[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
flights of fancy (or "I'm brave, but I'm chicken****")
There are really times when I wish I had more free time...
A lot has been said about the origin of flight and what modern animals
and fossils can tell us about it. I've been wanting to jump in for a
while, but only now find some time to actually do so. I'll try to be
brief, but I fear there's a lot I want to comment on...
First for an overall perspective: It seems to me that what we *don't*
know about how flight evolved in various animal groups *vastly*
outweighs what we do, and the certainty with which various statements
have been made on this list really bothers me. I'm glad Ronald jumped
on George for that, but I don't think George is the only guilty party.
In any event, on to specifics...
LN Jeff stated the opinion that protofeathers wouldn't help much in
protecting an animal from a fall, and George replied:
> Not necessarily. Any measurable amount of fluff will help.
I don't think this is a worthwhile assessment of the situation. While
it's probably true that any elastic and durable (or better yet
regeneratable) coating should help an animal survive impact, it's not
kosher to look at the feature's advantage(s) in isolation from its
disadvantages. "Any measurable amount of fluff" will also tend to get
tangled in branches leading to all sorts of problems if it is not also
controlled (e.g. with piloerector muscles) by its arboreal bearers.
Nevertheless, if George's idea were correct, one would expect that the
animals most likely to be involved in potentially injurious falls
would be those most likely to have protective coverings. Has anybody
ever considered testing such an idea by doing a comparative survey of
the density and mechanical properties of hair and feathers across a
large number of cursorial vs. arboreal animals? George, wouldn't you
think that sort of science is better than the "Just So story" you're
pushing with the above statement? Barring such an analysis, I fear
that George's statement is just so much fluff (God, the puns are
flying thick around here!)
> Bigger feathers _would_ help, IF the animal were arboreal. There
> _would_ be a significant difference. But how could feathers help a
> cursorial form in any way? They would just get in the way--slow it
This is a rather traditional assessment of the situation which Ronald
has also stated with an IMHO undue amount of certainty. Personally, I
think LN Jeff is on to something with his question about chickens.
I'm glad that Glen Kuban took him seriously, and I hope others will as
well. Like Betty, I grew up with chickens in my back yard (though I
never really thought of any of them as "pets"). I didn't torture them
like Jim Farlow did his emus ;-), but I certainly chased them
occasionally (even though this was before "Rocky II" was released ;-).
Based on what I've seen, I'd say that chickens use their wings in two
different ways when evading a "predator". Regardless of whether or
not they're "trying to fly", they do generate thrust with their wings.
It is thus NOT necessarily true that the presence of feathers will
slow down a running animal. If the feathers are tucked in while the
forelimbs are moved forward during the recovery phase and spread out
during the backwards directed power stroke, the animal can conceivably
propel itself with it's proto-wings even if it's not capable of
generating enough lift to overcome its weight. As for the statement
that this sort of behavior would tend to pull the feet off the ground,
I disagree. Before the behavior is well developed, there's no reason
to suppose that the animals would "try to fly" just because their
descendents eventually could. Keep in mind that spoilers on cars are
designed to keep them on the ground, so wings don't always make an
object go up.
The other behavior that chickens use their wings for is the rapid
change in direction. Roadrunners do this too. By throwing their
wings around and/or generating asymmetrical power strokes, these birds
can cut back better than any football player. It's those sort of
maneuvers that make roadrunners hard to catch (I've chased them too)
rather than their cartoon-fabled speed.
My point is that proto-feathers are not necessarily the hindrance that
they are normally purported to be for cursorial forms. I think the
"energy budget" argument that Ronald and George are pushing is
overblown. I think we know so little about how flight developed that
we can't exclude any particular avenue at this point.
And on the topic of assumptions, LN Jeff stated:
] New genetic variety for natural selection to chose from appears no
] faster in a species in a life threatening situation than in a
] species that can afford to evolve at leisure. An animals genes
] don't know what the species needs as far as mutations, the raw
] material for natural selection.
As Nick pointed out, we now have pretty clear evidence that the above
is false with respect to at least some forms of bacteria (yeast, too,
I think). While the specific mechanisms used by these organisms can't
be used in the same way by multicellular animals, it at least woke
people up to the idea that mutations may not be perfectly random in
their time of appearance. Our cells use various enzymes for error
correction in the copying of DNA strands -- there's no _a priori_
reason to believe that organisms can't control the functioning of
those enzymes (and/or the proteins involved in controlling
recombination events) with respect to environmental pressures. That
is to say, it could well be advantageous to up your mutation rate
during hard times. Granted this doesn't mean that organisms can
lengthen their descendents' necks by reaching for leaves in tall
trees, but it does mean you might be more likely to leave relatively
long-necked descendents when there are fewer leaves available to you.
I'm not saying this does happen, but only that you've got to keep an
In response to Ronald's claim that flight might have evolved for niche
expansion rather than predator avoidance, George claims:
> [reaching new sources of food was] Not as important as escaping
> predators, because the animals cannot know in advance that food will
> be found in new regimes.
To me this sounds like pure and simple arm waving. Animals also
cannot know in advance that feathers will help them fly away from
predators. Or put the other way, the mechanisms that result in
genetic variance didn't know that giraffes could reach more food if
they had longer necks than their ancestors did. That "argument" seems
like a non-starter to me -- one made more because George already
believes the conclusion than because it makes any real sense.
George seems to have understood the principles better when he wrote:
> Chickens have wings, albeit not particularly flightworthy ones, and
> they'll use them however they can.
As would their pre-flight-capable ancestors.
Also in response to LN Jeff, Kata McCarville suggests:
} In the case of a fleeing chicken, since chickens have a fairly high
} birth rate, maybe the intent is to draw the predator away from the
} flock, even at the cost of the individual's life.
I try to be open minded, but the above is an argument based upon group
selection, and evolution doesn't seem to work that way. Individuals
don't develop characters because they are good for the species as a
whole; they develop characters that are good for their descendents in
particular. You can see the distinction in the scenario reported
above: the animal with the trait in question is having its life cut
short, thereby making it less likely to leave descendents than its
conspecifics that don't have the trait...
On another subject, George claims:
> The problem is not with the evolution of flight, it is with our
> present inability to imagine the stages in which this evolution took
It seems to me that the problem isn't our inability to imagine, but
rather our inability to find enough evidence to support any one of the
pathways we have imagined to the exclusion of other possible paths.
Back to George:
> _Archaeopteryx_ undoubtedly had several (probably tens of) megayears
> of flight evolution in its ancestry in order to have evolved the
> flight feathers and other adaptations for flight observed in the
I'd lose the "undoubtedly" if I were George. While George's stance
does seem plausible, the pace of evolution is a neat area of current
research (I'll refer people again to Jonathan Wiener's _The Beak of
the Finch_), and it seems extremely premature to argue about the
strength of the pressures selecting for flight in birds when we can't
even comfortably conclude what the pressures were.
As for Glen's recommended experiments... I don't think you need to
pluck chickens to learn what you want to learn. All you need to do is
film them with a high speed camera and analyze their motion from frame
to frame. That is, you can plot their speed as a function of what
their wings are doing and verify my qualitative observations reported
above. That way all you need to do is chase them and film them.
Sounds like a perfect job for Jim ;-) (though he'd probably want to
get them to run across sand or a force plate and look to see if Ron is
right about them leaving the ground when they flap ;-)
Mickey Rowe (firstname.lastname@example.org)