National Geographic

Butting Heads Over Skull Injuries and Dinosaur Head-Butts

You don’t get to headbutt your way through life without picking up a few scars along the way. If you repeatedly ram your skull against your peers, you’ll pick up injuries, especially on the parts that suffer the most impacts. And when you die, your skeleton will preserve a record of your violent past.

Joseph Peterson from the University of Wisconsin has used this principle to study the lifestyle of a group of dinosaurs called pachycephalosaurs. The name comes from the Greek for “thick-headed lizards” and refers to the group’s most distinctive feature—a thick dome atop their skulls, usually fringed with small spikes. What was it for?

The most popular answer is that these dinosaurs used their skulls as battering rams, charging each other head-on to fight over mates, territories or both. The domes would have protected their brain during such collisions. But other dinosaur specialists, like Mark Goodwin and Jack Horner, have said that the dome was too brittle to be used as a ram. Instead, they think it was a billboard. The domes helped pachycephalosaurs to identify members of their own species; maybe they were even brightly coloured to patterned to attract the opposite sex.

The billboard idea, in turn, has its problems. Critics point out that the domes are much the same across different pachycephalosaur species, and change dramatically in shape as the animals grow up. They’re hardly not the best badges of identity.

Pachycephalosaurus skeletons. Credit: Kabacchi

Pachycephalosaurus skeletons. Credit: Kabacchi

Peterson approached this debate from a new angle. He was inspired by a specimen of Pachycephalosaurus at the Burpee Museum of Natural History in Rockford, Illinois, which has large dents on the top of the dome. Perhaps these were the result of a collision with another individual? “No one paid much attention to them until we started thinking about how common those types of features really were,” says Peterson.

Together with Collin Dischler and Nicholas Longrich, he spent five years studying the domes of 109 pachycephalosaurs from over 14 species, searching for irregularities. He found a lot, including fractures that had since healed, and thick, irregular surfaces that indicated past infection and inflammation. And to his surprise, 22 percent of the domes across 9 species showed signs of damage. “That’s much higher than we expected,” says Peterson.

Skull injuries on pachycephalosaur skull domes. Credit: Peterson et al. 2013. PLOS

Skull injuries on pachycephalosaur skull domes. Credit: Peterson et al. 2013. PLOS

Other palaeontologists had noticed these features before. “A lot of us, me included, had always assumed they were the result of erosion after the death of the animal,” says Andy Farke from the Raymond M. Alf Museum of Palaeontology. He means that the skulls might have been pitted by pebbles in a stream, or damaged by bone-eating worms, for example. “The new study pretty convincingly shows that this isn’t the case—these features are something that happened during the lifetime of the animal.” (Farke himself has used the pattern of bone dents in Triceratops skulls to show that these iconic dinosaurs locked horns in combat.)

If the pachycephalosaur skulls were eroded naturally, they would have holes everywhere. Instead, two-thirds of the injuries are on the frontal bone on the roof of the skull—the area that would suffer the most impacts during head-on collisions. This strongly suggests that the animals were indeed ramming each other. “We can equivocate over how solid the case for head-butting is, but I’m pretty confident that the skulls weren’t just for looks,” says Farke.

Possible pachycephalosaur combat moves. Up Up Down Down Left Right Left Right A B to pull off a finisher.

Possible pachycephalosaur combat moves. Up Up Down Down Left Right Left Right A B to pull off a finisher. Credit: Ryan Steiskal

Many pachycephalosaur species had these injuries, despite existing at different times and having differently shaped domes, which suggests that the entire group was a dynasty of head-butters. And only the fully-domed specimens showed signs of damage. Skulls with flatter domes, which Peterson thinks belonged to females or youngsters, were free of injuries. This implies that, as in modern goats or cattle, only the males charged each other.

In fact, Peterson found more support for the head-butting idea by looking at modern mammals. Goats, which usually hit each other in the sides, tend to have rib and spinal injuries. Bison, which lock heads and push, suffer a mix of head and spinal damage. But sheep, which do ram each other head-on, mainly suffer from head injuries much like those of pachycephalosaurs.

But there’s a slight flaw in this comparison—we don’t have many complete pachycephalosaur skeletons. Their skulls are easily found, but we cannot tell if they also sustained the same sort of rib and spinal injuries as bison or goats. “I’d really, really like to take a look at injuries in the rest of the body for pachycephalosaurs,” says Farke. “I’m not entirely convinced that head-to-head ramming definitely occurred in these animals, and wonder if they might have been primarily flank-butters.”

Horner is even less convinced. Based on his own analysis, he says that the dome’s internal structure and bone tissues were completely different to that of modern head-butting animals. “This suggests that pachycephalosaurs could only head butt once, and that trauma would have likely killed them,” he says.

