Australia: The Land Where Time Began

A biography of the Australian continent 

Tyrannosaurus rex behaviour

After studying T. rex for 100 years it is still being debated whether it was primarily a predator or a scavenger. Previously its behaviour has been inferred from its anatomy but this is now seen as inadequate to gain a true picture of the actual life of the animal. Palaeobiologists now use clues in the fossil evidence to try to discover what they were really like in life.

The interactions between T. rex and others of its kind as well as with other species such as the herbivores living in the same environment is being elucidated by studying the skeletons of multiple individuals for bite marks on bones and wear patterns on their teeth. Indications of which animals they ate can be found in their coprolites, i.e. fossilised dung.

An assumption that has been made is that it is possible that species that are closely related may have behaved in similar ways, which allows the finds for T. rex to be corroborated by comparing them with earlier members of the Tyrannosauridae, which includes related species that are from an earlier time such as Albertosaurus, Gorgosaurus and Daspletosaurus, which are collectively albertosaurs.

Social or antisocial

Tyrannosaurs have traditionally been depicted as solitary hunters but evidence has been accumulating that they may have actually been a more social animal than previously suspected, though the author¹ suggests it may have been social for only part of its life. 

When an adult T. rex was being exhumed from the Hell Creek Formation in eastern Montana in 1966 a second, smaller T. rex was unexpectedly found on top of the T. rex that was being excavated. The first identification of the smaller fossil skeleton was as a species of tyrannosaur that was more petite than the larger one. When the author¹ carried out an examination of the histological evidence of the microstructure of the bones his results suggested the smaller animal was actually a subadult T. rex. Sometime after Sue, one of the largest and most complete fossils to be found, had been excavated 2 other tyrannosaur skeletons were found in the same quarry, one a subadult and the other a juvenile. Workers at this site as well as the Hell Creek Formation think it is too much of a coincidence to find 3 loner tyrannosaurs in the same burial. The author¹ says it is a more parsimonious explanation to suggest that they were all part of a group.

Erikson¹ suggests that another find in 1910, a bone bed deposit that contained 26 individuals of albertosaurs, indicates Tyrannosauridae displayed gregarious behaviour.

A detailed study of the skeletons from the 1910 excavation was carried out by Phillip J. Currie and his team. When carnivores congregate in such numbers it is usually found that a prey animal is mired in somewhere like the mud at the edge of a water body of some kind and a succession of carnivores attempt to get to it, each in tern becoming mired in the mud. As no herbivore remains were found in this deposit, as well as the assemblage that included the T. rex that has been called Sue, it suggests the possibility that all the individuals in this group died together from some such cause as disease, drought or drowning.

Currie estimated, based on his examination, that among the remains that had been collected up to that time there were animals with lengths that ranged from 4-9 m (13-29 ft). Erikson¹ suggest that a group with such a range of sizes may have been composed of juveniles and adults, and based on his studies of growth lines; they had ages ranging from 2-28 years. Tyrannosaurs have been shown by some track evidence resulting from recent analysis to have been moving in pairs.

A new way to look at tyrannosaurs is as herds of the animals that formed complex interrelationships. That such herds were not composed of  a group or carnivores that were on friendly terms with each other is evinced by the number of bite marks that are partially healed, revealing that they had nasty interpersonal relationships, and this has been revealed by work of Currie & Darren Tanke. Tanke, a palaeopathologist, has found that there is a unique pattern of bite marks among thereopod dinosaurs that include the tyrannosaurs. The bite marks are in the form of gouges and punctures on the sides of the snout, on the sides and the lower surface of the jaws, though they sometimes occur on the top and back of the skull.

Currie and Tanke have reconstructed the way these carnivores fought by interpreting the wounds. They suggest that the animals faced off while mostly gnawing at one another with the teeth in one side of their jaw, instead of snapping. Currie and Tanke also suggest that peculiar bite marks on the sides of tyrannosaur teeth result from this jaw gripping behaviour. It is also implied by these bite marks that the heads of the combatants are held at the same height throughout the confrontation. It seems from the severity of some of these wounds that little reserve was displayed by T. rex in this intraspecific fighting. A tyrannosaur in their study has a tooth from another T. rex imbedded in its own jaw.

It is suggested that the cause of these clashes may have been food, mates and territory, and the evidence shows that such behaviour continued throughout the life of tyrannosaurs. Among the younger individuals injuries appear to have been more common, possibly because it was attacked by its own group as well as large adults.

The teeth of T. rex have been described as “lethal bananas” by Kevin Padian,  but it is generally believed among palaeontologists that it is rare to find bite marks of dinosaurs, the few reports of them up to the 1990s consist of brief comments in articles about other finds and the behaviour of dinosaurs wasn’t thought about.

Ralph F. Molnar began to speculate about the strength of T. rex, based on their shape, in 1973. Elaborate morphological studies were later performed on tyrannosaur dentition by James O. Farlow and Daniel L Brinkman, the results convincing them that the teeth of tyrannosaurs were robust as a result of their rounded c ross-sectional configuration, sufficient to withstand the bone-shattering impacts they would be subjected to during feeding.

