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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.


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.