Horned dinosaurs vs. theropods: how much did horns matter?

The hero of the hour, Triceratops horridus. But how often were those long horns stuck into predatory dinosaurs in defensive action? I feel a long discussion coming on...

A persistent idea around dinosaur biology is that the ceratopsids, or horned dinosaurs, were among the most formidable prey for predatory theropods. Though lacking armour, the large cranial horns and frills of these dinosaurs have been widely interpreted as having anti-predator potential, functioning like a knight’s lance and shield in their capacity to stab and parry attacking carnivores. Such notions are well over a century old and have taken on a life of their own in palaeontological media, especially thanks to widespread romanticising of the relationship between Triceratops and Tyrannosaurus, genera that we’ve decided represent the ultimate expression of dinosaurian predator and prey. Henry Fairfield Osborn wrote on such matters as early as 1917:

“The first of these [dinosaurs with anti-predator anatomy] are the aggressively and defensively horned Ceratopsia, in which two or three front horns evolved step by step, with a great bony frill protecting the neck. This evolution took place stage by stage with the evolution of the predatory mechanism of the carnivorous dinosaurs, so that the climax of ceratopsian defense (Triceratops) was reached simultaneously with the climax of Tyrannosaurus offense. This is an example of the counteracting evolution of offensive and defensive adaptations, analogous to that which we observe today in the evolution of the lions, tigers, and leopards, which counteracts with that of the horned cattle and antelopes of Africa, and again in the evolution of the wolves simultaneously with the horned bison and deer in the northern hemisphere.”

Osborn 1917, p. 224-225.

At this early stage in dinosaur research, the likes of Triceratops and Tyrannosaurus weren't viewed as laudable champions of evolution, but as animals so stupid and instinct-driven that their predatory and anti-predatory strategies had to be as simple and idiot-proof as possible. Tempting as it is to reduce this passage from William Matthew’s 1915 American Museum of Natural History book Dinosaurs to some choice soundbites, it’s such an amazing window into old-school concepts of dinosaurian stupidity that I present it here in its full glory.

“[Tyrannosaurus] probably reached the maximum of size and of development of teeth and claws of which its type of animal mechanism was capable. Its bulk precluded quickness and agility. It must have been designed to attack and prey upon the ponderous and slow moving Horned and Armored Dinosaurs with which its remains are found, and whose massive cuirass and weapons of defense are well matched with its teeth and claws. The momentum of its huge body involved a seemingly slow and lumbering action, an inertia of its movements, difficult to start and difficult to shift or to stop. Such movements are widely different from the agile swiftness which we naturally associate with a beast of prey. But an animal which exceeds an average elephant in bulk, no matter what its habits, is compelled by the laws of mechanics to the ponderous movements appropriate to its gigantic size. These movements, directed and controlled by a reptilian brain, must needs be largely automatic and instinctive. We cannot doubt indeed that the Carnivorous Dinosaurs developed, along with their elaborately perfected mechanism for attack, an equally elaborate series of instincts guiding their action to effective purpose; and a complex series of automatic responses to the stimulus afforded by the sight and action of their prey might very well mimic intelligent pursuit and attack, always with certain limits set by the inflexible character of such automatic adjustments. But no animal as large as Tyrannosaurus could leap or spring upon another, and its slow stride quickening into a swift resistless rush, might well end in unavoidable impalement upon the great horns of Triceratops, futile weapons against a small and active enemy, but designed no doubt to meet just such attacks as these. A true picture of these combats of titans of the ancient world we cannot draw; perhaps we will never be able to reconstruct it. But the above considerations may serve to show how widely it would differ from the pictures based upon any modern analogies.”

Matthew 1915, p. 52-53.

The image that launched a thousand Cretaceous daydreams... but not the version you know. This is the rarely-seen, 96 cm wide (presumably preparatory) version of Charles Knight's classic 1928 Triceratops vs. Tyrannosaurus mural, held today at Princeton University Art Museum. Knight's near-blindness meant that he could only execute the mural at scale; other artists then painted the better-known, full-size version. I don't need to explain why this image is included in this post.