In 1992 the author made orthodontic dental putty casts of some of the deeper holes about 12 cm long and several cm deep on the partial pelvis of a Triceratops that was 1.5 m long and 1 m wide, and the toe bone of an adult Edmontosaurus (duck-billed dinosaur) that both had many of these tooth marks on them.  The casts indicated that the teeth responsible for the holes were spaced about 10 cm apart, and the punctures they left had eye-shaped cross sections. According to Erikson¹ there were clear included of carinae, elevated cutting edges that were serrated, on the anterior and posterior faces of the teeth, and he suggests the totality of the evidenced pointed to T. rex being responsible for these indentations, and the first definitive evidence of bite marks from a T. rex.

The considerable implications for the behaviour of T. rex were conformation of the assumption that Triceratops and Edmontosaurus, its 2 most common contemporaries, were fed on by T. rex. The patterns of the bites also gave some indication of the actual feeding technique of T. rex, which apparently indicated 2 distinct biting behaviours, usually involving the “puncture and pull” strategy in which the teeth bite deeply into the food with great force then pulling the teeth through the penetrated flesh and bone, the result being long gashes. The predator that fed on this Triceratops appears to have detached the pelvis from the Triceratops and also used its front (incisiform) teeth to strip the flesh from narrow spaces between the vertebrae, places where only the muzzle could reach, leaving parallel furrows in the bone that were aligned vertically.

As many of the bite marks on the pelvis of the Triceratops were separated by only a few centimetres, which suggests the T. rex had worked its way methodically along the body, appearing to have removed a small section of bone with each bite. It was assumed the missing bone had been swallowed by the T. rex, which was soon confirmed.

In 1997 Karen Chin received a tapering mass that proved to be a T. Rex coprolite. It was 44 x 16 x 13 cm and weighed 7.1 kg, the first specimen that had been confirmed to have been produced by a theropod, was more than twice as large as any coprolite that had been reported for a carnivorous dinosaur, and it contained large amounts of pulverised bone. Chin and Erikson¹ used histological methods to conform that the bone had come from a young herbivorous dinosaur, showing that T. rex ingested parts of the bone with its food, and also that it had strong enzymes or stomach acids in its stomach to partially digest the bone.

Olson and Erikson¹ have argued strongly that there are probably many bite marks that were left by T. rex, in spite of the sparse record of them on known specimens. As absence of evidence is not evidence of absence, they believe there are 2 factors that account for the lack of fossil evidence of T. rex feeding habits. One is that bite marks have never been systematically searched for, and what is probably even more significant is that there has been a bias among collectors for museums towards specimens that are as complete as possible, so not specimens displaying bite marks, especially where part of the bone is missing. Museums have, according to Erikson¹, historically wanted complete skeletons and not single, isolated parts. Whole skeletons tend to be from animals that died from misadventure and subsequently been buried before they could be dismembered by scavengers. As in the case of this Triceratops pelvis, the shredded parts of bodies that are not wanted by museums are precisely the bones that are most likely to display evidence of feeding behaviour.

Isolated partial remains have been compared with those of nearly complete skeletons in Alberta by Aase Roland Jacobsen. Her findings show that for individual bones there were 3.5 times as many that displayed theropod bite marks (14 %) than the remains that had been less disrupted (4 %). Erikson¹ described most of the natural history museums of the world as “deserts of behavioural evidence” when compared with fossils that have been left uncollected in the field.

Hunter or scavenger

An ongoing debate concerning tyrannosaurs has been whether T. rex were primarily hunters or scavengers, and Erikson suggests the evidence for this feature of their biology is accessible by the study of their fossil record, trace fossils in particular.

From the very first discovery of T. rex fossils it has been believed to have been a predator, though this has been based mainly on their anatomy, they look like powerful predators. An example given by Erikson of an animal with a skull that makes it appear to be a predator is the bear, even though the majority of their food is not meat, but such food as berries. Lawrence Lambe, a Canadian palaeontologist, studied a partial skull of an Albertosaurus in 1917 and came to the conclusion that it ate soft, rotting meat, which suggested to him that they were actually giant scavengers. He based his conclusion on fact that the teeth were relatively water-free. According to Erikson, later research has found that 40 % of tyrannosaur teeth were broken and severely worn, and Erikson’s research has shown that such damage occurs in 2-3 years, based on his estimates on rates of tooth replacement in tyrannosaurs. Lambe’s conclusion has become the basis for the minority view that tyrannosaurs filled the role of vultures, very large vultures. The continuing argument has been mainly centred of the anatomy and physical strength of T. rex.

The “weak-tooth” hypothesis is supported by the scavenger proponents, suggesting that the elongate teeth to tyrannosaurs would have failed in struggles with prey if they were predators, or possibly their undoubtedly powerful bite caused their teeth to break when the impacted bone. The very small arms of T. rex are also claimed as evidence of the problems they would have when struggling with large and powerful prey, as well as being so large they would find it difficult to run down prey.

