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Re: Subject: Bird hibernation/torpor




On Sun, 23 Mar 1997, Ronald Orenstein wrote:
John Bois wrote:  This being true, desert ostriches enjoyed
greater reproductive success than tropical rain forest ostriches.

Ron Orenstein replies: 
> I doubt this VERY much.  First, in Africa there are far more potential
> ostrich predators outside the rainforest than in it (lions, for example, or
> cheetahs, or Cape hunting dogs, or hyenas, none of which occur in
> rainforest).  

I should have used a smiley face.  I was being facetious about tropical
rainforest ostriches.  You are, of course, right about this.

> Second, Ostriches are not purely desert animals but occur in
> savanna woodland in much of Africa.  

In fact they prefer the open savannah and avoid wooded areas.

>  Fourthly, ratites can do quite well in rainforest, as the cassowaries show.

If you allow me ostriches I'll get to work on emus and then cassowaries.

>> Now I have come up with a parsimonious idea which can well
> >account for the "terror birds" viz.., they could hide their nests
> >in the tall grasses of the savannahs. 

> But this does NOT fit the nesting patterns of either ostriches or emus, so
> it is only "parsimonious" if you ignore what the largest living birds are
> doing.

See my response to Tom Holtz or read Bertram's book for the effectiveness
of low grass in concealing ostriches when they are brooding.  During the
time he was studying a fire swept through the grasses--nest predation rose
rather dramatically.

> But there were cycads, ferns (ever been in a bracken thicket?) and other
> plants that could easily have provided plenty of cover, were any needed.
> There were lake islands.  There were forests.  Lots of places to hide
> nests... And of course eggs could have been protectively coloured (as are
> the eggs of birds that nest on sandy beaches today), or covered with
> vegetation when not brooded, etc etc.

I recently went for a walk in a mesozoic (cretaceous?) forest. 
It was a delightful exhibit at the National Botanical Gardens in
Washington.  And you are right.  There were plenty of
opportunities for a stealthy animal to hide its eggs.  But I
argue that dinosaurs used this opportunity less and less.  This
is my reasoning:

1. The average size of dinosaurs increased over the cretaceous
and there were none under chicken-size by the K/T.  There had
been previously.

2. There MUST be a reason for this.  By that I mean it is not
enough to say: "Well, dinosaurs were exploiting more large animal
niches and no longer needed small niches."  No.  If an animal
previously exploited a niche and is no longer found in it, then
that animal has been booted out of that niche.  Small dinosaurs
were outcompeted in the small-animal niche.  This was done,
presumably, by mammals, snakes, lizards, and (likely) birds who
are gifted at getting around in small places and flying off when
they need to.

3. Big egg-laying animals should best exploit the strategy of
defence over close-cover stealth.  Constant attention to a nest
means predator-alerting crackling in the bracken.  One might say:
"Well, but look at the emu.  Why couldn't a similarly-sized
dinosaur lay its eggs and go into torpor in deep cover?"  And I
would counter that predator density in thick vegetation is high:
discovery is inevitable.  Once discovered, small animals nip and
torment then hide untouched  (perhaps a good analogy to this is
the Vietcong vs. the mighty U.S. Army and how much it prefers a
_Desert Storm_).  And then, further evidence that this is an
inefficient strategy lies in the practical non-existence of such
forest creatures today.  Yes, mammals can do this but they are
stealthy reproducers (keeping baby safe inside they don't have
the fixed-site baby problem).  And birds that can fly are no
argument since they can either fly to secure nesting sites or
avoid extinction-driving life-time summation of predation--I mean
a wild turkey has nightmarish nest predation but it still exists
because the few that make it to adulthood have lower predation
rates because they can escape by a sudden burst of flapping.  It
is hard to imagine how any non-avian dinosaur could enjoy such a
differential advantage (such as flying) over another.  A wild
turkey-sized dinosaurs must suffer at both ends of its life
cycle: its nests are plundered by adept close-cover tormentors
_and_ its adult forms are preyed on by continents full of
dinosaurian bullies.  And so the success of this form diminished.

4. These forces led dinosaurs to lay their eggs out in open
country and defend them with increasing ferocity, tooth and
muscle power.  As such, the succor of bracken was irrelevant.
I have also argued that increasing size was then even more inevitable
because an open-field, fixed-site baby creates an arms race.

The hiding place you mentioned, lake islands, offers no differential
advantage, i.e., if one dinosaur can get there it is hard to imagine why
another could not.  As such, this is no hiding place.