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Re: cause of Gigantism in sauropods

On Feb 7, 2011, at 7:29 PM, <vultur-10@neo.tamu.edu> wrote:

> Yes, elephant-lion is rare.
> But Cape buffalo are a *major* part of lion diet, IIRC more than half of some 
> lion populations' diet (Lake Manyara National Park is one, I think <60% Cape 
> buffalo) and that's something like 3-4x the lion's mass; especially since 
> lionesses do most of the hunting, and they're often like 120kg animals, not 
> 200kg. *Allosaurus amplexus* or *Saurophaganax* were larger in comparison to 
> *Diplodocus carnegii*. 

Group attacks by lion on cape buffalo are common in some populations, true, but 
I am under the impression (admittedly from older literature) that solo attacks 
on such animals is extremely rare.  Taking into account that the average 
hunting group of lions is at least 3-4 individuals strong, that means that the 
mass ratio is actually close to 1:1, with the added advantages that a group of 
attackers naturally has on a single target.

> And what were *Allosaurus amplexus* and *Saurophaganax* --doing-- if they 
> weren't specialist sauropod, possibly big-sauropod, killers? Everything else 
> *A. fragilis* was quite big enough to deal with.

I suggest that perhaps they were killing larger juveniles, primarily.  Under 
that model animals such as A. fragilis would be killing smaller juveniles and 
the adults of small ornithopods, etc.  There's no reason to expect that any 
animals must have regularly predated the adults of large sauropods.  And there 
is no reason that the prey sizes could not overlap significantly; prey sizes 
taken by large carnivorans in semi-arid African habitats overlap quite a bit 
among multiple predators.  Far more is made of niche-partitioning that probably 
aught to be.

> But yes, the real titans were probably safe most of the time barring really 
> hungry / desperate theropods. Still, the idea you sometimes see that 'adult 
> sauropods were basically immune to predation' needs, at the least, 
> qualification.

True enough.  However, the reverse notion, that theropods regularly mauled 
giant sauropods to death, is seen very regularly, and seems highly implausible. 
 Adult sauropods were probably not immune to predation, but some of them were 
likely close.



> William Miller
> ----- Original Message -----
> From: "Michael Habib" <MHabib@Chatham.edu>
> To: vultur-10@neo.tamu.edu
> Cc: "dinosaur" <dinosaur@usc.edu>
> Sent: Monday, February 7, 2011 12:11:46 PM GMT -06:00 US/Canada Central
> Subject: Re: cause of Gigantism in sauropods
> 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

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