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RE: More about Erickson et al. 2004 and RE: Delayed "growth spurt" in T. rex
To throw in my two cents: the "twisted" (grab the middle of the curve and
turn counter-clockwise) growth curves of tyrannosaurs make sense both for a
small-pack scenario and tyrannosaurs as loners.
In the former case, it might be advantageous for a family-group pack
(pride?) to maintain a small stock of smaller juveniles, either to assist
in hunting (if that is what they did), or simply to ensure that a few of
them survived "training." With one or more large adults already present,
smaller juveniles means less food expended on them, which means fewer kills
are necessary (less danger, good thing). The need for more food that
probably coincided with the steeper portion of the growth trajectory might
then have coincided with an increased appetite (as it seems to in humans
:), monopolization of pack resources by the teenager, and might have
triggered expulsion from the pack. This, of course, NEVER happens in human
However, as others have noted, sigmoidal growth curves are the rule, rather
than the exception. Rapid growth may simply have been a way to rapidly
traverse the size gap between the "small" predator niche (1 ton... recall
the gap between small and large predators in Late K northern Great Plains
faunas), and that of the largest predators. If tyrannosaurs were
territorial, particularly abundant, or simply hungry, they might
preferrentially eliminate the competition while it is younger and smaller,
but just beginning to compete for the same resources (large herbivores),
rather than wait until it hits full size. It is possible that such rapid
growth to great height necessitates a long lead time; that makes sense
graphically, but I'm not sure what the biological reason would be.
As for why there aren't any really old tyrannosaurs, I very much liked the
point about r-strategy in tyrannosaurs. We should remember that these
animals are not large mammalian HERBIVORES, they are top predators. That is
a very rough life, with angry prey, other large predators, disease,
tripping and killing yourself, etc. If there truly wasn't much selection
for long life, there certainly would have been a large number of potential
means to shorten it!
Not that this is anything new, I just felt the need to pipe up. After all,
this IS the Theropod Mailing List!
P.S. Don't forget to cc me in replies!