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Re: Article: T. rex Had "some of the best vision in animal history"
Mike, I'll happily send you a pdf if you wish to read the article,
and do not already have access to JVP.
To estimate eyeball diameter I used a bracketing technique, with a
reptilian (alligator, from dissection) as the low-end estimate and a
modern raptorial bird for the high-end estimate. Mammal eyeball
diameter as a function of orbit diameter, incidentally, falls between
these brackets, but is of no real relevance to the theropod binocular
vision study.
It would be wildly optimistic to base tyrannosaurid visual
capabilities on a concatenation of high-end bracket values (of
eyeball-size-within-orbit, as well as modern raptorial retinal
receptor spacing and diurnal-vs-crepuscular receptor pooling, and
optical characteristics). The bracketed estimates fall out as a
mathematical exercise for both the optimistic and pessimistic bracket
values, of course, along with an intermediate avian model, the
ostrich. The Science News article, predictably, reports only the
high-end estimates based on raptorial vision characteristics (with
some cautionary wording, thankfully).
The bracketing approach to estimating spatial two-point acuity in the
great, beloved theropod is (I believe) a relatively straightforward
exercise. But binocular vision is harder to estimate. Binocular
vision in avians is quite specialized, and avian and mammalian
stereopsis differ greatly, not just in threshold measurements but in
function, and requires considerable caution in its interpretation.
For instance, recent comments about the binocular limiting far point
on this DML ignore the fact that in human vision, the limiting far
point is a threshold of negligible behavioral value, and that
camouflage-breaking (visually distinguishing figure-from-background
based on stereopsis) operates out only a tiny fraction of the range
of binocular threshold sensitivities. So it might well have been
that Tyrannosaurus rex had functionally (behaviorally) useful
stereopsis out a few hundred meters, which to me seems of great value
not only for predation but also three-dimensional perception of the
environment in general (as it is important to path planning and
obstacle avoidance in modern cursorial animals). Stereopsis is more
than range finding, and panoramic stereopsis, across 50 or more
degrees of frontal vision (measured horizontally) would have been
overkill [pun intended] for mere scavenging [obligate final comment,
sigh].
Again, if you do not have access to the JVP, I'll be happy to provide
you a pdf.
Kent
On Jul 5, 2006, at 7:43 AM, Michael Habib wrote:
Orbit size estimate: In extant reptiles and birds, how "tight" is the
correlation between calculated orbit diameter (as done by the "party
balloon" test in the Stevens paper) and the animal's actual eyeball
diameter? Is there any type of consistent ratio that fits this
relationship across taxa? And are there any extant exceptions to
this
rule (e.g., animals with a huge orbit with a small eyeball; or with a
small orbit with a huge eyeball)?
For birds, the correlation is very tight, but only if you have the
sclerotic ring to work with as well. Orbit size alone does not
have a particularly good fit; though it is better for birds than
for mammals.
There was an SVP talk, and a poster, on reconstruction of avian eye
size from osteological evidence. If I remember correctly, it was
at the most recent meeting; I will try to track it down. Does
anyone here know the paper/poster I refer to, and recall the author
(s)?
--Mike H.