T. rex – a scavenger?

Tonight I’m just going to write a short entry about the old idea of Tyrannosaurus rex being an obligate scavenger – i.e. incapable of feeding itself by other means than scavenging, or, in other words, incapable of killing prey. This hypothesis, proposed by John R. Horner, gained disproportional attention in the media (probably because T. rex is such a popular dinosaur, and such an iconic killer), and I think that is what spurred a (in my opinion) disproportional effort from the scientific community to respond with new evidence to counter this idea.

I think if I just go though the evidence for the scavenger hypothesis, you will understand my opinion that this should not have been taken seriously in the first place.

As a disclaimer, I must note that I have not done any research into this topic for two or three years, and what I am writing to you now is what I can remember from the sources I accessed ages ago. However, the focus of this post is on thinking and evaluating arguments. Therefore, if the facts seem a bit fishy, I urge you to do some research on your own, as my memory might not be as sharp as I hope it is. What I want with this entry is to inspire you to think about information that is presented to you, and to pay attention to how you can evaluate it critically.

For clarity, I will repeat the hypothesis: the idea is that T. rex would have been completely unable to kill its own prey, and therefore survived as a scavenger.

One line of evidence points to the short, atrophied arms of T. rex. (‘Atrophied’ basically mean greatly reduced, often due to neglect.) Horner argued that predators must use their arms to grab and hold prey in order to subdue and kill them. Think about this for a second. What does this imply? That all prey run away; none of them stand and fight. Considering that many dinosaur herbivores were rather heavy, and walking on all fours, with the front limbs shorter than their back legs, I doubt that would have been the case in the dinosaur era – especially not Triceratops, Ankylosaurus and the like. Think back to the old tv-shows with fight scenes between T. rex and some other dinosaur: how many of them involved chases? Seriously…

And what is worse, the idea that grasping forelimbs is a prerequisite for predation nothing more than a poor generalisation. Consider which types of land vertebrate animals use their forelimbs for grabbing prey. I can only think of mammals, a group with little connection to dinosaurs, being separated by 300 million years of evolution. In contrast, we have all the animal types that typically don’t use their forelimbs: birds, crocodiles (both being the closest relatives of dinosaurs), lizards, snakes, amphibians. Yes, as you can see, pretty much all animals except mammals are perfectly successful predators without really using their forelimbs for grabbing prey. How valid does Horner’s argument seem when you look at the spectrum of predators alive today?

A second argument is based on the shape of the teeth of T. rex: conical, designed to crush bones. In fairness, this is a rather useful adaptation for a scavenger, which cannot expect to be the first to feed from a carcass and might need to be able to crunch some clean bones to get to the nutritious bone marrow contained inside. However, since Horner is arguing that T. rex couldn’t have been a predator, this evidence misses the point by a mile. Crushing the bones of a prey with a powerful bite is a sure way of inflicting crippling pain and is almost guaranteed to stop the prey from either running or fighting back. I fail to see how a set of bone-crushing teeth could not be used to kill prey.

A third set of evidence reasons around what we guess about T. rex’s senses. Brain scans and other types of studies give clues that T. rex probably had poor eyesight, compensated by a really sharp sense of smell. This is indeed typical for a scavenger, in particular truly obligate scavengers – of which vultures are the only well-known, living examples. T. rex most certainly did not have binocular vision – i.e. the ability to judge distances – which is essential for many swift-striking predators, in particular those aiming for small prey. Now, I do not see T. rex as either a swift attacker or one that would go for prey so small it might miss them if it cannot judge distances effectively. While the lack of binocular vision definitely would have limited T. rex’s potential hunting prowess (if it was a hunter… let us keep an open mind), but doubtfully to the extent of complete incapacity. Similarly, a keen sense of smell is an excellent trait for a scavenger, since carcasses smell, and vultures have among the most acute sense of smell among land animals. But, there is nothing saying that a sense of smell is useless for a predator; there are in fact plenty of predators out there today that have smell as their primary sense.

I saved the best argument for last. I honestly mean that, though. This is the only piece of evidence I have seen presented for the scavenger hypothesis that I accept as a valid proposal of a serious difficulty T. rex might have faced when trying to kill prey. It points to the set of combined evidence suggesting that T. rex lacked speed and agility. Just looking at it, you can see it was a ponderous animal. Detailed studies of biomechanics and whatnot have shown, for example, that it could have taken T. rex as much as twenty seconds (I think it was, but I might not remember the figures accurately) to make a full turn. (When taking this into account, the problem of lack of depth perception might become a concern.) The speed at which T. rex could move has been debated about with such varied results I doubt there is any consensus even among the experts, but it is still challenging to imagine such a large animal chasing down a hadrosaur across a field. I doubt T. rex would have chased anything around in a tight forest either, again due to its size. Maybe it would have been better off as an ambush hunter? Again, I find difficulties imagining such a large dinosaur hiding effectively.

Still, this point is far from conclusive. Recall my argument that it is likely that many potential prey species would have tried to fight the predator off rather than try to outrun it? Now, these dinosaurs would have been well defended, with bone plates in their skin, bone shields, and offensive weapons such as facial horns, sharp beaks and bony tail clubs. Therefore, it would be ideal to get around them and attack any weak spot in these formidable defences. But without agility, that seems rather hopeless, so I can see another difficulty T. rex would have faced as a predator. However, some evidence points toward T. rex being potential team hunters, possibly working together in family groups. As a team, it would not be as difficult to surround the prey and strike at weak spots. It would mostly be a challenge if the prey live in tight herds as well. Triceratops probably did, but Ankylosaurus is thought to have been more solitary. Hadrosaurs, such as Edmontosaurus, on the other hand, were heard-living, but basically defenceless, and probably not particularly fast runners. It seems they might have been the preferred prey of T. rex, if it was a predator. When there is potential prey that is not impossible to subdue, it seems extreme to argue that T. rex would never have made a kill.

As a concluding remark, I want to add that while this evidence fails miserably to support the proposal of T. rex as and obligate scavenger, it does highlight some good reasons for why we should accept the possibility that T. rex may have been an occasional or opportunistic scavenger, while still perfectly capable of killing prey when an opportunity was presented. Indeed, this is where it seems the debate landed, and it further appears to have opened up our eyes to the potential benefits for predators of opportunistic scavenging, which has spread to many other predators, as you might see paleontolgists (proudly) talking about in tv-shows, etc.

I will finish by repeating my warning from earlier: please do not use this article as a source of information, but rather as a source of thoughts! If you are interested in this debate, check well-referenced sources for the details.