WORKING out how an extinct animal behaved when it was alive is tricky. But it is not always impossible, as Joseph Peterson and Collin Dischler of the University of Wisconsin explained on October 17th to the Society of Vertebrate Palaeontology meeting held in Raleigh, North Carolina. Pachycephalosaurs (literal translation: thick-headed lizards) lived 65m years ago. As their name suggests, they had large, bony, domed heads. For years, it was assumed they used these to bash one another in the way that rams do now. Recently, though, a second suggestion has surfaced - that rather than for fighting, the domes were for show; the dinosaur equivalent of a peacock's tail. Dr Peterson and Dr Dischler put these two theories into a head-to-head competition. To do so, they studied 102 pachycephalosaur skulls, of various species. They found that 23 of these skulls had pits in them, and that these pits were similar both to each other and to those found in the skulls of living animals that bash each other with their heads. That supported the head-butting theory. The clincher, though, came when they created computerised models of pachycephalosaur skulls and mapped the damage from each of their pitted specimens on to these virtual skulls. When they did this, they found that the pits were clustered - exactly as might be expected if they were the result of animals deliberately aiming at each other. Intriguingly, they found two patterns of clustering. Those pachycephalosaurs with highly domed heads had pits on both the fronts and the backs of their skulls. Those whose domes were lower had them only on the fronts. This suggests the two sorts of animal fought in different ways. Low-domes, it seems, simply charged at each other. High-domes did that too. But they also engaged in head-to-head wrestling of a sort that allowed them to get behind their opponents" skulls and do damage from the rear, in the way that modern bison do. None of which proves that pachycephalosaurs did not strut their stuff with their domes as well, just as a stag will often show off its antlers before engaging in combat, in order to give a less well-endowed rival the opportunity to withdraw. But it does demonstrate that the domes were serious weapons, not mere fripperies of fashion.