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Re: gigantism as liability



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