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on Allosaur brains and pack hunting



A couple of things...

Two weeks ago bh480@scn.org (Ben Creisler) wrote to the list about a
report discussing a recently described Allosaurus endocast:

http://members.xoom.com/prehistorics/News291.html

and the paper to which it points:

Rogers, S. W. (1998). "Exploring Dinosaur Neuropaleobiology: Viewpoint
     Computed Tomagraphy Scanning and Analysis of an _Allosaurus
     fragilis_ Endocast", _Neuron_, 21:673-679.

For those of you that only check out the description at the web site,
I'd like to point out some things there that probably go beyond what
the data suggest.  In particular, the title of the web article:

   Rare endocranial cast shows allosaurus was more
   closely related to crocodiles than to birds

is much less cautious than anything I saw in the paper.  Structures of
Allosaur anatomy which are more similar to crocodiles than they are to
other birds may very well represent the plesiomorphic conditions for
those characters in Archosaurs.  Bird brains and semi-circular canals
may very well be very derived relative to their homologues in their
ancestors -- it's not good to draw conclusions such as are drawn in
the article on this web page based on a small number of characters.
That's particularly true given that one of them -- the details of the
semicircular canal anatomy -- are presumed (in both articles) to
indicate that the animals are adapted to make particular head
movements.

But let me tackle that too...  Rogers claims that crocodilians are
adapted for making sideways snapping movements, and that birds
generally don't make such movements because if they did they would get
dizzy.  I don't buy it.  Crocodilians are known not to focus
underwater where they typically reside when capturing prey.  Indeed it
is known that at least some crocodiles capture fish just as well in the
absence of light as they do under what might be considered adequate
illumination.  Thus the argument that their semi-circular canals allow
them to avoid becoming "dizzy" seems somewhat irrelevant.  A submerged
crocodile needs to know where its prey is relative to itself, but if
it's not using visual cues and (my one major unsupported presumption
here) it doesn't need to worry about falling over while snapping at
prey then its state of "dizzyness" is just not a concern.

Furthermore, I would dispute the fact that birds generally avoid
making lateral head movements.  Birds are very different from us in
that their eyes are generally immobile in their sockets.  As such, if
a bird wants to direct its gaze then it must rotate its head.  On the
other hand, birds are like us in that they have foveas (actually many
if not most birds have two foveas in each eye, but that doesn't change
this argument).  Therefore they do need to orient their eyes in order
to get their best view of any particular object in their visual field.
Bird head movements are such a prominent part of their behavior that
Ken Carpenter once joked with me that a guy driving a pickup truck in
front of us moved his head "like a bird" because he kept darting his
head around to look at various objects around him.

So, either the peculiarities of alligator and Allosaur semi-circular
canals are adaptive and hence of dubious phylogenetic significance, or
they are phylogenetically significant but do not tell us as much about
the behavior of Allosaurs as the two articles might lead you to
believe.

I will concede, however, that the orientation of the canals probably
does suggest something about the normal orientation of an Allosaur's
head.  I found this bit particularly interesting because Rogers'
conclusion about _Allosaurus_ was the opposite of Kent Stevens'
conclusion about _Tyrannosaurus_.  Rogers concluded that Allosaurus
held its snout slightly upturned whereas Stevens concluded based upon
the fields of view of _Tyrannosaurus_ eyes that it held its snout
slightly downturned.  I'd like to see how this plays out if Stevens
analyzes (or has analyzed) an Allosaur skull, and if Rogers finds the
semi-circular canals of a _Tyrannosaur_.  I'm not sure which data
would sway me more if they were in conflict...

Anyhoo, on pack hunting I have three things to add.  First, we had a
somewhat acrimonious debate on the subject a while back, so I
recommend that nobody continue without making sure that they've seen
it (look for Brian Franczak's and Larry Dunn's contributions if you
really want to see a view opposed to the conclusion that dromeosaurs
hunted in packs -- also, in passing, the plural of _Deinonychus_ is
_Deinonychus_ or better yet, specimens of _Deinonychus_).  Second, as
others have suggested, to pack or not to pack is a difficult question.
Studies of lions and cape hunting dogs suggest that the amount of meat
per animal per unit time varies substantially dependent upon pack size
as well as external conditions.  Some studies have suggested that an
optimal group size is 2 under particular conditions...  Third, these
studies focused on mammals, and as was beaten to death the last time
around, dinosaurs were not mammals.  This brings me full circle -- if
I recall correctly, only one species of bird (Harris's Hawk) has been
shown to hunt cooperatively.  And yet bird brains are in some respects
much more like mammal brains than were the brains of other theropods.
There may or may not be some behavioral significance to this fact.

References:

Alcock, J. (1998). _Animal behavior : an evolutionary approach 6th
     ed._, Sinauer Associates, Sunderland, Mass.

Coates, K. J. (1998). "Through Dinosaur Eyes", _Earth_, 7:24-31.

Fleischman, L. J. _et al._, (1988). "Crocodiles Don't Focus
     Underwater", _J. Comp. Physiol. A_, 163:441-443.

Fleischman, L. J. and Rand, A. S. (1989). "_Caiman crocodilus_ Does
     Not Require Vision for Underwater Prey Capture", _J. of
     Herpetology_, 23:296-297.

Waldvogel, J. A. (1990). "The Bird's Eye View", _Am. Scientist_,
     78:342-353. 

--
Mickey Rowe     (mrowe@indiana.edu)

P. S. Note that I've sent this message to three of the authors I've
mentioned above.  I will forward to the list the meat of any responses
I receive.