Re: cause of Gigantism in sauropods

["Habib, Michael" <MHabib@Chatham.edu>]

On Feb 7, 2011, at 12:45 AM, <vultur-10@neo.tamu.edu> wrote:

>>> 
> 
> So in the Morrison, the sauropod-theropod size gap seems smaller than the 
> elephant-lion one. I see little reason to believe that Saurophaganax or A. 
> maximus could not take down even Giraffatitan or Supersaurus.

Neat comparison with the body mass estimates (thanks for punching the numbers!) 
but I'm not sure I quite agree with your conclusion.  It seems reasonable that 
something like Saurophaganax could take down something like Giraffatitan under 
very rare, extreme circumstances, just as living terrestrial macro-predators 
(or groups of them) very rarely kill much larger animals than themselves.  
However, I see no reason to expect that such events were common, or even 
occurred with a high enough frequency for us to seriously consider them as 
major factors in our  reconstructions of Mesozoic ecology.  Living terrestrial 
vertebrate predators rarely take prey even three times their own mass, much 
less 6-8 times.  

The elephant-lion size ratio probably does not represent the ratio at which 
predation is regular or ecologically important; at best it is a ratio at which 
a very rare predation event is still barely feasible - and that is for a 
specific guild of predators and herbivorous mammals.  The more important size 
ratio is the maximum predator:prey mass ratio among *regular* predation events. 
 Phrased as a question: Of those large terrestrial animals that are predated as 
adults with a high enough frequency for its impact on total population 
mortality to be measurable, how large are their smallest predators (or total 
mass of packs, if they are predated by groups)?

I don't know exactly what the answer to that question is, but qualitative 
observation suggests that the size gap is pretty small.  The vast majority of 
predators, even large ones, mostly take prey smaller than themselves.  Even 
animals like water buffalo, which are a fraction of the size of elephants, are 
large enough as adults to be predated upon rarely (albeit more often than 
elephants).  

Cheers,

--Mike


Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181