The image of dinosaurs hunting in a pack, in an intelligent and coordinated way, appears in countless films and documentaries and is almost an icon of our time. Did it really happen that way? Or are we imagining the world of dinosaurs too much like our own?
We can’t help it. Dinosaurs fascinate us, and those ancient and hollow skeletons, as beautiful as they are, fall short. What we want is to see those animals once again full of meat, once again showing off their best colors, once again bawling and grazing, fighting to live and for their own to live. It is inevitable: in reality, what we want is for the dinosaurs to come back to life.
Fortunately, there are scientific disciplines that can help us on this path: biomechanics can tell us what movements were possible, phylogeny tells us which living species are most closely related to them, ethology helps us understand the behavior of modern animals and formulate informed hypotheses about how animals of the past might behave. The meeting of all these disciplines has allowed us to get an idea of what some species of dinosaurs would be like if we could see them alive again.
For example, the discovery of dozens of skeletons of Iguanodon, a herbivorous dinosaur, deposited in the same place began to give us clues that certain species formed herds, like some current herbivores. Possibly they did it for the same reasons: because a large group commands more respect from predators and provides some protection against the action of a lone hunter. As for large carnivorous dinosaurs, such as Tyrannosaurus, what we observe is the opposite: the remains of more than one individual are rarely found in the same place. This has led us to think that they would probably act alone, like many current large predators. Perhaps also for the same reasons: because they are large animals, which need a significant supply of prey, and which are also very aggressive,
A recreation of what Deinonychus antirrhopus might have looked like, with its body covered in feathers and the characteristic retractable claws on its feet, which were surely formidable weapons during their hunting expeditions. PHOTO: Fred Wierum (Wikimedia)
For other predatory dinosaurs, however, especially the smaller ones with relatively large brains, it has been proposed that they could be social hunters. In other words, they could form packs, like current wolves, and hunt in a coordinated way in order to dominate prey larger than themselves, or to overcome the protection mechanisms of herbivore herds. The paradigmatic case of this idea is Deinonychus antirrhopus.
Small predators and large prey
Deinonychus was a hunter of modest size: bipedal, three feet tall and weighing between 150 and 220 pounds, it would have made an imposing foe for an average human. But of course, he lived in the lower Cretaceous, at a time when some predators reached five meters in height and seven tons in weight. Under these circumstances Deinonychus would have to admit that he was nothing but middle class.
As in the cases of Iguanodon and Tyrannosaurus, the most reliable source of information about the habits of Deinonychus is its remains. And in them it is observed, over and over again, that along with the Deinonychus fossils appear those of a herbivore from the same period, Tenontosaurus tilletti. The associations between herbivores and carnivores are common, and we tend to always interpret them in the same way: the carnivore is the predator and the herbivore is the prey. However, with the Tenontosaurus-Deinonychus pair, a problem arose: Tenontosaurus weighed more than a ton, between 10 and 20 times more than Deinonychus. It seemed impossible that an animal this large would allow itself to be hunted by a solitary Deinonychus.
Illustration of the size of Deinonychus (right) relative to a modern human and relative to its usual prey, Tenontosaurus (left). PHOTO: Alberto Aparici / Fred Wierum / Nobu Tamura / Linda Salzman Sagan / Tompw / Holek (Wikimedia)
As a solution to this problem, the paleontologist John Ostrom proposed in 1969 that Deinonychus had to hunt in groups, and in groups close enough to bring down prey ten times their weight. The social hunters we know today, such as lions or wolves, use highly coordinated strategies and can hunt animals five times heavier than themselves; in the case of African wild dogs, up to ten times heavier. Given that Deinonychus’ brain is relatively large by dinosaur standards, the hypothesis didn’t seem crazy. In the following decades, the discovery of new Deinonychus and Tenontosaurus remains and a more detailed understanding of modern social hunters only added to the argument: It was becoming more and more credible that this pair of predator and prey already staged in the Cretaceous what we see today in the Serengeti. The theory caught on, and has been appearing regularly in movies, novels, and documentaries ever since.
However, as is often the case in science, every hypothesis has its Achilles heel. In 2007, two researchers published an article in which they defended a totally different idea: that Deinonychus did not hunt in groups or, at most, did so in loosely cohesive groups that were formed for a single hunting episode. In this theory, the model was no longer wolves or lions, but the Komodo dragon, a reptile weighing more than 60 kilos and 2 meters long that lives on some Indonesian islands. Komodo dragons usually operate as solitary hunters, but sometimes groups of ten or more come together to harass large prey, and once it is down they compete for the right to be the first to enjoy the feast. Normally larger individuals have priority, as we also observe in an animal that we know better: the vulture, when a large group gathers around a piece of carrion. The hypothesis of these two researchers is that Deinonychus would act more like a Komodo dragon or a vulture than like a pack of wolves with well-established social hierarchies.
