Re: gigantism as liability

[Mike Habib <habib@jhmi.edu>]

 john bois wrote:

> But, unless sauropods vigorously protected their offspring, those  
> dinosaur babies faced a gauntlet of fresh predators at each size  
> class--growing from chicken size through adult! This should rather  
> be seen as a predator opporunity rather than a predator-resistant  
> strategy.

It would appear to be both: like any life history strategy, there are  
both costs and benefits.  R-type strategies can improve juvenile  
survival through sheer numbers, but they also tend to incur high  
mortality rates.  Whether there is a net benefit presumably depends on  
a wide range of factors.  We might hypothesize that the consistent  
production of many small offspring by most dinosaurs, including  
sauropods, implies that the r-type strategy was more effective than a  
K-strategy for those animals.

> I question the value of r selection alone as an effective means of  
> predator defence (although this argument is often made without  
> reference to parental protection), unless we're talking oysters and  
> oak trees.

Well, it sure is common among vertebrates, so the simple explanation  
is that the strategy works.  Perhaps the best study examples are in  
those squamate groups where oviparous species have viviparous  
relatives.  There is also a substantial range in clutch size and  
neonate body weight in some of the living squamate clades.  The upshot  
of it all being that both high-investment and low-investment  
strategies seem to work, and that r-type strategies are pretty clearly  
selected for in some taxa, under certain conditions.

>  So it might be argued that mammals have fewer advantages to being  
> large than sauropods, that selective pressures for large size are  
> not as great for them. If not, why not? Because sauropods defended  
> their nests; at this time of the year they likely faced the entire  
> decimation of reproductive output unless they were large enough to  
> engage in its active defence.

This in an interesting hypothesis, but I remain skeptical in that 1)  
many dinosaurs that also used an r-type strategy were much smaller and  
2) many (if not most) sauropods were probably well above the size  
limits required for active defense - an "overkill" problem, as it  
were.  There is also the issue that the hypothesis assumes nest  
protection, for which evidence is still somewhat vague.

> This was almost certainly an additional selective force operating on  
> them and absent from mammalian reproductive effort (i.e., mammals  
> can run and protect their offspring without the need to stand and  
> fight). And yet this factor is yet to appear in otherwise serious  
> discussion on the issue of why sauropods were so big.
> So I here humbly reassert its importance.

The call for further discussion is a good idea.  I remain skeptical,  
however, that the size differentials are a result of reproductive  
strategy differences.  To be more convincing, your model would require  
stronger evidence of intense nest protection by sauropod parents, as  
well as evidence that giant size would improve nest protection  
ability.  I think we also need more data about the impacts of high- 
care, high-investment viviparity in living mammals.  The strategy  
clearly has distinct advantages, but there are substantial costs as  
well.  The optimum conditions and size ranges for the operation of  
such strategies is, as far as I know, poorly typified (and is further  
confounded by phylogenetic history - there are simply no secondarily  
oviparous placentals to use as comparisons).  It seems intuitive that  
live-bearing and egg-laying should have radical effects on overall  
ecology, but I'm not sure if there are data to support this intuitive  
sense of things, or not.  It might be, for example, that the net  
survival of offspring would have been roughly the same for a giant  
mammal and giant dinosaur of the same mass, for example, despite the  
radical differences in life history.  We don't actually have a strong  
reason to believe the ultimate numbers would differ wildly - this  
could be a case of different solutions, similar effect.

Cheers,

--Mike


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