Pack Hunting

The idea that theropod dinosaurs hunted cooperatively in packs was first articulated by John H. Ostrom in his landmark 1969 monograph describing Deinonychus antirrhopus from the Lower Cretaceous Cloverly Formation of Montana, published in the Bulletin of the Peabody Museum of Natural History. At the Yale Peabody Museum quarry site YPM 64–75, Ostrom recovered the remains of at least three Deinonychus individuals in close taphonomic association with the much larger ornithopod Tenontosaurus tilletti. Given that an individual Deinonychus, weighing approximately 70–100 kg, would have been dwarfed by a Tenontosaurus estimated at roughly one tonne, Ostrom concluded that a solitary Deinonychus could not have dispatched such large prey on its own. Drawing an analogy with modern canids such as African wild dogs (which hunt prey several times their own body mass through coordinated pack behavior), Ostrom proposed that Deinonychus operated in cooperative groups. This interpretation was supported by the animal's cursorial limb proportions, relatively large brain for a dinosaur, and the presence of the formidable sickle-shaped claw on pedal digit II. Over the following decades, numerous additional sites in both the Cloverly Formation and the contemporaneous Antlers Formation of Oklahoma yielded similar associations of Deinonychus and Tenontosaurus, seemingly reinforcing the predator-prey pack hunting scenario.

Ostrom's pack hunting hypothesis gained enormous cultural amplification through Michael Crichton's 1990 novel Jurassic Park and Steven Spielberg's 1993 film adaptation. Crichton modeled his fictional "Velociraptors" closely on Deinonychus antirrhopus in almost every anatomical detail (as acknowledged by Ostrom himself), choosing the name Velociraptor because it sounded more dramatic. The film memorably depicted these animals as highly intelligent, wolf-like pack hunters capable of coordinated ambush tactics, problem-solving, and even door-opening. This portrayal cemented in public consciousness the image of pack-hunting raptors and profoundly influenced how an entire generation perceived dinosaur intelligence and social behavior. The Jurassic Park franchise's persistent depiction of raptor pack hunting continues to drive popular search interest in this topic and shapes lay understanding of dromaeosaurid behavior, often at odds with the more nuanced and skeptical positions of modern paleontology.

The most substantial challenge to the mammal-like cooperative pack hunting model came from Brian Roach and Daniel Brinkman in their 2007 paper published in the Bulletin of the Peabody Museum of Natural History. Upon reexamination of the original YPM 64–75 quarry assemblage, Roach and Brinkman found taphonomic evidence suggesting that at least one of the Deinonychus individuals at the site had been killed by conspecifics—an observation more consistent with agonistic intraspecific behavior than with cooperative pack dynamics. The authors systematically surveyed the behavioral ecology of extant archosaurs (crocodilians and birds) and found that true cooperative pack hunting of large-bodied prey is exceedingly rare among these groups. Among living birds, only the Harris's hawk (Parabuteo unicinctus) is well documented as engaging in coordinated group hunting, and even this behavior appears to be linked to specific environmental pressures (desert habitat with limited prey) and strong kinship bonds. In crocodilians, only rudimentary forms of feeding aggregation occur, with strict size-based hierarchies governing access to carcasses rather than any coordinated hunting effort.

Roach and Brinkman proposed that a more appropriate behavioral analogue for Deinonychus was the Komodo dragon (Varanus komodoensis). In Komodo dragon ecology, multiple individuals frequently converge on the same prey item or carcass, but this represents uncoordinated mob feeding rather than cooperative hunting. The largest individuals dominate through a strict size-based hierarchy, smaller animals feed at the periphery, and juveniles are excluded altogether—sometimes even at risk of cannibalism from adults. This model replaced the wolf analogy with an asocial reptilian framework.

A pivotal piece of evidence against mammal-like pack hunting in dromaeosaurids came from stable isotope analysis by Joseph Frederickson, Michael Engel, and Richard Cifelli, published in 2020 in Palaeogeography, Palaeoclimatology, Palaeoecology. The researchers analyzed carbon (δ¹³C) and oxygen (δ¹⁸O) isotope ratios in tooth enamel carbonate from Deinonychus antirrhopus teeth collected from two microsites: V706 from the Antlers Formation of Oklahoma and V1075 from the Cloverly Formation of Montana. They compared these data with teeth from contemporaneous goniopholidid crocodylians and Tenontosaurus tilletti.

