Re: cause of Gigantism in sauropods

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

On Feb 7, 2011, at 3:17 PM, Erik Boehm wrote:

> "> Living terrestrial vertebrate predators rarely take prey even three
>> times their own mass, much less 6-8 times.?
> 
> Might there be a problem with scaling laws here?

There is a scaling issue, but it's in the opposite direction of what you 
suggest below (though I completely understand your supposition; it does make 
some sense).  It's actually not terribly uncommon for arthropods to kill prey 
around their own size or larger, especially in marine settings, and some 
cephalopods do the same without a great deal of trouble.  These are still risky 
events - the predators are sometimes killed - but those taxa have life 
histories that can sustain high adult mortality.  

> I don't know of any arthropods that take prey larger than themselves without 
> the use of poisons.

Envenomation is pretty common, however, and it is not clear how critical the 
venom is in prey dispatch for some taxa (it may be more important for digestive 
purposes).  Still, we're looking at a great many centipedes, spiders, assassin 
bugs, and mantids that all kill large prey with some regularity.  In marine 
settings, predatory shrimp dispatch large prey, as do some smaller cephalopods 
(larger ones also kill big animals on occasion, but it's rare).  

Without having numbers applied to the whole thing we can't really pull out any 
firm conclusions, but generally speaking, I suggest that the scaling situation 
suggests that as absolute size of the predators increases, the allowable size 
gap with prey tends to *shrink*, not grow.  Again, this is merely "on average", 
plenty of exceptions exist, and even at small sizes the predators usually 
out-mass their prey.

> When an animal is approaching a physical size limit (just to be able to pump 
> blood, support its own, weight, and maintain mobility), I'd think its ability 
> to defend itself does not increase proportionately with an increase in size.  
> in other words, when on the scale of sauropods, perhaps there were 
> diminishing returns for an increase in size.

That seems intuitive, but do we have anything from ecology to back it up?  
Furthermore, we know from the existence of giant sauropods that most sauropod 
species were not approaching a physical size limit for that morphology.  The 
presence of 60 ton sauropods suggests that 20 tons was not near the mechanical 
limit for that general structure (it was, of course, well beyond the limit for 
other morphologies).

There is also a mechanical scaling argument for the opposite conclusion: the 
mechanical limits of bone and muscle to impact injury often do not scale as 
fast as the ability to support weight and generate power.  This will affect 
both the theropod predators and the potential sauropod prey, but this general 
increase in risk has a disproportionate affect on the predator, who must 
sustain repeated encounters at high risk - this is why, for example, the safety 
factor of cheetah limbs is much greater than that in antelope.  If the antelope 
falls, it's dead anyway (consumed), so breaking a limb makes little difference, 
but the cheetah must be able to sustain the occasional fall and hunt again.  

Yes, if a theropod bit a sauropod just right, it could kill it.  But if that 
neck swung and struck said carnivore in just about any way at all, it would 
pretty much crumple the theropod's skull.  At smaller scales, predators can 
often absorb blows from slightly larger prey because the ratio of power output 
to mechanical impact limit is lower.  At very small scales striking attacks 
don't work at all, in fact, which is why insects use acid, venom, grabbing, and 
piercing, but rarely utilize blunt force attacks, simple crushing, or ballistic 
weapons (a fist is essentially a ballistic weapon, as is a tail strike, etc).  
Vogel (2003) has a nice discussion of this scaling effect.  It's actually very 
difficult for a beetle to crack another beetle's armor by striking it, for 
example, but it can pick it up and and toss it, or burn it, etc. In a similar 
manner, small birds and bats seem "overbuilt" compared to larger ones with 
regards to maximum sustainable skeleton loads, though a few outliers (like 
falcons) do exist.

> I don't think they needed the bite force of a T rex to deliver a death blow 
> to a sauropod that let a theropod get too close to its neck.

True, but the encounter has to proceed "just so" for that to happen.  This 
problem came up a while back with a speculative "blow-by-blow" hypothesis of 
how large theropods should have been able to kill big sauropods without 
difficulty.  The problem with such a model is that it requires far more 
specifics than we have available, and quickly turns into a "ninjas versus 
pirates" type of argument.  Besides, the rarity of predation on giant 
herbivores seems to be relatively morphology independent - there might be the 
occasional big-game specialist in the Mesozoic, but I don't see any reason to 
think that the rarity of predator attacks on much larger prey across multiple 
clades and environments today should not also apply to Mesozoic systems for the 
most part.

In fact, I'll pose the reverse question: why should we suppose that giant 
herbivores in the Mesozoic were predated any more readily than giant herbivores 
today?  (noting, of course, that "giant" is relative, and so we really mean the 
size ratio of predator:prey).  The Mesozoic ecologies didn't have to work like 
modern ones, but given how widespread the size advantage trend is, I think we 
need more than what we have at present to overturn the null hypothesis that the 
Mesozoic size ratios had the same effect as present ones.

Cheers,

--Mike


> 
> --- On Mon, 2/7/11, Habib, Michael <MHabib@Chatham.edu> wrote:
> 
>> From: Habib, Michael <MHabib@Chatham.edu>
>> Subject: Re: cause of Gigantism in sauropods
>> To: "vultur-10@neo.tamu.edu" <vultur-10@neo.tamu.edu>
>> Cc: "dinosaur" <dinosaur@usc.edu>
>> Date: Monday, February 7, 2011, 10:11 AM
>> 
>> 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
mhabib@chatham.edu
(443) 280-0181