantothermy is homeothermy with bradymetaboly -- so a 
gigantotherm is a lumbering giant, an extreme slowpoke. With the exception 
of the only actually known gigantotherm, the leatherback, which cheats: it 
is extremely well isolated -- e. g. the working leg muscles are in the 
body core -- and moves very efficiently. Is not an option on land. 
 
> In fact, this is practically the antithesis of what so many metabolic  
> studies have shown. It costs a lot of energy to be a consistent  
> endotherm. Even facultative critters like honeybees, pay for it in terms 
> of nectar requirements. 
 
And get rewarded with the ability to collect more nectar. It is a bit 
circular, right. But think of it as an investment: you expend more energy 
to get back even more than you expended. 
 
> ___________________________________________________ 
>   
> >> I do have aerobic endurance info on varanids though (big surprise >:)  
> >>   
> >> Studies on varanids by Bennet (1972) showed that at mammalian body  
> >> temps, aerobic scope was essentially the same.   
> >> They incur oxygen date no faster  
> >> than mammals, and repay it just about as quickly.  
> >  
> >Is that about endurance? Or just about how high their exercise  
> >metabolism 
> >can rise and how good their remarkable respiratory system is (in short,  
> >they aren't really bradyaerobic)?  
 
(Interesting information on varanid lungs in another chapter of the Ostrom 
Symposium volume. Along with a plausible hypothesis about how air sacs can 
evolve. Unfortunately the author seems not to know about the evidence for 
air sacs in theropods and therefore probably puts that transition far too 
high in the theropod tree.) 
 
> ================================================== 
>  
> I almost wonder if we are debating two separate things here. 
 
Looks like it. Unfortunately I am capable of that. :-) 
 
> I'm certainly 
> not arguing that dinosaurs were bradyaerobic. I simply think they were 
> bradymetabolic, or eurymetabolic; if you will (i.e. they had a variable  
> MR, not a slow MR, or a consistently high MR).  
 
But... to use a high exercise MR, you need big muscles and a big heart, 
and there we are again. 
 
What about HP GSP's ilium length argument? 
 
> Tachyaerobic lifestyles are certainly possible on a  
> bradymetabolic energy budget. 
 
Hmmm... hm... 
 
> As you mentioned, varanids probably aren't bradyaerobic 
 
But they aren't exactly tachyaerobic either. 
 
> (in fact, Scleroglossa itself, seems to have many 
> species that show increased aerobic capacity), 
 
Sure. 
 
> yet they are bradymetabolic. 
 
I don't know exactly how bradymetabolic. For example, turtles have pretty 
high RMRs (because they can't run away from heat or cold). 
 
> As for the study, it did show aerobic endurance comparable to mammals. 
 
Compared to which mammals? Bears, which seem to be somehow comparable in 
build? 

-- 
+++ GMX - Mail, Messaging & more  http://www.gmx.net +++
Bitte lächeln! Fotogalerie online mit GMX ohne eigene Homepage!