Pack Hunting Raptors - What is the evidence?
Velociraptor, Deinonychus, Utahraptor, and all the 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 help in quick turns, and that famous “killing claw”! This large, highly specialized, sharp toe claw was held off the ground so as to always be sharp enough to effectively slash and cut its prey. Ever since Jurassic Park came out 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 amazing 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, a wonderful paleontologist that revolutionized dinosaur research, and a man I had the great pleasure to know. One of my most treasured possessions is a signed monograph of the original Deinonychus publication that he mailed to 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 (and most of that tail!) and roughly 200 lbs. It is unlikely a Deinonychus would be able to, on its own, bring down a Tenontosaurus. So the discovery of partial Deinonychus skeletons at almost 1/3rd of the Tenontosaurus fossil sites suggested to Dr. Ostrom that the 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 then test the hypothesis. Dr. Ostrom proposed the pack hunting idea and Hollywood and paleoartists embraced it wholeheartedly, providing wonderful images and vivid descriptions of how pack hunting raptors worked. However, many scientists began re-examining Dr. Ostrom’s proposal and began 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 for who gets the best parts. Komodo dragons, similarly and to an even greater degree, do the same. 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 begin viciously fighting 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 the Deinonychus skeletons were probably killed by other Deinonychus while fighting over food. The Deinonychus skeletons were of a smaller size, suggesting, like in the wild today, the larger predators didn’t share their food and, instead, at times mortally wounded the smaller, hungry ones.
When looking for behavior in the fossil record trackways provide wonderful information. Measuring the length between strides can determine how fast an animal was moving, looking at the arrangement of the 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 6 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 at the same time. This trackway has been cited as evidence that Deinonychus-like animals moved together in packs. However, Komodo dragons leave similar footprints because when they smell or hear that a Komodo dragon has taken down an animal all of the dragons in that area will head to the kill site. If this were fossilized the tracks would look like the China trackway, numerous individuals heading the same direction, but the reason for it is definitely not because they were hunting together as a pack. I love the image of a pack of Deinonychus working together but I don't take the track evidence as proof that they did, especially after watching Komodo dragons all run towards the prey.
In 2020 a group of paleontologists (Frederickson, et al., 2020) used modern chemistry tools (stable isotope analysis) to determine the Deinonychus diet using their teeth. They tested small and large teeth of Deinonychus, crocodilian teeth from the same environment and time as Deinonychus, and on modern crocodiles. Their results showed that in all three of these predators the smaller teeth and the larger teeth had very different isotope ratios. In modern crocodiles these ratios are different because baby crocs eat very different animals than adult crocodiles. Komodo dragons also show these 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 Deinonychus young ate different types of animals than adults. In modern predators baby and adult teeth share a similar isotope ratio because they are eating the same food, the young are being fed by the old. Since pack-hunting animals do not show such isotope differences in their teeth the study concludes Deinonychus did not live in packs because if they did the juvenile and adult teeth would have a similar isotope ratio.
If one is keeping score the evidence for pack hunting consists of the fact 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 the same direction at likely the same time, suggesting to the paleontologists the raptors were working together.
The evidence against pack-hunting includes the fact modern birds and reptiles don't hunt cooperatively, that Komodo dragons and crocodiles will opportunistically work to kill a larger animal but, once the prey is down, they then fight over who gets to eat it. Many Komodo dragons will rush to a kill site when a large prey is 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 automatically mean the trackmakers were hunting together, especially when one looks at modern Komodo dragons descending upon a kill. They are definitely not working together but a fossil trackway might lead one to think they were. Chemical analysis of modern and fossil teeth reveals that in animals that hunt in packs the young and the old teeth look very similar isotopically while in non-pack hunting animals the teeth of the young have a different chemical composition than those of the adults.
As a huge dinosaur fan I truly love the idea of raptors hunting in packs! As a scientist I don’t see any evidence that makes me think raptors were anything more 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, they weren’t making sure everyone caught a fish, rather they were taking advantage of their numbers to herd the fish into a smaller part of the lake where each bird ate what they could and when full, left the group. No teamwork, no sharing, and in fact 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 believe they were acting like mammalian predators where everyone in the group is given a turn at the table (as scrappy as it may be...).
I have watched many Harris's Hawks over the years and have wondered if they were working together while hunting jackrabbits. While I was writing this blog I learned about studies of Harris's hawks working together to catch prey (Coulson and Coulson 2013 and a number of papers I have thus far only skimmed). The authors indicate the hawks work together but I haven't (yet) found where they shared food mammal-style. One thought that came to my mind was how hawks lack teeth so an isotope study like the kind conducted on Deinonychus couldn't be run!
Thank you kindly for reading!