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Trex size and pack-forming (was Tyrannosaur age-popula...)



  Graydon wrote--
  The other thing this does is make a hash out of pack-forming; the
  breeding pair doesn't last long enough to raise their own offspring, so
  how does the pack form?
   
  Currie, et al., still have pretty good evidence of pack behaviour in
  Albertosaurs, with juveniles and sexually mature individuals in the same
  group.
   
  Did Albertosaurs have a different growth cycle?  Were tyranosaurine
  sibling bonds exceptionally strong?  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?  Were tyrannosaurine packs
  accreted on a basis of purely social bonds?
   
  I have no idea how to answer those questions, but I sure hope someone
  does.
   
  -- Graydon
   
  I agree that if one limits the concept of "pack" to the multi-generational 
wolf/lion model, and assume that an altricial lifestyle is a prequisite for the 
evolution of such, then the results of Erickson, et. al. (Sci. V313 7/14/06) 
seem to render an evolutionary scenario resulting in packs of tyrannosaurids 
improbable.  Therefore, I use the more generalized term "group-hunter" here, as 
tempting as the term "flock" may be.
   
  In my opinion--
  Given the large size of available prey, it is possible that tyrannosaurids 
were group-hunters at least until sexual maturity. Large prey create positive 
selective vectors for group-hunting; these include a big nutritional payoff, 
and increased likelihood of hunting success due to small predator/prey size 
ratios. These vectors could morph a basic species recognition package and a 
simple 'dinner bell' scenario (where the sight/sound/smell of distressed prey 
is the 'bell' attracting all local predators) into group hunting behavior, even 
in the absence of first order kinship bonds.
   
  A derived form could involve imprinting (on siblings and/or a parent) as seen 
in birds, a tendency to maintain sensory contact w/ other tyrannosaurids of 
similar size and age, and of course a tendency to attack prey en masse. It is 
certainly possible that juveniles might have known or followed their mothers 
until or even past sexual maturity, as wild turkeys do, but it is not necessary 
to the formation of a group-hunter lifestyle for tyrannosaurids to have even 
seen either parent.
   
  My personal cartoon of the evolution of top predator/top herbivore as it 
applies to sauropods and theropods, culminating in T. rex, is as follows--
  1. To the delight of many children, proto-sauropods and proto-theropods begin 
a classic predator/prey size race. As size increases, size adaptations appear, 
including long skinny necks for reaching food and water, and large jaws (for 
snapping necks) and tallness (for reaching them). 
  2. Although the theropods are small when hatched, there is a size-chain of 
herbivore species for them to 'eat their way up' (as long as they stick 
together), and an herbivore at the top that is large enough to support a 
group-hunting lifestyle when mature.
  3. Over time, the very largest prey become scarce, and the group-hunting 
lifestyle is no longer economically viable when the theropods reach maturity, 
leaving them stranded on an adaptive peak. Some of the top theropods respond to 
this by morphing their behavioral phenotype (probably by changing sexual 
behaviors) such that the group breaks up at (or somewhat before) maturity, and 
mature individuals begin a lifestyle of solo (or near solo) hunting and 
co-opting of fortuitous mortality. Although the prey are now somewhat smaller, 
they are still quite large, and have become armed and armored (long skinny 
necks are out of style, too). Further, the battle odds are now as low as 
one-on-one. This combines with another positive size vector, intra-specific 
competition, and results in the largest of all theropods _after_ the extinction 
of the largest sauropods.



And yes, they may even have ended as the ecological equivalent of a giant 
possum. Although I seriously doubt it. 

   
  Don