The key finding was that small Deinonychus teeth (representing juveniles, <4.5 mm crown height) and large teeth (representing adults, >9 mm crown height) showed significantly different average δ¹³C values. Small teeth were relatively more enriched in ¹³C (mean approximately −9.0‰), while large teeth were more depleted (mean approximately −10.4‰). This pattern closely paralleled the ontogenetic dietary shift observed in the contemporaneous goniopholidid crocodylians—and indeed in modern crocodilians—where juveniles and adults occupy distinctly different trophic niches. The isotopic values of adult Deinonychus teeth were close to those of Tenontosaurus, consistent with the hypothesis that adults fed upon this large herbivore, while juvenile teeth indicated a diet composed of smaller, trophically higher prey.

This ontogenetic dietary partitioning is characteristic of asocial carnivores in which juveniles must fend for themselves, as seen in Komodo dragons. In contrast, true cooperative pack-hunting mammals such as wolves show minimal dietary variation between juveniles and adults because the pack collectively provisions all members with access to the same large prey items. The isotope data from both geographic formations supported the conclusion that Deinonychus young were not being fed by adults and thus were unlikely to have been integrated into a cooperative hunting pack structure.

While the evidence against dromaeosaurid pack hunting has grown stronger, a parallel line of research has explored possible gregarious behavior in tyrannosaurids. Philip Currie first proposed evidence for gregarious behavior in tyrannosaurids in 1998, based on the Dry Island Buffalo Jump Albertosaurus sarcophagus bonebed from the Horseshoe Canyon Formation of Alberta, Canada, originally discovered by Barnum Brown in 1910 and relocated by Currie in 1997. This site preserves at least 12 Albertosaurus individuals of varying ontogenetic stages, interpreted as a mass mortality of a social group rather than a time-averaged accumulation. Additional monodominant tyrannosaurid bonebeds include a Daspletosaurus horneri site in the Two Medicine Formation of Montana, and the "Rainbows and Unicorns Quarry" (RUQ) described by Titus et al. in 2021, which contains at least four or five Teratophoneus curriei individuals from the Kaiparowits Formation of southern Utah, dated to approximately 76.4 million years ago.

The 2021 Titus et al. study in PeerJ was significant because its detailed taphonomic analysis demonstrated that the RUQ tyrannosaurid accumulation was unlikely to have resulted from a predator trap, drought-driven aggregation, or time-averaged deposition. The presence of multiple age classes buried together in what appeared to be a single mortality event strengthened the case for habitual gregariousness in tyrannosaurids. However, the authors and other researchers have been careful to note that gregariousness (living in groups) does not necessarily imply cooperative pack hunting; the tyrannosaurids may have aggregated for other reasons such as mutual tolerance during feeding, seasonal migration, or protection from environmental hazards.

Further ichnological (trackway) evidence came from McCrea et al. (2014), who described three parallel tyrannosaurid trackways (Bellatoripes fredlundi) from the upper Campanian–lower Maastrichtian Wapiti Formation in British Columbia, Canada. The trackways showed three large theropods walking concurrently in the same direction within an 8.5-meter-wide corridor, with similar depth and preservation suggesting contemporaneous formation. The estimated ages of the track-makers were 25–29 years, indicating fully adult animals. While these trackways provide ichnological support for gregariousness, they do not by themselves demonstrate cooperative hunting.

Trackway evidence for gregarious behavior in dromaeosaurids was reported by Li et al. in 2008 (published in Naturwissenschaften), documenting Early Cretaceous deinonychosaur footprints from Shandong, China. Six parallel deinonychosaur trackways, produced by two different-sized taxa, were found traveling in the same direction along a stream margin without overlapping, suggesting coordinated or at least simultaneous movement. This was the first trackway evidence supporting the idea that dromaeosaurids at least traveled in groups. However, as with the tyrannosaurid trackways, parallel movement does not constitute direct evidence of cooperative hunting behavior—the animals may simply have been moving through the same corridor for non-hunting reasons such as following a water source or migration route.

