Ah, the ol' T.rex scavenger debate.
Tyrannosaurus rex is, without doubt, the most well-known dinosaur of all time. It's the first dinosaur we learn as a kid and is the mascot for anything prehistoric. Part of the charm as that it was not only used so much in media, such as King Kong or Jurassic Park, but that it was also one of the biggest predators to ever walk the planet...or was it?
Tyrannousaurus rex – a movie icon
Everyone thinks of T.rex and thinks a great big two-legged lizard with scary sharp teeth chasing and hunting down its prey, mostly because it is an incredibly exhilarating thought. This made it all the more famous and controversial when it was suggested by paleontologists (such as Jack Horner, scientific consultant for the Jurassic Park films) that T.rex was actually a scavenger. This annoyed a lot of people, mainly because they didn't like the thought of of their favourite dinosaur cowering in a corner at the first sign of trouble and not partaking in the exciting behaviour that we see in media. This reaction is fairly common when something new is suggested about a famous dinosaur, the general public get very protective over the depictions they grew up with, often ignoring the scientific fact.
However, was T.rex actually a predator or a scavenger? Well, thankfully, we don't need a time machine to put this one to bed...
Was Tyrannousaurus rex a scavenger?
First off, let's look at why T.rex being a scavenger was suggested in the first place. Dr Jack Horner had four main arguments as to why he thought T.rex was a scavenger, those being: the arms, the teeth, the speed and the brain. The arms were an obvious one, they were simply too small to useful in a fight. The speed is also a bit of a hard one to argue with, since an animal of that size probably couldn't actually run (in the typical sense of the word) without breaking an ankle, making catching prey all the more difficult. The teeth and skull of a T.rex, however, are prime examples of how, despite everyone thinking it is a fairly typical two-legged dinosaur, it's actually really weird for a dinosaur and very unique from its other theropod cousins.
The shape of the skull and teeth of Tyrannosaurus rex are built for one type of bite: a bone crushing one. The teeth, in cross section, are very round and thick, as opposed to the more blade-like teeth of other theropods. This means they can take much more strain without breaking. The skull is also very thick and robust, suggesting possibly the hardest bite force in Earth's history! The ability to chew through bone like butter is much more typical of a scavenger. Since not all of the animal will necessarily be there when the scavenger finds it, they need to be able to extract as much nutrition as possible.
How big was T.rex's brain?
Another feature typical of scavengers can be seen when we look at T.rex's brain. A CT scan showed that the olfactory bulb (the part responsible for smell) of a T.rex's brain was unusually large, meaning that it had an extraordinary sense of smell. Many predators have a fairly good sense of smell, but they have nothing on scavengers. It is much more useful for a scavenger to have a sense of smell this good as it means you can sniff out food that is miles away.
Horner finished all of this off with the suggestion that Tyrannosaurus rex's tremendous size had evolved for nothing more than bullying anything else off from a carcase, which isn't unheard of in nature.
However, there were many arguments against Horner's theory of T.rex being a scavenger.
Did T.rex have good eyesight?
The first is that Tyrannosaurus rex had simply evolved and perfected too many tools that were suited to being a predator. Those big bulky teeth were actually curved backwards, which is a feature that helps dig the teeth in further once they've sunk into the animal if it struggles. Now, I'm no vet, but I'm pretty sure that for an animal to be struggling in the first place it has to be...y'know...alive. Another unusual feature that hints to T.rex being an active predator is its eyesight. Yes, Jurassic Park lied to you, T.rex actually had excellent eyesight. In fact it was most likely even better than a hawk's. The unusual skull shape of a T.rex actually meant that it had binocular vision (i.e. it could focus both eyes on a certain point at the same time like we can), meaning it had excellent depth perception.
Believe it or not, many non-avian theropods who were known predators were unable to to do this. This kind of tool was actually a relatively new one at the time and most certainly evolved for hunting.
How many calories did T.rex need to survive?
Then we can come back to its size. When you are a scavenger, you have to travel a lot. You can't simply lounge around all day and just wait for something to drop dead right next to you. A scavenger must travel for many miles for a scrap to eat and that burns a lot of calories. It burns a lot more calories when you're hoisting around 6-8 tonnes like T.rex would have been. An animal the size of T.rex would need around 200,000 calories a day just to survive and if T.rex spending even more than that trying to walk around all day looking for a dead animal with the likelihood that there isn't that many calories left in it, it would be in trouble. It costs far fewer calories to ambush an animal with a much higher amount of food at the end of it.
Did T.rex have a good sense of smell?
So, if a T.rex is a predator and not a scavenger, why did it have such an excellent sense of smell and bone-crushing teeth? Well, the sense of smell may not have evolved for hunting, instead being used for communication. It's far easier to spot a potential mate or rival wandering into your turf when you can sniff them from miles away! Another explanation is my own (albeit untested) hypothesis:
It was found that the serrations in T. rex teeth were shaped in such a way that they could have stored harmful bacteria, giving it a septic bite, much like a komodo dragon. If this were true, it could explain such a good sense of smell. Predation can be very dangerous, not just for the prey, especially if the prey has three sharp horns sticking out of its face! A T. rex could lower its chances of injury by only biting its prey once, crushing and tearing as much as possible in a single ideal bite. After this, the T. rex could simply retreat and let the animal escape, ideally succumbing to injury, shock and infection caused by the septic bite. The T. rex would then use its sense of smell to track the dead/dying animal, with its bone-crushing teeth becoming useful for extracting as much nutrition as possible in case other scavengers had already begun consuming the animal.
But, alas, we'll never truly know for sure. I guess we'll just have to wait until some irrefutable evidence to come alo-... oh wait, it has!
Was T.rex a scavenger? A conclusion
In 2008, the arguments were finally settled when a Triceratops was found with healed bite marks in its bones from a T.rex. The animal would have needed to survive the encounter for the injury to heal, meaning it was indeed alive when the T.rex attacked.
So, was Tyrannosaurus rex a scavenger or a predator? Well, it was both. While it had the tools best suited for a predatory lifestyle (one we have evidence for it partaking in), most animals are opportunistic. T.rex certainly wouldn't have turned its huge nose up at a free lunch that just happens to already be dead and its massive bulk would have indeed helped to scare off any other animal who was tucking in. When you're as jacked as a T.rex, you're gonna need all the protein you can get!