Pack Hunting Raptors - What is the evidence?

Velociraptor, Deinonychus, Utahraptor, and many other dinosaurs colloquially known as “raptors,” are bipedal (two-legged) carnivores (meat-eaters) possessing sharp teeth and claws, legs that let them run fast, a stiff, straight tail that helps in quick turns, and wield that famous “killing claw”!  This large, highly specialized toe claw remains perpetually sharp as tis always held off the ground.  Ever since Jurassic Park, raptors have been portrayed as pack hunters, small(ish), fast-moving, intelligent predators working together to overwhelm animals much larger than their individual selves.  Though exceptional to watch on the big screen, see in artwork, and read in wonderfully written stories, where did the pack idea come from, and what scientific evidence exists for such behavior?

The argument for raptor pack-hunting behavior was introduced by Dr. John Ostrom, an outstanding paleontologist who revolutionized dinosaur research and a man I had the great pleasure of knowing.  One of my most treasured possessions is a signed monograph of the original Deinonychus publication, which he mailed me after we first met at Yale University.  Dr. Ostrom found four Deinonychus skeletons in Montana, scattered amidst a Tenontosaurus he was excavating.  A Tenontosaurus is a large (20’+, over 2,000 lbs) plant-eating dinosaur.  In contrast, a big Deinonychus was 11’ long (most of that tail) and roughly 200 lbs.  It is unlikely a Deinonychus would be able to, on its own, bring down a Tenontosaurus.  The discovery of partial Deinonychus skeletons at almost 1/3rd of sites containing Tenontosaurus led Dr. Ostrom to conclude that Deinonychus were like wolves, working together to take down much larger prey and often being mortally injured in the process.  From this idea, the Jurassic Park author breathed vivid life into such a scenario, and the movie has us all rooting for pack-hunting raptors by the end!  

Science is great because a person can propose an idea, called a hypothesis, and others can test it.  Dr. Ostrom proposed the pack-hunting idea, and Hollywood and paleoartists embraced it wholeheartedly, providing fantastic images and vivid descriptions of how pack-hunting raptors worked.  However, many scientists began re-examining Dr. Ostrom’s proposal, using numerous lines of evidence to test the belief that raptors hunted in packs.  Let’s take a look at these findings.

Occasionally, crocodiles are seen attacking the same wildebeest, but they aren’t communicating with one another and coordinating their attacks like wolves, lions, African dogs, chimpanzees, and many other mammals that truly work together do.  With the crocodiles, one grabs a leg, the other grabs, say, the head, and they pull.  Once the large animal is brought down, there is massive conflict amongst the crocodiles over who gets the best parts.  Komodo dragons do the same, to an even greater degree.  On the surface, they appear to be working together, but in reality, they aren’t communicating; they are opportunistically attacking the same animal.  Once the animal is down, they viciously fight amongst themselves over the spoils.  Crocodiles and Komodo dragons are sometimes killed during these fights.  Relatively recent interpretations of the Deinonychus skeletons being found amidst Tenontosaurus skeletons lead some paleontologists (Roach and Brinkman 2007) to conclude that the Deinonychus skeletons were probably killed by other Deinonychus while fighting over food.  The Deinonychus skeletons were smaller, suggesting, like in the wild today, the larger predators didn’t share their food and, at times, mortally wounded the smaller, hungry ones.  

When looking for behavior in the fossil record, trackways provide wonderful information.  For example, measuring the length between strides can determine how fast an animal was moving. The arrangement of tracks can demonstrate that some dinosaurs stayed with their young (Lockley and Christian, 1994).  One trackway, found in China and belonging to cousins of Deinonychus, showed six individuals moving in the same direction (Li et al. 2007).  The geology suggested that the tracks were laid down very close in time to one another, potentially even simultaneously.  This trackway has been cited as evidence that Deinonychus-like animals moved together in packs.  However, Komodo dragons leave similar trails because when they smell or hear that a Komodo dragon has taken down an animal, all the dragons in the area head to the kill site.  If this were fossilized, the tracks would look like the China trackway, with numerous individuals heading in the same direction, but that is definitely not because they were pack hunting.  I love the image of a group of Deinonychus working together, but I don't take the track evidence as proof that they did, especially after watching Komodo dragons all run toward prey.

