Tyrannosaurus Hunter versus Scavenger
| Experts continually debate where the
tyrannosaurs were hunters, scavengers opportunistic feeders. The
discussion about the feeding patterns of T. rex and other large
carnivorous dinosaurs remains active. Most paleontologists have
portrayed them as highly active predators, while others see them as
obligate scavengers (Lambe 1917; Colinvaux 1978; Halstead and Halsted
1981; Barsbold 1983; Horner and Lessem 1993; Horner 1994; Horner and
Dobb 1997). The scavenger hypothesis has been re-proposed by Jack
Horner in the 1990s and appeared in Horner's 1993 book "The
Complete T. rex".
The ocular cavities of Tyrannosaurus are positioned so that the
eyes would point forward, like a lion or a human. A scavenger rex
would not need the advanced depth perception that stereoscopic vision
allows. Stereoscopic vision is essential for predatory animals who
catch other animals (owls for example), but has secondary importance
for animals who are chased (such as rabbits or deer).
T. rex had an incredibly large and powerful jaw with serrated
teeth, one bite could break most anything in half. This was a
terrible weapon. Certainly if T. rex was a scavenger, it
would not have to be so well armed..
There were also the large powerful legs. Some scientists
believe that T. rex could run as fast as 30 miles per hours.
This ability would prove very helpful to a predator chasing prey.
It was not needed by a scavengers whose meal was already dead and just
waiting to be devoured.
Some scientists think that T-Rex didn't chase down its prey at all,
but was merely a scavenger. As a scavenger he fed off of already dead
animals, killed by old age, disease, or other carnivores,
Paleontologist Jack Horner of the Museum of the Rockies is leading
proponent of the theory that T. rex could not have been a predator. He
maintains that its eyes were too small to see distant prey; its arms
were too small to hold victims; and that is huge legs would have
resulted in it moving slowly. He also points out that its large
olfactory lobes provided it with an acute sense of smell.
Vultures also have large olfactory lobes which they use to smell dead
bodies at great distance.
There is no fossil evidence for predation. Bones have
been found with tyrannosaur teeth embedded in them or scratched by
them, but there is no evidence to show that the animals were not
already dead. A bone that showed evidence of a healed
tyrannosaur bite would be strong evidence for predation.
Horner's main argument, which is generally agreed upon, is that T.
rex was a slow walker and not a runner. Therefore, Horner said, it
is more likely to be a scavenger. However, predators do not
necessarily have to be swift. Speed can be measured in some ways,
using an analogy with living animals and sports (the femur/tibia
ratio), using biomechanics, or using footprints (trace fossils). For
instance, bicyclists with longer thighs are said to have better
endurance. Horner claims that the femur (thigh bone) to tibia (shin
bone) ratio (>1, like in almost all large theropods) suggests a
specialized walker, rather than a runner. Thus it was a slow
scavenger rather than a fast running predator. However, T. rex's
legs were better designed for speed than its probable prey.
Another argument from Jack Horner regarding T. rex's
slow speed are its useless forelimbs mentioned above. It could not
catch itself, should it fall over in a high speed hunt, (and perhaps
sustain severe injuries due to its heavy skull size) and would
therefore have to play it safe by walking rather than running. This
claim has been substantiated by Farlow et al. (1995): they used a
mathematical model using impact forces and decelerations for an animal
weighing 6000 kg to gauge that a fall at very high speed (20 m/s or 72
km/h, the top speed used in most models) would kill it. They speculate
a top speed of adult individuals of about 10 m/s (36 km/h).
The claim that T. rex's legs were not suitably
adapted for high speed is an important point independent of the
predator/scavenger debate. A paper in Nature (Hutchinson amd Garcia
2002) — Tyrannosaurus was not a fast runner — used a
mathematical model (based on chickens and alligators) to gauge the leg
muscle mass needed for some top speeds. They found that some proposed
top speeds (40 km/h or 25 mph, or even 72 km/h or 45 mph) are quite
unfeasible, because they require very large leg muscles (needing ~86%
of total body mass as leg muscles). They specify a very rough upper
estimate of 18 km/h or 11 mph.
However, those figures depend on the popular assumption that
T. rex weighed around 6-8 tons. Some have suggested that "T.
rex" may have weighed a mere four tons, since its bones were rather
hollow and its breathing style would have required less mass. This
would probably result in a much higher top speed. Even if T. rex
or other large theropods were rather slow (as argued by Farlow et al.
1995; Hutchinson and Garcia 2002), it does not necessarily mean they
were incapable of hunting prey. The dinosaurs they probably hunted
were likely even slower, and ambush tactics cannot be ruled out.
However, the fact that T. rex had longer femurs and
legs than other theropods points to the fact that T. rex could
indeed reach higher speeds than implicated. Secretary birds had long
femurs and legs to improve the distance covered by each step, so even
when taking a "slow pace" T. rex would still be moving 12 to 15
mph (the speed of a sprinting human). (Hutchinson and Garcia 2002) The
mathematical model of chickens and alligators was not an accurate
parallel of T. rex in most respects. For one thing, T. rex
was a bipedal carnivore (unlike alligators), and the skeletal
structure was more streamlined to decrease wind resistance (unlike
chickens). The most important point is that no person in this time has
examined the muscle structure of the T. rex, so one cannot be
completely sure of the speed of T. rex.
Other scientists argue that scavenging for food and hunting aren't
mutually exclusive activities and T-Rex might have done both depending
on what was easiest. Such is the case with lions in modern Africa.
They hunt when they have to, but are happy to steal a carcass from a
smaller predator, like the hyena, when they can.
They point out that its panoramic vision and great sense of
smell would aid both a predator and a scavenger. As far as the
lack of a fossil bone showing a healed T. rex bite, one asked, "What
is going to survive a T. rex bite?"
There it not
enough evidence exists to say for sure how T. rex earned its daily
bread. Most paleontologists believe that tyrannosaurs were
eating machines that would consume protein where they could find it.
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