Peterson acknowledges that the bone in the domes is unlike anything in living animals. “Simply put, the domes are just weird!” he says. However, he points to work from Eric Snively at Ohio University, who scanned several domes and put them through the same virtual crash-tests that engineers use on cars. The result: “these domes could withstand quite a wallop”.

He says that he and Horner have agreed to disagree. “At the end of the day, we will never know what pachycephalosaurs used their bizarre domes for because we obviously cannot directly observe their behaviour,” he says.

If the domes were truly rams, they could have been billboards too. After all, prominent weapons like antlers or horns also make inherently good ads for an individual’s strength and vigour. “I think that the idea of the domes being used as a display structure is very likely,” says Peterson. “A display structure is great for communicating about how tough animals are… but sometimes they have to back that up.”

Reference: Peterson, Dischler & Longrich (2013) Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae). PLoS ONE 8(7): e68620. http://dx.doi.org/10.1371/journal.pone.0068620

 

There are 7 Comments. Add Yours.

  1. Don
    July 19, 2013

    The last paragraph draws the best conclusion, most things domes or ideas can have several purposes or effects. Perhaps the next big evolutionary jump in humans or another species is the ability to inuitively see the world not as one or the other, but with many possible outcomes.

  2. Doug
    July 19, 2013

    Well they must have rammed head on at least some of the time. I remember the paper on the Burpee specimen, how they found the same kind of injury on the skull of a bird who collided with a window. Someone commented about flank butting, but it was pointed out that to produce that kind of injury in a flank collision would have required a ridiculous amount of force that probably wouldn’t have left much of the other guy (the episode of Mythbusters where they smashed a car with a rocket sled comes to mind)

    But who’s to say they had to do either one or the other? Many modern animals don’t just go for the head. Bighorn sheep, the animal pachys are most often compared with, attack head on, butt flanks, and even kick each other in the nuts (believe it or not): http://www.youtube.com/watch?v=zj8istSAMoY&list=PL757E454EE565EC50&index=42 . Of course, we may never be able to properly test this since pachycephalosaurs are so poorly understood. Only a few are known from more than skulls or (more commonly it seems) skull domes.

    I don’t put much stock in species recognition. It works for one or two groups, but not others. Darren Naish has argued that sexual selection, rather than species recognition, best explains the weird structures in dinosaurs. Of course, this fits with the idea that these were the dinosaur equivalent of antlers, both weapons and advertisements.

  3. psweet
    July 20, 2013

    It seems odd that animals that are going to butt heads would have a dome shape — if you put that much force into a glancing blow, you’re probably going to give your own neck a bit of a tweak, I would think.

  4. Jonny O
    July 22, 2013

    Any number of possible uses of those domed heads, including head-butting but not at full force. Pachys could have indulged in some kind of territorial head-butting that fell far short of the force that bighorn sheep employ but was considerably more forceful than modern-day gopher tortoises.

    Their heads may also have been used for some sort of noise-making or mating ritual, with the animals banging their heads against trees and not other pachys.

    Or the domed head may have been some sort of defense mechanism – particularly considering the spikes that many have.

  5. marc verhaegen
    July 23, 2013

    I don’t know what the heavy skulls were for in pachycephalosaurs (eg, Goodwin MB & Horner JR 2004 “Cranial histology of pachycephalosaurs reveals transitory structures inconsistent with head-butting behavior” Paleobiol.30:253-267), but in archaic Homo (Java, Petralona etc.) they were not for intraspecific head-butting or head-banging, but – as in other pachyostotic mammals – simply for ballast during shallow diving: when Pleistocene Homo populations dispersed to different continents & islands, they trekked along the coasts (& inland along rivers) where they no doubt frequently beach-combed but also dived for shell- & crayfish, perhaps seaweeds etc.
    – Verhaegen M & Munro S 2011 “Pachyosteosclerosis suggests archaic Homo frequently collected sessile littoral foods” HOMO J compar hum Biol 62:237-247.
    – Munro S & Verhaegen M 2011 “Pachyosteosclerosis in archaic Homo: heavy skulls for diving, heavy legs for wading?” p.82-105 in Vaneechoutte M, Kuliukas A & Verhaegen M eds “Was Man More Aquatic in the Past? Fifty Years after Alister Hardy” eBook Bentham Sci.Publ.
    For more info on this castal dispersal model, please google
    – Greg Laden verhaegen guest post
    – econiche Homo
    – pachyosteosclerosis
    – Attenborough Rhys Evans
    marc verhaegen

  6. dan balash
    July 23, 2013

    I enjoyed the Nintendo pass code (Up Up Down Down Left Right Left Right A B to pull off a finisher) under the artists drawing of possible combat moves. Clever!

  7. Philip
    September 8, 2013

    I’m not a scientist, just a goat owner. And I have never seen my two male goats butt each other in the side, but they constantly collide in a head butt. For whatever its worth. (article said goats don’t head butt)

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