In 2020, paleontologists (Frederickson et al., 2020) used modern chemical tools (stable isotope analysis) to determine Deinonychus's diet from teeth.  They tested small and large teeth of Deinonychus, crocodilian teeth from the same environment and time as Deinonychus, and modern crocodiles.  Their results showed that in all three of these predators, the smaller and larger teeth had very different isotope ratios.  These ratios differ in modern crocodiles because baby crocs eat very different prey than adults.  Komodo dragons also show such isotope differences.  In Komodo dragons, the baby Komodos spend their early lives in trees, as they are considered food to adult Komodos!  The study suggests that young animals ate different types of animals than adults did.  In modern cooperative predators, baby and adult teeth share a similar isotope ratio because they eat the same food; the young are fed by the old.  Since pack-hunting animals do not show such isotope differences in their teeth, the study concludes that Deinonychus did not live in packs because, if they did, the juvenile and adult teeth would have similar isotope ratios.

If one is keeping score, the evidence for pack hunting consists of the fact that Deinonychus skeletons are often found with large Tenontosaurus skeletons, and since a single Deinonychus likely couldn’t take down an adult Tenontosaurus alone, Ostrom proposed they hunted in packs.  A trackway of six individual raptors walked in the same direction at likely the same time, suggesting to the paleontologists that the raptors were working together. 

The evidence against pack-hunting includes the fact that modern birds and reptiles don't hunt cooperatively, and that Komodo dragons and crocodiles will opportunistically work together to kill a larger animal, but once the prey is down, they fight over who gets to eat it.  Many Komodo dragons rush to a site where a large prey has been, or is being, taken down, resulting in injured or killed dragons in the ensuing squabble over who gets to eat.  Tracks can be difficult to interpret. For example, animal tracks moving in the same direction do not automatically indicate that the trackmakers were hunting together, especially when one considers modern Komodo dragons descending upon a kill.  Komodos are definitely not working together, but a fossil trackway of their behavior might lead one to think they were.  Chemical analysis of modern and fossil teeth reveals that, in animals that hunt in packs, young and old teeth are isotopically very similar. In contrast, in non-pack-hunting animals, the young's teeth have a different chemical composition than the adults'.

As a huge dinosaur fan, I genuinely love the idea of raptors hunting in packs.  As a scientist, I don’t see any evidence that suggests "raptors" (technically dromaeosaurs) were anything other than solo hunters, acting more like modern Komodo dragons than Jurassic Park raptors. 

I have personally observed cormorants seemingly working together to eat fish.  The cormorants were not working together in the mammalian sense, and they weren’t making sure everyone caught a fish; they were taking advantage of their numbers to herd the fish into a smaller part of the lake, where each bird could more easily catch fish. A "feeding frenzy" ensued, and when each individual was full, it left the group.  No teamwork, no sharing, and sometimes two or three chased the same fish, which usually resulted in the fish escaping.  Could dinosaurian raptors work in such a fashion?  I believe they certainly could have, given that birds descended from dinosaurs, but even if so, I don't think they were acting like mammalian predators, where everyone in the group gets a turn at the table (as scrappy as it may be...).

I have watched many Harris's Hawks over the years and wondered if they worked together while hunting jackrabbits.  While writing this blog, I learned about studies of Harris's hawks working together to catch prey (Coulson and Coulson 2013). The authors indicate the hawks work together, but I haven't (yet) found where they shared food mammal-style.  One thought that came to mind was that hawks lack teeth, so an isotope study like the one conducted on Deinonychus couldn't be run.

Thank you kindly for reading!  

BC