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Re: Tyrannosaur age-population distributions



On Sun, Jul 16, 2006 at 09:27:41AM +0000, Phillip Bigelow scripsit:
> On Sun, 16 Jul 2006 08:46:02 -0400 Graydon <oak@uniserve.com> writes:
[T. rex died soon enough after reaching sexual maturity that the first
clutch wouldn't have been out of 'childhood']
> > The other thing this does is make a hash out of pack-forming;
> 
> Not necessarily.  But it may give insight into the nature of tyrannosaur
> pack-forming (if it existed at all).

Everything we've got examples of that forms packs does so around at
least one breeding pair; it's pretty much purely a kin-selection
mechanism.

> > the breeding pair doesn't last long enough to raise their own
> > offspring, so how does the pack form?
> 
> Pack-forming may still have been an important part of _T. rex_ behavior,
> but it may have occured only part of the year.  Alternatively, pack
> behavior may have been the norm for most of the year, except for a short
> period where mate selection was taking place (when all of those injuries
> in the adults may have accrued).

You still can't get from there to the kin selection mechanism; by the
time the first clutch's offspring are 10 years old, the parents are
dead.

Which adults have genetic commonality with those offspring, and why do
they exhibit social behaviour with them, rather than treating them as
competitors?

> > Currie, et al., still have pretty good evidence of pack behaviour in
> > Albertosaurs, with juveniles and sexually mature individuals in the
> > same group.
> 
> I realize that Currie and others have published a lot on this, but I'd
> like to see even more data.  There is still the possibility that
> taphonomic concentration was occuring.

The possibility exists, but it's not, to my mind, the least hypothesis.

> > Did Albertosaurs have a different growth cycle?
> 
> Apparently, yes.  According to Erikson et al. (2004), p. 774, fig. 2, the
> _Albertosaurus_ growth curve was shallower (lower-angle slope); with a
> much less obvious "growth spurt" phase within the longer-period juvenile
> phase; but with a similar [end of childhood -- beginning of juvenile
> years] transition point.  This _Albertosaur_ childhood-juvie transition,
> like T. rex, occurred around age 10 or so.
> 
> In summary, the entire _Albertosaurus_ grow curve is more subdued that
> that of _T. rex_.

So they're *not* evidence of T. rex pack-forming behaviour.

> > Were tyranosaurine sibling bonds exceptionally strong?
> 
> In the juveniles?  Maybe.  According to the new paper that came out just
> last week, juveniles led relatively "safe" lives up to about age 20 (body
> bags for the juvies are almost non-existant).  So perhaps the youngun's
> didn't hurt each other.  Their longevity also suggests (weakly) that the
> juvies didn't hunt dangerous prey.  Perhaps they raided baby nests or
> scavenged carcasses.

Well, not only did the juvies not hurt each other, the adults didn't,
either, and not just close relatives.

That's decidedly odd.

> > Strong enough to work over several years worth of siblings, so that
> > the oldest had the help of their younger siblings in managing to
> > breed?
> 
> Sort of like the Wilson brothers (the actors) all going to a singles bar
> together and fixing each other up with dates?  ;-)

If you're on the leading edge of a set of sibs spread out of over eight
or ten years, it's possible to imagine a case where, by the time your
last aunts and uncles have died, you're old enough to take care of your
younger siblings.  By the time you reach breeding age, the age cohort
behind you can help take care of the offspring so that by the time your
cohort is all dead, the front edge of your offspring cohort made it to a
size where they can help take care of the siblings.

This doesn't consider if, or when, the aunts and uncles breed; it's
possible it was a sort of iterative ostrich communal nesting thing
within a fairly large kin group.

> > Were tyrannosaurine packs accreted on a basis of purely social
> > bonds?
> 
> Heh.  Keep in mind the size of a tyrannosaurid's brain.  IF pack
> activities occured, was it instinctual?  Yes.  Instinct usually drives
> behavior in non-mammalian vertebrates.  But was it "social" behavior,
> sensu lion prides?  Who knows, but I doubt it.

I don't doubt it at all; chickens have social behaviour, and chickens
aren't any smarter than a tyrannosaurid and arguably less.

-- Graydon