The recent documentary Dino Gangs has met with the usual mixed reaction. The general opinion appears to be that the program (in the UK) was too long by at least an hour and suffered the usual mix of repetitiveness and also that the amount of CGI used was excessive. Still, any new programme on dinosaurs is always welcome and Phil Currie is very watchable.
When discussing the programme with Mark Graham, a colleague of mine, and the newly appointed preparator at the Natural History Museum in London, Mark made a comment which made me take a step back and do a little thinking. He said “How is it that pack hunting in dromaeosaurids seems to be generally accepted but whenever pack hunting is suggested for tyrannosaurids there is always a huge outcry declaring it unlikely?” Good point.
The focus in Dino Gangs is too concentrated on pack hunting tyrannosaurs – something akin to pack hunting in today’s extant mammal populations and yet it is wrong to compare. Don Henderson remarked that the dinosaurian brain was small and relatively underdeveloped when compared to mammals. His observation that "I doubt the thought would cross their tiny reptilian brains" when discussing the possibility of co-ordinated pack hunting tyrannosaurs is probably correct. And yet we should not be too dismissive about the possibility.
It was interesting, however, that throughout the programme, there was not one dissenting voice arguing against tyrannosaurs displaying ANY gregarious behaviour – not one. I accept that the onus was on pack hunting but never the less, perhaps we can take this that there has been a slow acceptance that tyrannosaurs (and other theropods) did gather together for an unspecified amount of time even if it was only briefly, to mate or perhaps to sort out territorial disputes.
Looking at the fossil evidence, there are now many examples of bone beds that contain the associated remains of multiple theropods. Multiple, it should be mentioned, may actually be only two animals but generally speaking when recovering predatory dinosaurs, two is indeed a bone bed. The most famous in recent years is the Albertosaurus bonebed in Alberta, which we now know contains the remains of at least 12 individuals although there are revised figures suggesting more than 20 individuals are represented . There are, however, other examples of tyrannosaurid bonebeds that include the taxa Tyrannosaurus, Tarbosaurus, and Daspletosaurus.
Other theropods found in bonebeds ranging throughout the Mesozoic include Coelophysis in the Triassic, Saurophaganax in the Jurassic and Mapusaurus in the Cretaceous (for an extensive list see Currie 2010). Other evidence for theropods perhaps living in groups includes what I consider to be some of the most compelling evidence and that is the fossilised trackways that are found worldwide. Some of these display tracks of multiple individuals moving together in a uniform direction and which, significantly, do not overlap thus indicating the likelihood that the animals were walking or running together. You also have to bear in mind that other related dinosaurs such as hadrosaurs and ceratopsians have also been found in mass bonebeds containing hundreds, even thousands of individuals. So there is significant fossil evidence that is suggestive that tyrannosaurs did cohabit for an unspecified amount of time.
Moving on to phylogenetic inferance using the extant phylogenetic bracket. Dinosaurs fall between crocodiles and birds and, by inferance alone, can suggest something about the behaviour in theropods, indeed dinosaurs as a whole.
Crocodiles often live together in large numbers but this is not strictly a communal group and is more often than not just an area where they happen to be together usually due to environmental conditions. But at certain times of the year they do gather together in vast numbers to intercept migrating herds of wildebeest and zebra. However they do not pack hunt and prey animals are taken by individuals but the dismembering is very much a joint effort. The seething mass of animals that bite, hold and twist the carcass apart looks as primitive as it gets and yet it is a very efficient way for a number of animals to share the spoils around a large number of animals.
Crocodiles also display amazing courtship, parental care and even a deftness of touch that can be almost regarded as genteel. Courtship is surprisingly complex and includes bubble blowing, slapping the water and vocalising before coupling takes place. The nest of eggs, once laid, is protected by the female and when the hatchlings appear the young are actually carried to nursery pools in the jaws of the female and they benefit from the protection of the mother for a further period of time.
Crocodiles are often imagined to be slow moving, dim witted, cold blooded animals that are throwbacks to the time of the dinosaurs. Well, they outlived the dinosaurs and continue to thrive, often in extreme conditions and are actually a very successful and complex animal. This brings us to the birds.
