Re: Addendum to Trex vision as support for scavenging...

[don ohmes <d_ohmes@yahoo.com>]

Evidently I failed in the attempt to state my point clearly. Or perhaps you 
didn't read my original post. So I will recap and hopefully clarify-- 

IF there are visual cues such as are offered by carrion birds (ie, little dots 
off in the distance that tell of a RECENT death), THEN exceptionally keen 
eyesight is advantageous for a terrestrial top predator in that hyperkeen 
eyesight enables efficient utilization of fortuitous food resources not 
obtainable by other means. If the evolution of habitat is appropriate, then the 
percentage of total food resources that are obtained by scavenging may become 
increasingly significant, and alter the evolutionary path of the predator. 

If visual cues DO exist, then the nature of the terrain is not a factor in 
carcass detection (ASSUMING A CLEAR FIELD OF VISION), and is only important in 
that it alters the area that can be utilized (ie, alters arrival time). And 
yes, in a first approximation, "ideal" terrain is a desirable simplifying 
assumption.

Timeliness of carcass acquisition is critical to the viability of the 
mega-scavenger concept, IMO. This why olfaction is a relatively inefficient 
method of carcass detection, and in fact is why I never liked the concept of 
Trex as obligate scavenger. By the time a carcass starts to stink, it is TOO 
LATE to utilize it efficiently; the good parts have been eaten by others and 
decomposition is advancing rapidly. Also, the "nasal" scavenger is hampered by 
the geometry of air circulation (NEVER 360 degrees, unlike a visual cue), and 
the vagaries of the weather, including the 24 hour heating/cooling cycle. 
TRAILING prey can be very efficient, but pinpointing the (random) location of a 
stationary odor source from any great distance (FORGET 6 km) is very iffy, 
difficult, and time-consuming. 

Also, I guess I need to stress that I do NOT think that Trex was an OBLIGATE 
scavenger. My personal take on Trex lifestyle has always been that Trex was, in 
effect, a bipedal terrestrial crocodile (ie, ambush predator/opportunistic 
scavenger). The addition of hyperkeen vision and the perhaps not unreasonable 
assumption of "tattle-tale" volant scavengers to the scenario just alter the 
attainable ratio (ie, predation/scavenging can become a smaller number).

Where I part company with (judging from the press comments I read online; 
haven't had the opportunity to read the paper) Stevens (and Holtz?) is this; it 
has proved impossible for me to imagine (I am open to suggestions) a realistic 
"pure" (terrestrial) predation strategy that would select for a far point limit 
of 6 km. Not so for scavenging, IF the visual cues provided by volant 
scavengers exist. However, no visual tattle-tails, no efficient scavenging 
based on, and selecting for, hyperkeen vision. 

I feel that the existence of volant scavengers in the period of time occupied 
by Trex is not an unreasonable assumption. Therefore, I conclude that hyperkeen 
vision actually SUPPORTS the idea that Trex obtained a significant portion of 
its food by scavenging. 

Don

----- Original Message ----
From: Dann Pigdon <dannj@alphalink.com.au>
To: DML <dinosaur@usc.edu>
Sent: Monday, July 3, 2006 10:48:36 PM
Subject: Re: Addendum to Trex vision as support for scavenging...

Quoting don ohmes <d_ohmes@yahoo.com>:

> If my figures are correct--
>
> Taking a Trex stride as 1.76m, assuming a stride frequency of 2   
> seconds (I believe 2 sec is conservative), and using Stevens' far   
> point of 6 km, Trex could walk to his limiting far point in ~1.9   
> hrs. This means that, assuming visual cues in the form of volant   
> scavengers were available, Trex could monitor, from a stationary   
> position, ~113 sq km (11340 hectares) with a two hour arrival time...


Of course, this assumes that T.rex is striding about in a completely  
flat, treeless desert. :)

Sigh. No, it assumes a stride frequency of 2 sec.

It would seem to me that especially acute eyesight would only be of  
any use to a purely scavenging animal if it could fly. If like most of  
us you're stuck crawling about on terra firma (with all it's hills,  
trees, etc), then eyesight is probably not the best way to spot  
carcasses - at least not directly. Watching for carrion birds (or  
pterosaurs) congregating might be an exception.

See above.

If T.rex was an obligate scavenger (and that's a pretty bit 'if'),  
then no doubt it's enormous nasal cavity played a larger part in  
tracking down carcasses. The nose knows...

See above.
-- 
___________________________________________________________________

Dann Pigdon
GIS / Archaeologist         http://heretichides.soffiles.com
Melbourne, Australia        http://www.geocities.com/dannsdinosaurs
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