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Re: Pterosaur size

1). What you say about the ptero launch technique seems to make sense
if the image I have taken from context is correct. I am sure that I am
behind on my reading. Do you have a reference that delineates this
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 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. 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. I can tell you more about what I, specifically, am doing. That would be a long post, however, and may have to be an off-list correspondence. --MH

No, herons are nowhere near any limit. Yet they use most of their potential stroke amplitude till they get going. --Don

True, but their 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

My take is most easily expressed by re-stating two of
your sentences. Quantifying launch abilities _from a skeleton(s)_ is a
mechanical problem. The _probability_ of achieving the phenotype for a
given launch technique through natural selection is critically affected by the
"ecological part". 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

2) I agree entirely that limits vary per flightstyle, but there are absolute limits, per medium. It is not possible to infinitely improve locomotive phenotype. Further, I posit that, because any living system must engage in multiple processes, theoretical limits for a given process can be approached (rarely), but not achieved. --Don

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, rather than trying to estimate the maximum size for a given morphotype, it is generally easier to simply evaluate the structure of observed taxa and work out their individual performance. We talked about possible maxima earlier, but all were very speculative. By contrast, the performance of large pterosaurs is much less speculative. Other than a gut reaction to Cretaceous pterosaurs being huge, there isn't actually anything to suggest that they were at a maximum size, that they had any trouble launching, or that they would have trouble launching in a modern atmosphere. In fact, because the density of the Cretaceous is still not known with certainty, the lift strip models generally just use modern air parameters. 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

That may be true, to a point. However, they probably come close to being optimized at both altitudes because they can adjust planform and kinematics accordingly. Birds and bats do so quite frequently during flight at any given altitude, based on any number of conditions, so that is their known "first line" accommodation. The accommodation between the altitudes appears to be quite minor, at least based on video footage at higher altitudes. 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

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

3). I see the relevance. The point is that there is not an observed change i
g birds across very different atmospheric densities. You suggested previously that a 15% atmospheric difference would be significant for large vertebrate flyers... --MH

3). I don't. This conversation started out w/ mega-volants. 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' anecdotes. --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 haven't read any author that feels that the extant maximals show any potential for doing well or better (as you seem to imply) at larger sizes. I grant you, they continue to be volant, but just barely (in still air). --Don

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.

I don't know of many authors that have really considered the problem, in the sense that we are here. I also haven't read any author that has demonstrated that extant maximals are at any kind of global size maximum for volancy. Several case studies have suggested that some modern taxa are at the local maximum for their morphotype. That is to be expected. --MH

Also: Reference on Quetz. performance, please. (Heh. You sound like you got one in the backyard. Kin I see him? }: D.) --Don

See above. Papers will be forthcoming as possible; currently finishing some bird stuff first. See Jim Cunningham's abstracts for more details. I am also more than happy to discuss the specifics of the mechanics of pterosaurs and/or giant birds at length. Obviously I'm not hiding any pterosaurs in my backyard, but the material I have seen is pretty telling. It isn't that difficult to calculate the launch speed requirements for a given planform and mass, either. --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

4). I like "maximal volants". Actually, "... the largest observed volant species of their time intervals..." is an excellent definition of what maximal volant refers to. --Don

Okay; if that's the definition then I like it just fine as well. Just needed to clear up its usage. --MH

Plot wingspan vs time,
are the minimums... it is a math thing, as you know. As the specimens are obviously "observed", the burden of distinction falls on those who use the term "maximal" in the theoretical sense. In the same vein, the declining trend of maximal wingspan exists, and as such is real, unless new data destroys it. Relevance or statistical significance may not exist, but I don't think the term "real" should be used as a substitute for "relevance" or "significant". It may sound cool, but it limits the language, and causes confusion. --Don

Well, I used the term "real" because I'm not certain there really is a trend. To put it another way, I think there is already data that breaks that trend, and that is phylogenetic data. I think you have a great first-line observation, but if you take into account kinship and shared traits, then there probably is not a trend, after all. I tend to think about such analyses in statistical terms. So, if a trend is very far from having relevance or statistical significance, then I have a habit of saying that the trend is not "real", because I generally consider the term 'trend' to imply a certain degree of significance. If you prefer using "significant" or "confirmed", etc. then that is fine by me; they are more precise terms anyway. I got used to using reference to trend reality in my training as a comparative biologist. Some comparative biologists say that a trend is only apparent (and therefore, not real), if pseudoreplication removal or similar corrections eliminate the trend. --MH

Cheers,

--Mike H.