The Komodo dragon is the largest lizard on Earth today, measuring more than two meters in length. Their main hunting technique is ambush, but sometimes larger or smaller groups can be seen encircling deer or water buffalo, and then giving a good account of their meat. In these gatherings, the larger dragons claim priority, and if they perceive the attitude of other individuals as a challenge, they may resort to violence to assert their position. PHOTO: Mark Dumont (Wikimedia)
Three main ideas supported this new theory: First, at least one Deinonychus fossil showed evidence of having been attacked by another Deinonychus. Specifically, a claw from the hand seemed to be embedded in one of the vertebrae of the tail. By itself this only indicates that there were episodes of violence between Deinonychus, but it is true that among social animals it is rarer for disputes to reach open violence, because social hierarchies allow these differences to be settled in a less bloody way. On the other hand, among animals with less social structure, such as Komodo dragons, it is common for disputes, especially over food, to degenerate into violent aggression until one of the contenders withdraws.
Second, the authors point out that although the relationship between Deinonychus and Tenontosaurus is very well established, it is not so clear that the Tenontosaurus that appear at these sites are necessarily adult specimens. Some indications suggest that the Tenontosaurus that appear together with Deinonychus remains are mainly juveniles, less corpulent animals that perhaps would be within the reach of a solitary Deinonychus.
Finally, the strongest argument in favor of the new hypothesis is that there are hardly any known cases of social hunting in the lineage of dinosaurs and their closest relatives. Among birds, hunting in coordinated groups is very rare, and among the next closest relatives of the dinosaurs, the crocodiles, non-existent. Crocodiles, like the Komodo dragon, do sometimes gather in groups to harass large prey, then compete for the right to be the first to eat the feast. Thus, if Deinonychus, or any other dinosaur, hunted as wolves or lions do today, this would be a behavior invented by that group of dinosaurs and then “rediscovered” by mammals. Although that is not impossible.
The inheritance of teeth
Until this point, the debate moved in the field of theoretical arguments and reasonable evidence. Objectively, it is difficult to obtain more information from remains that only give us circumstantial information about the behavior of the animals. But a few days ago an article appeared in the journal Palaeogeography, Palaeoclimatology, Palaeoecology that aims to provide a little more objective evidence on this matter. The fundamental idea is that in social species the young and the young usually have a diet identical to that of the adults, because they are protected within the group. In non-social species, on the other hand, the young must find their own food, and differences in diet between young and adults are often observed.
To check which of the two categories the dinosaurs fit into, the authors of this new article analyzed the carbon in the teeth of Deinonychus, to see if there was any difference between the smaller teeth, corresponding to juvenile specimens, and the larger ones. Carbon has two stable varieties: carbon-12, which has six neutrons, and carbon-13, which has seven. Chemically they are almost identical, but some biological reactions have a certain preference for carbon-12 over 13, so different living things can have a different proportion of one and the other. Through the diet these differences are transferred to the composition of the body of the dinosaurs and this, through the fossilization process, to the composition of the fossils that have come down to us.
A recreation of a family group of Utahraptor, in which one of the parents seems quite aware of the young. Since birds tend to care for their young at least for a while, it is likely that dinosaurs did something similar. But that care could be very close and include a certain “education” in hunting strategies, as wolves do, or be much more lax and limited to scaring off predators, as ostriches do. PHOTO: ABelov2014 (Deviantart)
The result of the study has been clear: Deinonychus teeth show differences between the diet of juveniles and adults. These differences are consistent with those seen in crocodilians, both modern and extinct, with Tenontosaurus teeth showing uniform composition regardless of age. Although the statistics are small, because unfortunately we have to destroy the teeth to be able to analyze them, this new piece seems to support the hypothesis that the social structure of Deinonychus was not as established as in modern social mammals.
Does this mean that we have already solved the mystery of Deinonychus’ behavior? Unfortunately not. Although each of the hypotheses is very well argued and it is extraordinary that we can extract so much information from rocks that were once bones, the truth is that all this evidence is still very indirect. However, this is how science works: this is the best we have today, and right now the balance may be tipping in favor of not very social carnivorous dinosaurs. What does the future hold? Only the bones know that.