Re: Resources, energetics and dinosaur maximal size

[Mike Habib <habib@jhmi.edu>]

I look forward to reading this paper, and it would appear to have some  
very interesting ideas contained within.  What I am most interested to  
see, though, is how McNab supports the second statement in the  
abstract: " The factors most responsible for setting the maximal body  
size of vertebrates are resource quality and quantity, as modified by  
the mobility of the consumer, and the vertebrate's rate of energy  
expenditure".

That sounds very reasonable at first, but upon a second look, it's  
actually a remarkably bold statement.  I would, for example, expect  
maximal body size to be set (at minimum) by an interaction of resource  
usage and morphology. For example, basic structural limits come into  
play at large sizes.  The consumer mobility aspect mentioned by the  
author does include morphology, of course, but I'm not sure it  
includes the full range of relevant variance attributable to shape.   
There is also a certain matter of dumb luck, it would seem: there is  
no guarantee that an evolving lineage will actually "find" the  
morphospace that includes shapes capable of hitting the maximal body  
size, which leads to my second big question going into the paper: how  
does McNab support the implicit assumption that the lineages in  
question actually reached their maximal body size?

Cheers,

--Mike H.


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280-0181
habib@jhmi.edu



On Jul 26, 2009, at 11:19 PM, GUY LEAHY wrote:

> DMLers,
>
> -------------------------------------------------------------------------
> Proc Natl Acad Sci U S A. 2009 Jul 21;106(29):12184-8. Epub 2009 Jul 6
> Resources and energetics determined dinosaur maximal size.
> McNab BK.
> Department of Zoology, University of Florida, Gainesville, FL 32611,  
> USA. bkm@zoo.ufl.edu
> Some dinosaurs reached masses that were approximately 8 times those  
> of the largest, ecologically equivalent terrestrial mammals. The  
> factors most responsible for setting the maximal body size of  
> vertebrates are resource quality and quantity, as modified by the  
> mobility of the consumer, and the vertebrate's rate of energy  
> expenditure. If the food intake of the largest herbivorous mammals  
> defines the maximal rate at which plant resources can be consumed in  
> terrestrial environments and if that limit applied to dinosaurs,  
> then the large size of sauropods occurred because they expended  
> energy in the field at rates extrapolated from those of varanid  
> lizards, which are approximately 22% of the rates in mammals and 3.6  
> times the rates of other lizards of equal size. Of 2 species having  
> the same energy income, the species that uses the most energy for  
> mass-independent maintenance of necessity has a smaller size. The  
> larger mass found in some marine mammals reflects a greater resource
> abundance in marine environments. The presumptively low energy  
> expenditures of dinosaurs potentially permitted Mesozoic communities  
> to support dinosaur biomasses that were up to 5 times those found in  
> mammalian herbivores in Africa today. The maximal size of predatory  
> theropods was approximately 8 tons, which if it reflected the  
> maximal capacity to consume vertebrates in terrestrial environments,  
> corresponds in predatory mammals to a maximal mass less than a ton,  
> which is what is observed. Some coelurosaurs may have evolved  
> endothermy in association with the evolution of feathered insulation  
> and a small mass.
> -------------------------------------------------------------------------
>
> One apparent caveat... some coelurosaurs did reach 8 tons... :-)
>
> Guy Leahy