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Re: What is an "active ectotherm"?



About endothermy in aquatic vs. terrestrial environments, Jonathan
Wagner wrote:

>         If I recall specific heat correctly, it is irrelevant in
> this case.  If the fish were surroundeed by cold Fritos(tm), it
> would get just as cold.

Sorry for the pun, Jonathan, but you're out of your element.  Others
have addressed this, but I want to add some discussion to make sure
it's clear.  If you'd only taken equilibrium thermodynamics, what
Jonathan wrote above would sound perfectly reasonable.  However, if
you subsequently took a course in heat transfer you'd see where he's
erred.

Think of an animal as a little furnace which burns sugars, fats and
sometimes proteins.  The temperature of the animal is going to depend
on the rate at which heat is produced by the burning and the rate at
which heat is being lost to the environment.  The animal's temperature
is stable when these two rates are equal.  The driving "force" which
determines how rapidly heat is lost to the environment is the
temperature difference between the animal and it's surroundings, but
the rate of heat transfer is also proportional to how well the
surroundings can take the heat away.  We stay warmer in air of a given
temperature than we would in water because the water takes heat away
better than air.  That means that a smaller temperature difference is
required between you and the water in order to get the same heat
transfer that you would get between you and the air.  If your
metabolism continues to burn the same amount of energy when you've
moved from air to water, your temperature will drop until the
temperature difference between you and the water is small enough that
the rate of heat transfer equals the rate at which you're producing
heat via metabolism (assuming you were in steady state -- i.e. that
the rate at which you were losing heat to the air was equal to the
rate at which you were producing heat via metabolism).  Ok, I think
I've beaten that horse to death.

This is pretty basic stuff...  If you want to get into some more depth
on the subject, then in addition to the Ruben article Terry Jones
mentioned, you should look for articles by Barbara Block who studies
the evolution of endothermy in fish.  In particular:


 Author(s):      BLOCK BA; FINNERTY JR
 Title:          ENDOTHERMY IN FISHES--A PHYLOGENETIC ANALYSIS OF CONSTRAINTS,
                   PREDISPOSITIONS, AND SELECTION PRESSURES

 Source:         ENVIRONMENTAL BIOLOGY OF FISHES V0040 N3 JUL 1994 pp. 283-302.

And while you're in the library you might want to look for:

 Author:         Heinrich, Bernd, 1940-
 Title:          The hot-blooded insects : strategies and mechanisms of
                   thermoregulation / Bernd Heinrich.
 Published:      Cambridge, Mass. : Harvard University Press, 1993.
 Description:    601 p. : ill. ; 25 cm.

Looks like there are a few other recent articles you might want to
check out too.  Here's a couple:

TI: The evolution of endothermy: Testing the aerobic capacity model.         
 AU: Hayes-J-P; Garland-T-Jr                                                  
 CS: Dep. Biol., Univ. Nevada, Reno, NV 89557-0015, USA                       
 SO: Evolution 49(5): 836-847                                                 
 PY: 1995                                                                     
 IS: 0014-3820                                                                
 LA: English                                                                  
 AB: One of the most important events in vertebrate evolution was the         
 acquisition of endothermy, the ability to use metabolic heat production to   
 elevate body temperature above environmental temperature. Several verbal     
 models have been proposed to explain the selective factors leading to the    
 evolution of endothermy. Of these, the aerobic capacity model has received   
 the most attention in recent years. The aerobic capacity model postulates    
 that selection acted mainly to increase maximal aerobic capacity (or         
 associated behavioral abilities) and that elevated resting metabolic rate    
 evolved as a correlated response. Here we evaluate the implicit evolutionary 
 and genetic assumptions of the aerobic capacity model. In light of this      
 evaluation, we assess the utility of phenotypic and genetic correlations for 
 testing the aerobic capacity model. Collectively, the available              
 intraspecific data for terrestrial vertebrates support the notion of a       
 positive phenotypic correlation between resting and maximal rates of oxygen  
 consumption within species. Interspecific analyses provide mixed support for 
 this phenotypic correlation. We argue, however, that assessments of          
 phenotypic or genetic correlations within species and evolutionary           
 correlations among species (from comparative data) are of limited utility,   
 because they may not be able to distinguish between the aerobic capacity     
 model and plausible alternatives, such as selection acting directly on       
 aspects of thermoregulatory abilities. We suggest six sources of information 
 that may help shed light on the selective factor, important during the       
 evolution of high aerobic metabolic rates and, ultimately, the attainment of 
 endothermy. Of particular interest will be attempts to determine, using a    
 combination of mechanistic physiological and quantitative-genetic            
 approaches, whether a positive genetic correlation between resting and       
 maximal rates of oxygen consumption is an ineluctable feature of vertebrate  
 physiology.                                                                  


 TI: Paleohistological analysis of a growth series of Lapparentosaurus        
 madagascariensis (Middle Jurassic): Essay on growth dynamics of a Sauropid   
 Dinosaur.                                                                    
 AU: Rimblot-Baly-F; De-Ricqles-A; Zylberberg-L                               
 CS: URA 11 37, Lab. d'Anat. Comparee, Case 70 77, Univ. Paris 7 Denis        
 Diderot, 2, Place Jussieu, F-75251 Paris Cedex 05, France                    
 SO: Annales de Paleontologie 81(2): 49-86                                    
 PY: 1995                                                                     
 IS: 0753-3969                                                                
 LA: French                                                                   
 LS: French English                                                           
 AB: Histological examination of a growth series of humeri (ranging from 30   
 to 155 cm in length) of the Mid-Jurassic sauropod dinosaur Lapparentosaurus  
 demonstrates the bone-tissue changes occurring during growth. Those changes  
 are quantified by computerized image analysis. The speed of primary bone     
 deposition was very high during early life then tapered somewhat towards     
 still high values (7 micrometers a day of radial apposition of cortical      
 bone) during later ontogeny, with a somewhat cyclical pattern of deposition  
 suggesting yearly cycles of growth. On qualitative grounds, the extent and   
 intensity of the remodeling process of bone, both in compact and spongy      
 regions, appears to have been very high but the actual rate of bone turn     
 over per time unit could not be assessed. Details of epiphyseal structures   
 show precise adaptations to active longitudinal growth, even at large body   
 size, and to the support of heavy strains among adults. Growth appears to    
 have been of the indefinite pattern. Adult condition may have been reached   
 at an age of two decades but ultimate longevity could not be assessed. On    
 the other hand, observation of demineralized sections with the TEM has shown 
 clear evidence of well preserved collagen fibrils in bone, and probably also 
 in cartilage. Overall, the data are accordant with the current               
 interpretation of Sauropod biology, namely one of land dwelling (rather than 
 amphibious or aquatic) tetrapods with an active, indefinite growth and       
 specialized thermo-metabolical physiology (i.e. mass endothermy or           
 gigantothermy).