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Re: Pterosaur size


----- Original Message ----- From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Friday, December 15, 2006 7:41 PM
Subject: Re: Pterosaur size


launch cycle in detail? That would help a lot. --Don

Jim has worked the most on this launch cycle model; it was presented at the 1999 SSA conference, and perhaps elsewhere as well.

I forgot about that one. That was the first time, an hour talk at the request of Paul MacCready. Then, I touched on it at Toulouse in 2001, and at the 7th International conference on Vertebrate Morphology in Florida a couple of years ago. I don't remember if I brought it up at the Advance Physics Conference in New Jersey or not.

I don't know if he's had a chance to write up a full paper on it. I am also working on the launch mechanics, but that is a project in progress and also has not hit print yet.

I have a presentation scheduled on it for the coming year. I'm not going to say too much about it in detail until then. I will be sharing my information with Mike if he wishes. His work is totally independent of mine. JimC

Jim may be able to supply you with copies of his abstracts, I don't have them with me at the moment. I could give you a much more detailed description, but seeing as how it is mostly Jim's work, I will leave that to his discretion. MH

Mike, feel free to say anything you like. The more of us working on this stuff, the better. JimC

True, but their [herons] use of high amplitude strokes is mostly a result of their particular kinematics, especially the use of a ring vortex gait during launch (typical for most birds). That gait requires a higher amplitude than would be used by a launching pterosaur (much higher). Interestingly, size increases can lower stroke amplitude if the change in size places the animal into a flow regime where they use a high speed gait with more continuous flow. Such animals also have a higher speed launch window, however, and must either use a very powerful launch (pterosaurs) or run quite fast, at least for a short distance (running launch birds; including most seabirds). Just to give you a good visual, imagine the difference between a pigeon launching and a large gull launching. The gull, despite being larger, has a lower launch stroke amplitude. --MH

Were mega-volants the only phenomena difficult to explain relative to current conditions I would be much less hard-headed (by the way). --Don

Fair enough; the probability of achieving a given phenotype depends on many ecological factors. However, evaluating a phenotype for its flight performance is still a mechanical problem. Determining how selection produced it is an ecological problem again. It turns out that mega-volants aren't actually hard to explain mechanically, in the sense that it is seems apparent now how pterosaurs were able to launch at large sizes. The mechanics also strongly support the conclusion that modern conditions would be comfortable for pterosaur launch and flight. As to why they became so large, that is more difficult. There are a number of flight advantages for a soaring animal if it is large-bodied, so I hypothesize that this strong selective advantage played a role. Food resources, etc. must be sufficient as well, however. When they are, large size can be very helpful for a pulse flapping high-speed soarer utilizing a highly loaded planform morphology. --MH


All true. However, there is no single limit. There is a limit for each morphotype, for any given condition. It turns out that morphotype variation explains the variance in maximum observed size between clades.

In fact, because the density of the Cretaceous is still not known with certainty, the lift strip models generally just use modern air parameters.

I have density and altitude as parameters, but generally just leave them set at today's values. There is so little proposed difference that it just don't make two whoops and a holler which one you use. JimC

So the models are really run for a pterosaur flying in modern conditions, and the results strongly suggest that they would perform admirably. --MH

High altitude migratory birds aren't optimized for sealevel flight, even if they nest there, as high altitude selection obviously occurs. Those examples demonstrate very little about density effects. (Heh. Wind blows like hell up there. V^2 and all that.) --Don

Insert -- Again, what does wind have to do with anything, once you are in the air? JimC

So while it is true that all flying vertebrates are playing a trade-off role, most of the accommodation for density changes is probably made at the individual level. Just the same, getting some high frame footage at known altitudes of marked birds might be a cool project. --MH

Yes, it would. JimC

At _any_ weight, the effects of medium density change are NOT easily compensated for if the other critical variables (circulation, temperature, and composition) are controlled. The effects are easily observable in lab in both wing kinematics and various metrics of power, especially lift generated per power expended. --Don

Well, I'd expect that is true for insects, but larger-bodied flyers should compensate pretty effectively. Do you have a particular study in mind that shows otherwise? --MH

Further, the flight morphologies of birds that are optimized at
5500' are measurably different relative to sealevel birds (per flightstyle/species), ditto w/ insects. (See Feinsinger P, Am Nat v 113, #4, 481-497 for Andean hummingbirds). --Don

True, for hummingbirds and insects the differences can be measured, and it is not surprising. However, hummingbirds fly in a flow regime rather like that for large insects, and very different from that of large birds (not to mention they have vastly different kinematics). That is a cool study, and I am not particularly surprised with the results. --MH

Nor I. It's not at all what we've been discussing. JimC

Now, you seem to be saying that because a 27 lb goose can fly in uncontrolled conditions, that is support for a 150 lb whatever flying in standard air. I am surprised you put forward these 'in vivo' necdotes. --Don

Actually, I only suggested that large-bodied living flyers (birds esp.) fly at various altitudes with little to no difference in kinematics. The differences should be felt less, if I'm not mistaken, for larger-bodied flying animals. It is not a direct connection, I grant, but there was a method to the madness. The comments regarding emergency landings were Jim's, not mine. I found them enlightening, myself, but cannot comment on them further than that. --MH

I was referring to a 4 hour flight at maximum performance in a gale, not an emergency landing. It was near the end of a flight from Scotland to Iceland, and the animal was at severe risk of missing Iceland entirely. The paper describing it is an enthralling read (as is most of Colin's stuff). JimC

It depends on what you mean by better. Many performance characteristics improve with size, others do not. There are only a few living morphotypes within flying vertebrates that are good candidates for selection for larger size. Fast-soaring seabirds are one such group, and albatross-type birds can achieve some performance advantages from increasing in size, *if* they scale their relative hind limb strength accordingly to accommodate launch. This is what pseudodontorns appear to have done. I am the only person who has looked at that for pseudodontorns, so the only reference at the moment is my abstract from the Calvert Meeting this past November. Full paper forthcoming. MH

As to size limits, please allow me to communicate by re-statement again. I think of the mega-volants as _approaching the practical size limits for their flight morphology at that time_. And I feel you should say, "_theoretical_ mechanical size limits" no matter how much modeling you've done. --Don

Okay, fair enough. In that case, why do you think they approached the practical size limits for their flight morphology at that time? I wonder this because large azhdarchids fall below the theoretical mechanical size limits for their morphotype. While ultimately theoretical, this is based on substantial physical evidence, is quantitative, and is based on known (non-theoretical) aerodynamic principles and observations of animal flight. --MH 

I have no problem with insertion of  'theoretical' either.  JimC