Over the last century, our views on dinosaur physiology and intelligence have (perhaps thankfully) changed, but the concept of horned dinosaurs protecting themselves with their facial ornaments has not. Some authors (e.g. Colbert 1948) have seen anti-predator functions as the primary role of ceratopsid horns, and even those who view these structures as evolving under different selection regimes (e.g. Hone et al. 2011) assume some predator defence was possible. Seminal figures like Robert Bakker have worked sweeping hypotheses from concepts of long-standing predator-prey interactions between dinosaur species, rephrasing Osborn’s “counteracting evolution of offensive and defensive adaptations” into the catchier “Mesozoic arms race” (Bakker 1986). Of course, Triceratops and Tyrannosaurus are considered the final, ultimate example of this era-spanning feud. On their relationship, Bakker wrote:

“No matchup between predator and prey has ever been more dramatic. It’s somehow fitting that these two massive antagonists lived out their co-evolutionary belligerence through the very last days of the very last epoch in the Age of Dinosaurs.”

Bakker 1986, p. 240.

Gregory S. Paul, also a fan of the idea that Triceratops was the apex challenger to theropod aggressors (Paul 1988), has further worked the concept of ceratopsid predatory combat into other hypotheses. Specifically, in 2008 he proposed that such dangerous prey items were a factor in the short (c. 30 year) lifespans of tyrannosaurids, while also echoing Bakker’s concept of an “arms race” between these clades (“the upgrading the weaponry in tyrannosaurids and ceratopsids… may represent a Red Queen arms race”; Paul 2008, p. 344.).

Today, a large body of evidence has challenged the idea that ceratopsid evolution was driven by developments in predatory dinosaurs. Instead, it points to ceratopsid skulls being primarily shaped by their own intraspecific behaviour. First proposed in the 1970s, this concept arose after researchers noted the many similarities between the sexually-selected horns of living animals and the ornaments of horned dinosaurs. These shared features include their exaggerated and often complex shapes, their functional peculiarity (i.e. that many seem maladapted for other activities, including predator defence), their positive allometry (that they grow faster than the rest of the skull), their high amount of intraspecific variation, and their high morphological diversity between species (e.g. Farlow and Dodson 1975; Spassov 1979; Sampson et al. 1997; Horner and Goodwin 2006, 2008; Hone et al. 2011, 2016; Knell et al. 2012). Evidence that horned dinosaurs injured each other in ways consistent with modelled ritualised combat (e.g. Farke 2004; Farke et al. 2009; D’Anastasio et al. 2022) supports the hypothesis that their horns were employed against one another, not necessarily other dinosaurs, and, along with their fossilisation in huge monospecific bonebeds, we can readily rationalise horned dinosaurs as boisterous animals with somewhat bovid-like behaviours.

Centrosaurus aperatus, a ceratopsid that's so familiar nowadays as to seem unremarkable. But look at that face anew, dear reader: what a crazy animal. Skulls like those of ceratopsids are ripe contenders for dinosaur anatomy shaped by sexual, or at least intraspecific, selection pressures.

The concept of ceratopsid horns serving primarily as anti-predator devices idea has not, however, entirely been set aside despite these data. Nowhere is this more obvious than in popular culture, where horned dinosaurs frequently employ their ornament in life-or-death struggles against predators, and we routinely discuss “armed” dinosaurs as being more dangerous prey than their "unarmed" relatives. But are these action-packed scenarios really a defensible alternative to their horns being used in intraspecific display and aggression? How realistic, really, are these predatory scenarios? Rather than taking the tried and tested route to address this by looking into ceratopsid skull form and function, we’re going to look at the use and evolution of horn-like structures (horns, antlers, ossicones etc.) in living vertebrates. Our view on extinct animal behaviour, after all, is seen through the lens offered by living species and it’s generally our assessment of modern taxa that dictates behavioural models for extinct animals, not the other way around. The anti-predator behaviour of living animals, even just those with horns, is a huge topic that is far too broad and multi-faceted to cover in detail here — especially as I need this to be a relatively short article* — but even in this brief visit, I hope we can hit a few key points that may give food for thought on horn function in Mesozoic animals.

