Gargling galahs!

[Colin McHenry <s172579@student.uq.edu.au>]

> > > Note, in one reference I have a case is made that dinosaurs, even
> > > the large ones. were r-selected, rather than K-selected like large
> > > mammals.  If this is really so, than it may never have been the case
> > > that protecting young was economically viable (individual young are
> > > not as valuable to a r-selected animal, and so are less often protect > > 
> > My own suspicion is that, as in, say, ostriches, the dinosaurs were 
> > R-selected as eggs and K-selected as chicks ...
>
>Or, mor precisely, a hybrid stategy. 

As far as I can recall my ecology lectures, 'r' and 'k' selection are
properties of a population, and do not generally vary with respect to the
age of an organism.  Survivorship does change with age, and 'r' selected
organisms tend to have high levels of mortality when young, followed by very
decreased levels of mortality after, say, the first year.  This is known as
a Type 3 survivorship curve.  As an example of such a 'r' selected large
archosaur, in my "Webb's Crocodiles of Australia", the following figures are
given for a cohort of eggs laid by a population of Crocodylus porosus...

Eggs laid:                      1000
Eggs hatched:                   250

(and remember, these eggs are vigorously defended by a large female
crocodile!.  After hatching, though, parental care is minimal - indeed,
adult conspecifics are often the biggest threat to young crocodiles)

Juveniles at 1 year old:        135
ditto     at 2 years old:        41
ditto     at 3 years old:        24
ditto     at 4 years old:        14
ditto     at 5 years old:        8
   (% eggs surviving to 5 years = < 1%)

At 5 years they are 1.7 metres long (15 kg), and their probability of
survival to breeding age (at least 12yrs for females, 16 yrs for males) is
thereafter fairly high.

>
>On the other hand, the evidence from the Texas Chasmosaurus
>mass-death site is that ceratopsian herds were mixed-age, ranging
>from very young to full adult.  his suggests parental care, and
>supports mixed-strategy reproduction in at least Chasmosaurus.
>.  

Not necessarily; if mortality is heavily biased towards younger age (and
therefore smaller size), then once past these dangerous early years the age
classes of the population will be of roughly equal size (in terms of
numbers), all the way from juvenile to adult.  If these ages classes mix
into herds (this may be more probable with herbivorous species) then the
pattern you describe will result.  If the adults are defending their nests
(and even new hatchlings) then the herd (or at least the parents) aren't
going to be too far away,  giving the young animals a chance to join up with
the herd.  This doesn't necessarily imply parental care of young once out of
the nest - the only protection would be as part of the herd.


The Webb book also shows a picture of the Galloping Freshie...Well, it's
defintely moving at quite a pace - looks like a young animal, though.  This
picture also shows how an animal with shorter forelimbs than hind can
gallop...by moving the fore limbs together, then the back legs, the back
feet touching the ground outside and in front of the forefeet..just like a
running greyhound or cheetah.