Re: follow-up on follow-up on sauropods

[ccp@okway.okstate.edu]

{My comments and questions in brackets; sorry if I missed some of this in 
earlier posts}:

______________________________ Reply Separator _________________________________
Subject: follow-up on follow-up on sauropods 
Author:  <cadams@hh.gpz.org> at SMTP
Date:    6/5/98 10:50 AM
     
     As I said, birds and mammals are more limited in their ability to 
     adjust their growth to food intake than ectotherms.  
     
Dwarfism also occurs in the African elephant, 

{Of course these are examples of two very different phenomena, one a 
phenotypic, acute response to variation in food supply ("phenotypic 
plasticity"), the other (presumably) a chronic, genotypic, evolutionary 
response to the myriad selection pressures that can affect body size.}


{from a previous post:}
Energy efficiency is of prime importance. 

But I can envision no adaptations that would obviate the need for an 
endothermic metabolism.
     
     {You know, these statements can be viewed as mutually contradictory. 
     If energetic efficiency is paramount then _ectothermy_ is called for. 
     Actually, your use of "endothermy" here suggests you mean 
     "tachymetabolic." Fact is, an animal the size of a big sauropod would 
     be endothermic (sensu stricto) no matter what its metabolic rate was. 
     There are two necessary and sufficient conditions for endothermy: high 
     rates of heat production and low conductance. There are two ways to 
     achieve high total rates of heat production: tachymetabolism (tissues 
     with high rates of metabolism per gram) or large size (lots of tissue 
     producing heat, even if at a low mass-apecific rate). There are 
     several ways to reduce conductance, the most common being insulation, 
     specialized vascular heat exchangers, and low surface area-to-volume 
     ratios (e.g., large body size). A big enough animal has low 
     conductance (heat loss) and produces lots of heat, and is therefore at 
     least partially endothermic, by virtue of its large size alone.
        So what is this "need" for high metabolic rates? If a rapid growth 
     rate is selected for, once again _ectothermy_ is called for, because 
     food energy can be converted directly to new tissue instead of being 
     "wasted" on heat production. Really fast-growing altricial birds are 
     ectothermic during the period of most rapid growth. Support of the 
     body mass against gravity? Thsat might require a constant cost of 
     maintained muscle tension (ameliorated, I'm sure, by columnar legs 
     supporting a suspension-bridge-like axial skeleton), but I don't see 
     why the whole animal would have to crank up its metabolic rate just to 
     stand.
        Can you convince me?
     
     OK,
     Chuck Peterson}