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Social Behaviour

Philip J. Currie suggested that Tyrannosaurus may have been pack hunters, comparing T. rex to related species Tarbosaurus bataar and Albertosaurus sarcophagus, citing fossil evidence that may indicate gregarious (describing animals that travel in herds or packs) behaviour. A find in South Dakota where three T. rex skeletons were in close proximity may suggest the formation of a pack. Cooperative pack hunting may have been an effective strategy for subduing prey with advanced anti-predator adaptations which pose potential lethality such as Triceratops and Ankylosaurus.

Currie's pack-hunting T. rex hypothesis has been criticized for not having been peer-reviewed, but rather was discussed in a television interview and book called Dino Gangs. The Currie theory for pack hunting by T. rex is based mainly by analogy to a different species, Tarbosaurus bataar. Evidence of gregariousness in T. bataar itself has not been peer-reviewed, and to Currie’s own admission, can only be interpreted with reference to evidence in other closely related species. According to Currie gregariousness in Albertosaurus sarcophagus is supported by the discovery of 26 individuals with varied ages in the Dry Island bonebed. He ruled out the possibility of a predator trap due to the similar preservation state of individuals and the near absence of herbivores.

Additional support of Tyrannosaurid gregariousness can be found in fossilized trackways from the Upper Cretaceous Wapiti Formation of northeastern British Columbia, Canada, left by three tyrannosaurids traveling in the same direction. According to scientists assessing the Dino Gangs program, the evidence for pack hunting in Tarbosaurus and Albertosaurus is weak and based on group skeletal remains for which alternate explanations may apply (such as drought or a flood forcing dinosaurs to die together in one place). Others researchers have speculated that instead of large theropod social groups, some of these finds represent behaviour more akin to Komodo dragon-like mobbing of carcasses, even going as far as to say true pack-hunting behaviour may not exist in any non-avian dinosaurs due to its rarity in modern predators.

Feeding Behaviour

Paleontologist Jack Horner has been a major proponent of the view that Tyrannosaurus was not a predator at all but instead was exclusively a scavenger. He has put forward arguments in the popular literature to support the pure scavenger hypothesis:


 * Tyrannosaur arms are short when compared to other known predators. Horner argues that the arms were too short to make the necessary gripping force to hold on to prey. Other paleontologists such as Thomas Holtz Jr. argued that there are plenty of modern day predators that do not use their forelimbs to hunt such as wolves, hyenas, and secretary birds as well as other extinct animals thought to be predators that would not have used their forelimbs such as Phorusrhacids.
 * Tyrannosaurs had large olfactory bulbs and olfactory nerves (relative to their brain size). These suggest a highly developed sense of smell which could sniff out carcasses over great distances, as modern vultures do. Research on the olfactory bulbs of dinosaurs has shown that Tyrannosaurus had the most highly developed sense of smell of 21 sampled dinosaurs. Other paleontologists claim that this does not suggest a pure scavenger diet as an advanced sense of smell may be used to track live prey or behaviour unrelated to feeding altogether. Holtz also points to modern predators as well as other predatory dinosaurs with excellent smelling ability such as wolves and dromaeosaurs.
 * Tyrannosaur teeth could crush bone, and therefore could extract as much food (bone marrow) as possible from carcass remnants, usually the least nutritious parts. Karen Chin and colleagues have found bone fragments in coprolites (fossilized feces) that they attribute to tyrannosaurs, but point out that a tyrannosaur's teeth were not well adapted to systematically chewing bone like hyenas do to extract marrow. This idea was challenged on the basis that such a potent bite force would be advantageous for subduing and consuming prey. Holtz also notes that while the teeth of Tyrannosaurus and Hyenas share several similarities, all known hyena species are known to actively hunt prey, with the largest, the Spotted Hyena (Crocuta crocuta), obtaining most of its food by hunting (In some areas hunting as much as 95% of their food). Holtz also points out similarities with the teeth of other modern predators such as crocodiles, cats, and orcas.
 * Since at least some of Tyrannosaurus' potential prey could move quickly, evidence that it walked instead of ran could indicate that it was a scavenger. On the other hand, recent analyses suggest that Tyrannosaurus, while slower than large modern terrestrial predators, may well have been fast enough to prey on large hadrosaurs and ceratopsians.

