Re: How The Giraffe Got (or didn't get) Its Long Neck

[dinoboygraphics@aol.com]

> > > In that pace we will go back to "lamarckism".<<<

Huh?  You can derive elongate necks that aid in feeding with a 
Darwinian selective regime.  Just because people use the case of 
giraffe necks to contrast Darwinian selection with Lamarckian change 
(or rather a caricature thereof) doesn't imply that both can't be used 
to explain it (note, I am not endorsing Lamarck's ideas, merely 
agreeing with S.J. Gould that the textbook description of his idea is a 
caricature of the position Lamarck actually held).

> > > And what about fossils? Are there more or less compleat fossil 
skeletons from ancient Giraffidae?<<<

Yes, there are several species of fossil giraffid, some with long necks 
and they didn't all live in Africa.  That's something that people seem 
to overlook when trying to explain giraffe necks, as nearly every 
discussion I've seen presumes the modern African setting is the one 
that lead to the evolution of their long necks, but of course this may 
not be true.  It could be that in other environments with different 
feeding regimes, or even in the current one but when competing against 
now-extinct herbivores that the long neck conferred a greater advantage 
than it does now.  It's also worth noting that an adaptation can be 
more or less neutral most of the time but play a key role during times 
of stress (eating above other herbivores might be _very_ advantageous 
during times of drought, for example) and if=2
0those periods of selective 
pressure are common enough then they will still drive phenotypic 
change, even if during the majority of the animals life that selective 
pressure isn't acting.  And while I think the current study is correct 
that sexual selection wasn't the driving factor, it could well be a 
maintaining selective pressure, as mate selection often favors (and 
therefore serves to reinforce) characters that have traditionally 
played an adaptive role.

I don't know enough about giraffid biogeography off hand, but 
personally my first inclination would be to look for the likely origin 
of long-necked species and see what THAT environment was like before 
attempting to speculate on what factor(s) drove the evolution of neck 
elongation.  Most current discussions (even in technical papers) ignore 
the phylogenetic and temporal context that giraffes evolved in when 
trying to solve this mystery.

Scott



Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333

www.skeletaldrawing.com

-----Original Message-----
From: Roberto Takata <rmtakata@gmail.com>
To: rtravsky@uwyo.edu
Cc: dinosaur@usc.edu
Sent: Tue, 19 May 2009 11:06 am
Subject: Re: How The Giraffe Got (or didn't get) Its Long Neck








What about okapi? No useful information from comparative anatomy and 
embryology?


[]s,

Roberto Takata=0
D

On Tue, May 19, 2009 at 1:49 AM, Richard W. Travsky <rtravsky@uwyo.edu> 
wrote:
> with maybe some relation to long necked dinosaurs...
>
> http://news.bbc.co.uk/earth/hi/earth_news/newsid_8050000/8050298.stm
>
> For centuries experts have argued over how the giraffe got its long 
neck.
> Some said it helped the giraffe feed on leaves other animals cannot, 
while
> some suggested it evolved as a consequence of giraffes evolving long 
legs.
> But evidence for such ideas remains flimsy. Now one of the more recent
> hypotheses has also bitten the dust.
>
> Giraffes did not grow elaborately long necks as a sexual signal, 
scientists
> have shown, leaving its origins a mystery.
>
> In the journal Zoology, Professor Graham Mitchell of the University of
> Wyoming, in Laramie, US, and colleagues Professor John Skinner and Dr 
S J
> van Sittert of the University of Pretoria in South Africa report that 
there
> is still no consensus on the origin of the giraffe's neck.
>
> The theory which most support, they say, is that the neck confers a 
feeding
> advantage, allowing the giraffe to reach leaves beyond more numerous 
smaller
> browsers such as gazelles or antelope.
> [...]
> More recently, another popular idea emerged: that sexual selection, 
rather
> than natural selection, drove the evolution of the giraffe's neck.
>
> The idea is that down the generations, males evolved ever longer 
necks to


> dominate rivals for the affections of female giraffes. The fact the 
male
> giraffes uniquely wrestle for dominance by "necking" and "head 
clubbing" one
> another, with males with the longest necks and heaviest heads tending 
to
> win, has been forwarded as evidence to support the hypothesis.
>
> So Professor Mitchell and his colleagues decided to put the sexual 
selection
> hypothesis to the test by examining 17 male and 21 female giraffes.
>
> If long necks were a sexually selected trait, they expected to find a 
number
> of things:
>
> Â Â* Long necks should be more exaggerated in males than females
> Â Â* They should evolve to be bigger in size more than other parts of 
a
> giraffe's body
> Â Â* They should confer no immediate benefit to survival, and may 
come at a
> cost
>
> Their results didn't support any of these propositions.
>
> They could find no significant differences in the relative size of 
male or
> female necks. [...]
> But they can say that any advantages that were gained don't appear to 
have
> been sexual in nature. "Better explanations for neck elongation must 
be
> sought elsewhere," they write.