Birds (which are, of course, surviving avian theropods) also live and breed together in sometimes enormous numbers. In fact the birds often play the numbers game with great success since the more of your fellow birds you breed with, live with or fly with the greater your chances of survival for you and your young are. Birds also display high degrees of courtship and parental care and include some of the most amazing displays of ritual, nest building and care on the planet.
Birds are also known to hunt in groups and, again, complex behaviour is often displayed. Harris hawks hunt in small groups of two to six birds to maximise their chances of a successful hunt in desert terrain. Boobies and gannets also display both visual and oral communication combined with superb diving techniques in often large numbers, again to maximise their efficiency in catching fish. Vultures are very adept in spotting carcasses on the ground and also use their excellent eyesight to spot when one of their number has found such a carcass and join in on the feast almost immediately.
So what about dinosaurs other than theropods? I’ve already highlighted that many species of dinosaur have been found together in large numbers in vast bonebeds. Although these too are studied intensively, there is more acceptance that animals such as ceratopsians and hadrosaurs moved around in large numbers and herds. The numbers of animals in some sites defies belief and that in itself is a huge pointer to gregarious animals moving together.
Of course just why they were moving together cannot be wholly explained because extinct animals and their behaviour cannot be observed as you would observe in extant animals today. It’s easy to imagine migration, breeding and safety in numbers as possibilities for these dinosaurs moving together but it has to remain a point of conjecture that remains impossible to prove.
Parental care in dinosaurs has long been established – to a certain degree. Large nesting sites are known all over the world and in such numbers that almost certainly points to large numbers of animals nesting communally. The Maiasaurs of Egg Mountain are amongst the most famous and fossils here suggest complex parental care. Additionally, and more relevant in this case, are the egg brooding oviraptorids found in the Jiangxi Province of China , which are not that distantly related from tyrannosaurs.
So using the available evidence in combination with phylogenetic inference, it appears to me that the chances of tyrannosaurs living gregariously are very high, indeed maybe should even be expected. For how long a period of time they may of have been together is a matter of conjecture and is impossible to say without direct observation and, until a time machine is invented, that is how it will remain. Quite possibly it was only for short periods such as a mating and breeding season but, on the other hand, there may have been permanent groups of animals living together throughout the year – there is just no way of knowing.
Personally, I do see some tyrannosaurs living in groups for an indeterminate amount of time – but not all. I would also expect a degree of parental care, maybe not as advanced as some birds but nest protection seems likely and maybe feeding and protecting the young as well to some degree.
But what about pack hunting? Impossible to prove and it would appear to be a level of behaviour and coordination above what could be expected of dinosaurs. As things are right now, I see the possibility of communal hunting and feeding which is quite different – more akin to the crocodiles of today. Maybe one tyrannosaur would attack a prey animal and others would join in or, if the prey was injured or cornered, then more tyrannosaurs may have joined the attack and help despatch the unfortunate beast. And again, just like crocodiles, multiple tyrannosaurs ripping in to a carcass would make short work of it and a lot of meat and bone would get shared out real quick. Perhaps the face biting pathologies found in tyrannosaur skulls are actually indicative of this (Tanke 200), as intraspecific squabbling broke out during feeding.
So for me, it’s time to accept that tyrannosaurs and other theropods probably lived in groups for at least part of the time and all the available evidence, albeit circumstantial, suggests there may have been a degree of group hunting but I find it hard to accept that they could hunt in a coordinated, complex manner similar to a pride of lions although, of course, even that cannot be ruled out. Perhaps its time to turn the argument on its head and ask what evidence is there that tyrannosaurs were solitary hunters?
Currie, P.J. & David A. Eberth. On gregarious behaviour in Albertosaurus. Canadian Journal of Earth Sciences, 2010, 47:1277-1289, 10.1139/E10-072
Tanke, D.H. & Philip J. Currie. Head-biting behaviour in theropod dinosaurs: paleopathological evidence. Gaia 15: 167-184