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Waiting for a giant bird and dino physiology revisted



David Marjanovic writes:

"> and they [birds] have IMHO not yet had a chance to grow
> to gigantic sizes."

I don't see the mystery here about the absence of multi-ton birds.

1) Most birds fly and so there has been an evolutionary "pressure" to keep most of them small enough to get off the ground. I am aware of giant flying condors, etc., but there is probably an upper limit to how big you can get and still fly. Before I am pointed to large pterosaurs, keep in mind that pterosaurs and birds have a different wing "membrane": one is skin and tendon, one is feathers. Maybe this has something to do with how big one group got over the other as far as flying vertebrates are concerned. I don't know, but it seems like an interesting thing to look at. To fully expose my ignorance, has anyone working on pterosaur and/or bird flight explained size differences in terms of feathers vs. skin? Anything about bats?? And let's not forget skeletal differences either. Hmmm ... I'm interested now.

2) In the case of ground birds, their hindlimbs were already modified so that they walk or run with bent knees all the time. This means that their femur is constantly being loaded in tension, not in compression. As bone is stronger in compression than tension, maybe there is a size limit to how big you can get with bent legs. After all, the biggest animals we know of on land have columnar limbs, not bent legs: check out elephants or sauropods. In contrast, even moas have bent knees. Further, all of these big ground birds had no easy option for returning to all fours. Whereas sauropods were probably quadrupedal secondarily and able to modify the forelimbs of their bipedal ancestors back to support structures, the wings of these giant ground birds are pretty reduced.

Thus, size in birds is probably limited by two factors of their evolutionary history: flight and bent knees. The contingencies of history can sometimes limit the development of certain body plans. A good example of this is turtles: they develop the shell early, are successful and radiate into numerous forms, but in over 200 million years there have been no flying turtles or running turtles. Evolutionary contingency is probably also at work on limiting bird size.

Thus, we can wait for a giant bird to tern up (pun intended), or think about this: the two groups of tachyaerobic endotherms living today do not approach sauropod sizes, and even though some extinct mammals did get very large (small sauropod size), dinosaurs have everyone licked in terms of average size, hands down. Even factoring in the estimated masses of the biggest extinct mammals, the average mass of most mammals is under a kilogram, whereas the average mass of most dinosaurs was in the 10kg to 100kg range (from Farlow et al., 1995).

Now, play devil's advocate with yourself. What is going on here? Why are the dinosaurs getting so big and no one else? If physiology is the answer, what's the deal with mammals and birds? How much time do they need to get dino-sized? Or are we waiting in vain? Maybe dinosaur size has to do with MANY factors, not just physiology, and maybe tachyaerobic endothermy alone can't explain dinosaur success at large size. I'm not saying dinosaurs were cold-blooded (boy, I miss that hyphenated word) gigantotherms. What I am saying is that we have to consider many, many factors, including functional morphology, before deciding that dinosaurs are gettin' really big simply because they are warm-blooded (boy, I miss that hyphenated word, too). Dinosaurs are DIFFERENT from mammals and birds in many ways, especially the big ones, and we do these fantastic animals a disservice by trying to shoehorn them into a mammal or bird pigeon hole (pun intended again).

I am very interested in WHY and HOW dinosaurs got so big on the average, and why some became incredibly gigantic ON LAND. The minute we settle on a single factor as the reason for dinosaur success at large size, we blind ourselves to so much else. Sure, maybe dinosaurs were warm-blooded and that was a part of their success. But that's the point: a PART. Life history, reproduction, functional morphology, historical contingencies, the archosaurian body plan, climate, etc., must have all played a part. Otherwise, we should have gigantic land mammals and birds, right? Why don't we? That's what's interesting to me and why I remain cautious about putting so much stake in physiological models: they are a good start, but we shouldn't stop there.

Matt "warm-blooded" Bonnan
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