A skeleton of the hadrosaurid Edmontosaurus annectens has been described from Montana with healed tyrannosaur-inflicted damage on its caudal (tail) vertebrae. The fact that the damage seems to have healed suggests that the Edmontosaurus survived a tyrannosaur's attack on a living target, i.e. the tyrannosaur had attempted active predation. Despite the consensus that the tail bites were caused by Tyrannosaurus, there has been some evidence to show that they might have been created by other factors. For example, a 2014 study suggested that the tail injuries might have been due to Edmontosaurus individuals stepping on each other, while another study in 2020 backs up the hypothesis that biomechanical stress is the cause for the tail injuries. Additional fossil evidence of active predation on Edmontosaurs was described by David Burnham in 2013 from a pair of caudal vertebrae discovered in 2007. These vertebrae had the tip of a Tyrannosaurus tooth embedded in them with new bone growth around the tooth suggesting the individual survived an encounter with a Tyrannosaurus, providing direct evidence of predatory behaviour in T. rex.

A 2004 study also suggested that due to the lack of selection pressure it is unlikely for a large terrestrial carnivore to evolve exclusive scavenging behaviour, with opportunistic feeding behaviour being much more advantageous.

Most paleontologists now accept Tyrannosaurus was an opportunistic carnivore displaying both predatory and scavenging feeding strategies like many modern day carnivores, rejecting the idea of T. rex as an pure scavenger.

A study by Eric Snively and Anthony Russell suggested that Tyrannosaurus could use its powerful neck muscles to feed by tearing pieces of flesh from a carcass, tossing them in the air, and swallowing, much like some of its modern archosaur relatives (birds and crocodilians).

Hunting Behaviour

Holtz has suggested Tyrannosaurus hunted prey by seizing and killing it with its jaws, similar to canids (dogs) and hyenas. Its stout, strong, deep rooted teeth were mechanically stronger than other theropods’ teeth, and it possessed a bony secondary palate similar to that of a crocodile which is uncommon in other theropods. These adaptations would allow Tyrannosaurus to resist the twisting loads of struggling prey. Holtz suggested it punctured and pulled its prey rather than slicing its flesh like many other theropods. Paleontologists believe that like most modern carnivores, Tyrannosaurus likely preferred to prey upon smaller, weaker animals, including juvenile, sick, and elderly individuals. Hone and Rauhut conducted an analysis in 2010 which suggested Tyrannosaurs preying upon juvenile dinosaurs which may partially explain the rarity of juveniles in the fossil record. Tyrannosaur adaptations specializing in the crushing and ingestion of bone would leave fewer skeletal remains, leading to a lesser abundance of fossils. Some studies also suggest that at different life stages Tyrannosaurus likely preferred different types of prey. Evidence suggests its powerful bite only developed in adulthood, while juveniles were nimble long-legged runners. This would allow adults to prey upon megaherbivores such as hadrosaurs, while juveniles likely hunted much smaller prey, granting a huge evolutionary advantage by reducing intraspecific competition for food resources.

Parenting

While there is no direct evidence of Tyrannosaurus raising their young (the rarity of juvenile and nest Tyrannosaur fossils has left researchers guessing), it has been suggested by some that like its closest living relatives, modern archosaurs (birds and crocodiles) Tyrannosaurus may have protected and fed its young. Crocodilians and birds are often suggested by some paleontologists to be modern analogues for dinosaur parenting. Direct evidence of parental behaviour exists in other dinosaurs such as Maiasaura peeblesorum, the first dinosaur to have been discovered to raise its young, as well as more closely related Oviraptorids, the latter suggesting parental behaviour in theropods.