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Fw: avian flight



> <<A living being has a certain way
> of life, a mutation appears, this way of life provides selectionary
> pressure
> too keep this mutation, and afterwards the mutation is called an
> adaptation.
> And adaptations like wing feathers (or limbs, for that matter, in the
> origin
> of tetrapods) can be exapted for use in other ways of life.>>
> The  mutation happens----> the caracter(s) that that are the espression of
> the genetic mutation are under a selective
> pressure(with the whole animal bearing it) and if this mutations gives
> some advantages to the animal, against other animal of > the same
> species,( and in general against all the "forces" that will deny it
> survivaland thus reproduction ) with the "old"
> caracter(given the possibility of  the permanence of the same
> enviromental conditions for some generations), then it will
> probably become the most present "version" in the population;
>
> you wrote., <<this way of life provides selectionary pressure
> to keep this mutation>>I don't understand what you mean by "selctionary
> pressure to keep this mutation"
> the selective pressure is on, the mutation, meaning( i know  i write not
> very well, i'm sorry; hope you understand what i
> mean) that it's not acting to keep it( it sounds like "trying to preserve
> the mutation"), but against it.

"Selective pressure to keep a mutation" is a bit convolutely worded by me. I
mean the circumstances that make the mutation an advantage rather than a
disadvantage or a neutral character.

> <<Remember the tail of *Archaeopteryx* that prevented it from gliding? >>
> Mr Cunningham answer to this...

In his answer (thanks for the explanation of glide ratio) HP James R.
Cunningham writes:

> However, it will arrive at the
> touchdown point in only 70.7% of the time it took at the lighter weight.

That's it. Ebel has addressed this mathematically. First he refutes the
possibility that *Archaeopteryx* could have parachuted: When assuming
present air density (small changes there would hardly alter the outcome), a
weight of 250 g and a wing area of 479 cm² (too low), Archie's speed of
descent was 9.13[8233248 in my calculation with the same formula] m/s,
"corresponding to a free fall from a height of 4.2 m, would hardly be
acceptable to a bird with delicate bones"; more recent estimates of wing
area (754 cm²) still give 7.283570407 m/s. This is still very much. (754 cm²
is probably too high, as it assumes a full set of tertials instead of a gap
between the wings and the body, but on the other hand it does not take into
account tail area; these two effects more or less cancel each other out.)
Then he addresses gliding. Unfortunately, he does not give the formula here
(it can probably be derived from the figure, but I can't do that); however,
he writes:
        "Even if the rate of descent may be rather low in a good glider, in
any case the total kinetic energy must be annihilated when arriving on the
ground or on a branch. An impression of this problem is given by an
albatross which has to land in calm air, but often tumbles over. The speed
of *Archaeopteryx* during gliding would be approximately v = 12.9 m/s, with
an estimated lift coefficient of cL [index L] = 0.5, which is surely not too
small [I can't decide this]. This corresponds to a free fall from a height
of 8.5 m and would probably lead to an injury [rather, death] if the speed
could not be reduced prior to touch-down. However, the available mechanisms
for speed reduction are restricted, since the maximum lift coefficient
during landing could hardly be greater [what an understatement] than the
maximum drag coefficient of cD-max = 1,4 of a cup-shaped parachute. To be
able to achieve a lift coefficient of cL-max = 1,4 an airfoil section must
be perfectly developed. This cannot be expected at the beginning of the
[sic] bird flight evolution. Here also the not yet evolved wingstroke cannot
serve as an escape from the dilemma [means: Gliding can't be a precursor
stage to flapping if gliders already need to flap]. Furthermore,
considerations regarding the origin of flight must take into account that at
the beginning of the evolution the wing area must have been substantially
smaller than that of *Archaeopteryx*. To be able to fly up a landing site in
a curve so that the speed is completely reduced at arrival, as Recent birds
can do, requires completely developed flight abilities. [Coffin for
trees-down.] [...] it appears very doubtful whether the flapping flight of
bats (RAYNER 1985, PENNYCUICK 1986) might have evolved in gliders. [Ebel
offers no alternative, though.]"

Once more, I strongly recommend getting the paper:

Klaus Ebel: On the origin of flight in *Archaeopteryx* and in pterosaurs.
Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 202(3)
(December 1996), 269 -- 285

I could send scans, though copyright... :-]

> <<Prey in
> the mouth would have pulled the center of gravity forward and thus made
> flight easier. >>
>
> has someone tried to quantify this "easier"?

No, not even Ebel. Good idea.

> << in the air above
> small ones, there are the same insects as on the dry land around.>>
> Infact I said that , given the possible use of this scenario( an aquatic
> theropod trying to get something in the air from the
> water surface) as one of the reasons that possibly led to the evolution of
> flight, it would be nothing different from the
> scenario imagined as a support to the ground-up theory.

bit complicated...

> Have you ever talked about large masses of whater?

Yes. *Archaeopteryx* was found in sea sediments. And about small bodies of
water... there is no advantage a semi-aquatic insectivore would have over a
terrestrial one in there.

> <<The wild form of the domestic chicken that lives somewhere in South East
> Asian jungles doesn't fly more.>>
> what are their predators?

Hm... there are all sorts of small carnivorous mammals around (well, small,
there are leopards).

> Big flightless birds evolved in australiaand other isolated(big or small)
> enviroments because of the absence of big
> predators..you know how the story ended.

Hah! Big flightless birds evolved in Australia: Dromornithidae have just
turned out to be carnivores!
Big flightless birds evolved in Laurasia: Gastornithidae ("Diatrymidae")
have been confirmed to be carnivores!
And phorusracids...
All these were the big predators themselves!

 Carnivorous mammals were around in all these areas all the time. They just
didn't grow larger.

Omni- or herbivorous emus, nandus (*Rhea*) and ostriches have evolved on
continents too. The only examples of your idea are moas and elephantbirds
(as well as several smaller-sized groups like moa-nalos).

> <<You know, ostriches and rheas have evolved flightlessness in
> environments
> that were always full of predators>>
> I'd like to know something more about this, if possible.
> since I've always thought big flightless birds evolved in relatively
> "hunterpoor" enviroments...

I don't know much myself. I just can tell that for much of the Neogene
ostriches were present in all European and Asian steppes.

Lots of species of flightless rails have involved on predator-free islands.
This just isn't the only factor that can lead to the evolution of
flightlessness.

> however do you want to compare the running abilities of chickens and
> ostriches?

I don't...

> "In a jungle, hiding from predators is very easy."
>
> oversimplifying...
> predators can hide as well....

True, but no counter-argument.