The proponents of the predator theory base their arguments of biomechanical studies, citing Erikson’s studies on bite force which show that the teeth of T. rex were actually “quite robust”, though Erikson has not committed himself to either side in the debate while he waits for further evidence to decide the issue. Kenneth Carpenter and Mathew Smith have estimated that T. rex arms could have curled (lift a weight using only the hands, wrists and forearms) of almost 180 kg. The work of Per Christiansen, which is based on the proportions of the limbs, suggested that T. rex may have been able to sprint at 47 km/hr (29 miles/hr). Later studies have suggested that a more likely speed would have been 24-37 km/hr, and that adults probably ambled rather than ran. Erikson suggests that even at the slowest of the estimated speeds T. rex would have been faster than any of its large contemporary herbivores. Though as Erikson points out there are other factors to consider in predation, such as acceleration, endurance, guile, stealth and agility. There are also the characteristics of the prey species, such as differences in size, and the availability and their vulnerability.

According to Erikson the issue cannot be solved by biomechanical studies, suggesting instead that searching for evidence that it was a predator based on its physical characteristics the crucial determinant is finding to what degree it utilised the animals in its environment, and not working on the basis of the presumption of its physical ability to kill. Extant animals such as lions scavenge as well as hunting, and hyenas hunt as well as scavenge. And the evidence is mounting that tyrannosaurs also did both, hunt and scavenge.

Bone beds have been found in same areas where the remains of T. rex have also been found that contain the skeletons of hundreds to thousands of edmontosaurs that died as the result of floods, drought or other causes that don’t include predation. Evidence that these edmontosaurs were scavenged by T. rex is in the form or bite marks and shed tooth crowns. Comparable evidence for scavenging by albertosaurs has been found by Jacobsen. Solid evidence has also been found of hunting by T. rex by Robert A. DePalma, David Burnham, and others consisting of unsuccessful attacks, in one case on what was probably an adult Edmontosaurus that was attacked by a T. rex, in which the prey had escaped with a bite mark on its tail that subsequently healed with a broken tooth still embedded. Other edmontosaurs and possibly Triceratops have also provided evidence of hunting by T. rex.

Erikson suggests it would be helpful if standard definitions of predators and scavengers were decided upon, such as the definition of a predatory species being a species in which most individuals acquired most of their food from animals that were killed by themselves or their peers. For scavengers it would be most of the food consumed by a species would have been killed by other species.

A systematic approach to the predator-prey controversy could be helped by the study of trace fossils, testing hypotheses of entire patterns of tyrannosaur feeding preferences could produce a resolution of the problem. It has been pointed out by Jacobsen that evidence of a preference for animals that are less dangerous or more easily caught would indicate a carnivore was a predator. Carnivores with no apparent preference for a particular size or defensive capability would most likely be a scavenger.

When this logical framework is used Jacobson’s data appears compelling that tyrannosaurs were indeed predators, as when she surveyed thousands of dinosaur bones collected from Alberta her results indicate that hadrosaurs, which lacked armour, are more than twice as likely to have tyrannosaur bite marks than were the horned ceratopsians, and no bite marks had been on the ankylosaurs that were built like tanks.

Jacobsen has cautioned that her findings are confused by other factors such as most hadrosaur bones being from isolated individuals, whereas in her study most ceratopsians were found in bone beds. There are more whole, unscathed animals in these beds which creates a kind of tooth mark bias that has been discussed earlier. It is suggested that it would be enlightening if isolated ceratopsians were surveyed. Analysis of more bite marks that indicate failed attacks by predators, as have been reported by DePalma, Burnham and their colleagues might also indicate preferences for prey that was less dangerous.

Jacobsen also found that cannibalism was rare among tyrannosaurs, at 2 % of albertosaurs bones with signs of albertosaurs bite marks, while among herbivores it was 14 %, might possibly also support the predatory preferences rather than scavenging for T. rex, particularly if they were gregarious. If it is assumed they weren’t averse to eating flesh of their own species it would be expected that there would be as many T. rex bones with signs of the animal being eaten by another T. rex as there are on herbivore bones, and if T. rex travelled in herds it would be easy to find conspecifics that were freshly dead.

Valuable evidence of the eating habits of T. rex may also be found in coprolites, any bone in the coprolite can be analysed histologically and the approximate stage of development of the animals consumed can be determined. It has been suggested by Chin and Erikson that examination of the bone in coprolites may indicate if T. rex concentrated its hunting on vulnerable members of herds, such as the very young, as such a bias would indicate that it was indeed a predator, and if it had more impartial feeding habits, which matched the normal attrition pattern, it would indicate it was primarily a scavenger.

Enough physical remains of T. rex have been recovered over the last 100 years to give a good idea of the appearance of these large animals. What it was actually like in life is a different matter, as this requires the fossils to be examined in detail, searching for clues to their daily activities. It has now been recognised by palaeontologists that previously ignored remains of T. rex need to be re-examined with the aim for searching for these clues that have been overlooked in the past, such as the bite marks on the snouts of individual tyrannosaurs that match the pattern of bite marks made by conspecifics, and the collection biases have also now been recognised. Erikson suggests that knowledge of palaeobiology should be gained more rapidly by intentionally looking for behavioural data.

Sources & Further reading

Gregory M. Erikson