Re: gigantism as liability

[Michael Habib <mhabib5@jhmi.edu>]

john bois wrote:

> The article's authors count gigantism as some sort of 
> achievement...one that mammals have been unable to attain. I believe 
> this perspective is flawed, biased (perhaps) by the infernal 
> anthropomorphic attitude that bigger is better.

It depends on what sort of metric one uses - large size invokes special 
mechanical constraints, and thus represents a sort of biological 
"solution" to interesting problems.  At the same time, I agree with 
your sentiment that "bigger is better" is a subjective tendency that 
probably clouds some analyses.  Particular size ranges are only "best" 
in the context of specific circumstances.

> I find most of the actual explanations in the article well considered; 
> but there is, in my view, a major omission, an omission in whose light 
> mammals relative small size may be seen as more impressive.

It all depends on your point of reference.  Mechanically speaking, 
extremes of size (both large and small) are most "impressive" because 
they invoke special constraints.  Ecologically speaking, I wouldn't 
tend to consider the 'impressiveness' a key feature.  In this case, the 
interesting bit is that terrestrial mammals don't achieve sauropod 
sizes, even when under selection for gigantism (observed by the 
presence of rapid size increases in some mammal lineages).

> The article does refer to sauropod reproductive biology, but does not 
> talk about the likely need to defend the nest. Sauropods were too 
> large to hide. This surely made nest defense imperative, providing a 
> huge selection advantage in even larger size. Comparing the 
> imperatives of nest defense with the breeding biology of large mammals 
> is instructive: sauropods must either stand and fight or lose babies; 
> mammals can run away with baby on board. This is a critical difference 
> between the two taxa, a difference that should at least be noted in 
> such a discussion.

I haven't read the article yet, so I can't comment on whether or not 
that should have been included in the discussion, but it does seem like 
the relative advantages and disadvantages of oviparity and live-bearing 
might interact with size selection in interesting ways.  I'm not sure 
that oviparity is quite as purely negative for large dinosaurs as you 
seem to imply, but it does have a unique set of costs and benefits 
relative to the placental condition.  One of the primary size effects 
is probably seen in neonate size, in fact, more than adult size - 
sauropods apparently maintained a relatively "r-selected" type breeding 
strategy in terms of juvenile size, juvenile growth, and clutch size.  
This probably works very well under a wide range of conditions, but may 
become a liability under specific circumstances.

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