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Re: vaulting pterosaur launch, questions



I'm sure he wasn't the first, but the 'running take-off' was discussed by Speakman (1993). It's based on the relatively puny pectoral muscle mass of _Archaeopteryx_. The argument runs that having flight muscle mass at or below 16% of total body mass renders the bird incapable of a stationary take-off. _Archaeopteryx_'s is estimated at 9% - so it's well below the cut-off. Grebes are said to have a flight muscle mass below 16%, which is why they require a requiring a "taxiing" run in order to become airborne. Speakman doesn't necessarily endorse this line of reasoning; and the grebe example comes from Marden (1987).

Ah, interesting. I'm afraid that they seem to have misinterpreted. In fact, looking at the Magnan dataset I have here in front of me (as reported in Greenewalt's 1962 compilation), I note that many running launchers have flight muscle ratios above 16% (most anseriforms, in fact) while many standing launchers have flight muscle fractions below 16% (many raptors, owls, and some herons).


Grebes do run to launch, and they do have small muscle fractions. What those authors seem to have overlooked is that grebes also take off from water, have hindlimbs placed far back on the body, have short hindlimbs unsuited for leaping, and have very high wing loadings. The low muscle fraction actually plays into the same gestalt: aquatic birds are often endurance flyers, with low muscle fractions composed of highly oxidative muscle - good for steady, fast flight at cruising speed (but not so good for bursts of power). All of these factors are related to the running launch in grebes (and similar semi-aquatic birds). Running launch in birds is a derived condition (at least among modern forms), and appears to have evolved as a way to maintain takeoff ability in the presence of several other morphological and ecological derivations, usually associated with aquatic living. The cost of requiring a runway is largely offset by the open habitat.

The only trait from the list that Archaeopteryx seems to have demonstrated was low muscle fraction. However, seeing as how there are living owls that can standing launch, with prey, at a flight muscle fraction of just over 10%, I fail to see this as a reason to assume a running launch. In fact, I fail to see how the running launch would help it very much.

It would seem (though this is merely an impression) that Speakman and Marden made the error of assuming that standing launches are more wing-based while running launches are more hindlimb-based, with regards to initial power impulse. In fact, early stage launch in both is mostly hindlimb driven - standing launches in birds use mostly the hindlimbs for initial power (see Earls, 2000).


Cheers,

--Mike

Earls, K. 2000. Kinematics and mechanics of ground takeoff in the starling Sturnus vulgaris and the quail Coturnix coturnix. Journal of Experimental Biology. 203: 725-739

Greenewalt, CH. 1962. Dimensional relationships for flying animals. Smithsonian Miscellaneous Collection 144: 1-46


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