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Tyrannosaurids scavengers?/Nutritional value of rotting meat?

 The ongoing topic of whether tyrannosaurids were predators or a scavengers (or both) recently came up here at Tyrrell. Given the rapidity of consumption of prey animals on the African plain, it is fair to suppose that most "carrion" there is still fresh as many of the Zebra/Wildebeest and smaller-sized animals are consumed by primary/secondary carnivores and scavengers within hours of death. So a scavenger in these instances is still eating relatively fresh meat. But what of the carcasses of larger animals like Elephants? They cannot be eaten to the bare bones before their flesh goes "bad", yet animals still scavenge their putrified carcasses.
 There was some discussion on the nutritional value of rotting/rotten meat and scavenging generally, which brought up some unresolved questions:
 1. Nutritional value of rotting/rotted meat vs. fresh meat? Anyone have any thoughts on this? Have any studies been done on this issue? I'd be interested in getting copies of such papers if they exist. Someone here suggested that the rotting/rotted meat would be more nutritious due to the secondary ingestion of insect eggs, larvae (maggots), and adult insects. Another person thought rotten meat was more protein-rich but could not substantiate this claim.
 2. Why scavenge rotting meat anyway? 1. Lack of available prey? 2. Easier to digest? 3. More "tasty"? 4. Injured predator cannot catch and attack more "normal" prey? 5. Easier to "catch"?
 #'s 1, 4 and 5 are self evident, but what about #2? Any other reasons missed here?
 3. Near as we could tell, vultures are the only extant true scavenger. But is this really so? If a hungry vulture was standing around and an insect or small animal happened by would it ignore this potential meal completely?  Are any vultures known to take live prey? If so, then they cannot be considered pure scavengers. Maybe there really is no such thing as a "pure predator" or "pure scavenger"? (but see #6 below).
 4. If given the choice would fresh or rotten meat be preferred?
 5. An interesting observation that was pointed out is that Black Bears generally kill and eat their vertebrate prey (then bury leftovers for later) whereas Grizzly Bears tend to kill, then bury their prey and wait for it to "ripen" before returning to feed. Anyone have any thoughts on why this is so?
 6. Are there any pure predators in the world today? I've personally seen Weasels and Red-Tailed Hawks "catching", "killing" and eating Richardson's Ground Squirrels that had just previously (5-10 minutes) been killed by rifle fire. I've also seen several raptor species eating roadkill. I once read on a bird chat-group where apparently an Osprey (which is reputed to to be a pure predator) had learned to regularly steal smoked Herring hanging out to dry. As kids me and my friends would catch and keep Frogs in pails. My friends "pets" aways died as they just threw in dead insects for the Frog to eat. Because the insects would not move, the Frog would not see it and thereby starve. I'd put insects on the end of a long bristle snapped out of a corn broom and twirl the impaled insect in front of the Frog's nose and never had any problems feeding them in that way. Would this movement-related feeding activity make Frog's pure predators? If so, this would make a Frog more of a "pure predator" than a Hawk!
 Personally I think tyrannosaurids were both predators and scavengers, but leaning more towards the predator mode. Phil Currie has co-authored a paper with me on face-biting behavior in some large and small theropods including tyrannosaurids. This paper is coming out in an upcoming issue (this year I hope!) of the journal GAIA. This issue is dedicated to theropod paleobiology. We have healing bite marks on the heads of tyrannosaurids- 43% specimens in Tyrrell are so affected. Clearly they used their teeth on each other when alive- so why not use the same teeth on the more fragile head of a meal- say a hadrosaur? We also have a specimen I mentioned on this chat group months ago that shows scavenging of a very rotten tyrannosaur carcass; probably scavenged by another tyrannosaur-- but, that's another story........ :)
Darren Tanke, Tech. I
Dinosaur Research Program, Royal Tyrrell Museum of Paleontology, Drumheller, AB, Canada
Senior Editor, Paleopathology and Recent Dento-Osteopathology Bibliography; see homepage
at:  http://dns.magtech.ab.ca/dtanke