The Mapusaurus bonebed, described by Coria and Currie in 2006 from the Huincul Formation (Cenomanian–Turonian) of Neuquén Province, Argentina, represents another case of a monodominant large theropod accumulation. Multiple individuals of the giant carcharodontosaurid Mapusaurus roseae were found together, leading to speculation about possible gregarious behavior in this lineage. Other relevant sites include the Cleveland-Lloyd Allosaurus quarry from the Upper Jurassic Morrison Formation, though this has been alternatively interpreted as a drought-induced or predator-trap assemblage rather than evidence of social behavior.

A fundamental challenge in this debate is the distinction between genuinely coordinated cooperative hunting and uncoordinated group feeding. True cooperative (pack) hunting, as observed in wolves and African wild dogs, involves role differentiation (drivers, blockers, ambushers), coordinated spatial positioning, and shared provisioning of kills across the group including dependent young. Mob or communal feeding, as seen in Komodo dragons, crocodilians, and many scavenging species, involves multiple individuals independently converging on a prey item or carcass with dominance-based access and no coordinated roles.

The fossil record inherently struggles to distinguish between these behavioral modes because behavior does not fossilize directly. As Frederickson et al. (2020) noted, "behavior like pack hunting does not fossilize, so we can't directly test whether the animals actually worked together to hunt prey." Instead, paleontologists must rely on indirect proxies: taphonomic associations, trackway evidence, encephalization quotients (brain size relative to body mass), phylogenetic bracketing with extant archosaurs, and geochemical signatures of diet. Each line of evidence has significant limitations and interpretive ambiguities.

The extant phylogenetic bracket for non-avian dinosaurs consists of crocodilians and birds. Among crocodilians, cooperative hunting is virtually absent; feeding aggregations occur but are agonistic and hierarchical. Among birds, cooperative hunting is exceedingly rare. The Harris's hawk is the best-documented case, hunting in family groups of 2–6 individuals in desert environments. Some corvids show pair-based cooperative foraging, and a few raptor species occasionally hunt socially under specific conditions. The rarity of pack hunting in both branches of the extant bracket has led many paleontologists to argue that attributing wolf-like pack hunting to non-avian dinosaurs requires extraordinary evidence that currently does not exist.

As of the most recent literature, the scientific consensus is nuanced. There is growing acceptance that various theropod dinosaurs, including some dromaeosaurids and tyrannosaurids, likely exhibited gregarious behavior—meaning they tolerated conspecifics in proximity and may have traveled, roosted, or fed in loose groups. However, the specific claim of mammal-like coordinated cooperative pack hunting in the style of wolves or African wild dogs is widely considered unsubstantiated for any known non-avian dinosaur. The available evidence is more consistent with Komodo dragon-like mob feeding behavior in dromaeosaurids, and with some degree of social tolerance or loose gregariousness in tyrannosaurids. The question remains an active area of research, with future isotope studies, trackway analyses, brain endocast studies, and newly discovered bonebeds likely to further refine our understanding.

Ostrom (1969) — First proposed pack hunting in Deinonychus based on taphonomic association with Tenontosaurus.

Roach & Brinkman (2007) — Reevaluated the evidence and argued for a Komodo dragon-like asocial model, found evidence of intraspecific killing.

Li et al. (2008) — Reported parallel deinonychosaur trackways from China suggesting gregarious travel.

Currie & Eberth (2010) — Described the Albertosaurus bonebed as evidence for gregarious behavior in tyrannosaurids.

McCrea et al. (2014) — First tyrannosaurid trackways showing parallel movement of three adults.

Frederickson et al. (2020) — Stable isotope analysis showing ontogenetic dietary shift in Deinonychus, inconsistent with pack hunting and more similar to asocial reptilian patterns.

Titus et al. (2021) — Teratophoneus bonebed providing additional evidence for tyrannosaurid gregariousness.