*Yeah, nice try, Past-Mark.

An obvious place to begin is with well-known examples of horn-like structures being used as predator deterrents. It’s absolutely true that some species, like muskox, African buffalo, various rhinos and red deer use their cranial ornament aggressively against predators (Geist 1966, 1999; Schaller 1972; Kruuk 1972) and it is assumed that predator deterrence may explain the presence of horns in a great number of bovids (e.g. Packer 1983; Bro-Jørgensen 2007; Stankowich and Caro 2009; Metz et al. 2018). However, the idea of widespread horn use against predators has been challenged because field observations show such behaviour is rare among many species, and often of limited effectiveness (Estes 1991; Roberts 1996; Gerstenhaber and Knapp 2022). Some groups, like antelopes, are rarely or never witnessed using their sometimes enormous horns in defence against attacking carnivores, even when faced with certain death (Schaller 1972). Most deer seem to behave in a similar fashion, preferring to run or hide from predators despite their capacity to gore and kill conspecifics with their antlers. Indeed, there are indications that antlers may have a deleterious effect on prey species, with Geist (1966) reporting that antlerless moose are more capable opponents against wolves than their "armed" relatives. This may not be the case for all deer, however, with American elk proving more vulnerable to predators once their antlers are cast (Metz et al. 2018). But sometimes losing cranial weaponry makes no difference to predator vulnerability at all, as is the case for black rhinoceros. The necessary act of dehorning these animals to deter poachers shows that both adult and calf survivability are little affected by the removal of their horns (Chimes et al. 2022), suggesting that these structures have a non-essential role in thwarting predatory efforts.

When discussing anti-predation strategies involving horns, the African buffalo Syncerus caffer is one of the go-to species. And yet, these large, formidably armed animals are some of the preferred prey of lions, and are sometimes subdued by single individuals. Photo from Wikimedia, by Diamond Glacier Adventures, CC-BY 2.0.

Perhaps against expectation, not all animals with horn-like structures employ them in defence. Moose generally kick attackers, a behaviour they share with giraffes, who also lash out at predators with their powerful legs rather than bludgeoning them with their armoured, tri-horned heads (Gesit 1966, 1999). Kicking strategies are, of course, also available to species that we might mistake for being “defenceless” from their lack of horn-like anatomy. In some cases, these animals can be far more aggressive than their better-armed contemporaries. Horses, especially zebras, exhibit pronounced anti-predator aggression where they bite and kick attacking cats and hyenas (Kruuk 1972), and with such force that they may explain sightings of lions with shattered jaws (Schaller 1972). Zebras are also recorded as charging towards predators in a fashion that neutralises predatory effort. I like ecologist George B. Schaller’s account of this behaviour where he describes lions simply watching zebras running at them, as any effort to grab them would be “like jumping on a fast-moving train from a standstill” (Schaller 1972, p. 265).

We can augment our discussion further by switching our focus from prey species to their predators. If cranial weapons are effective predator deterrents, we might expect predators to avoid species with horn-like structures or, at least, those more likely to wield them aggressively. And yet, prey preferences seem largely determined by the energy investment demanded in animal capture (Schaller 1972) rather than the presence or absence of cranial armaments. Pouring cold water on romantic notions of life-and-death battles of horns and hooves vs. claws and teeth, field ecology suggests that large predators preferentially target species that are abundant, live in dense populations, and are of a size that provides a suitable reward against the effort of capture. Potential prey species are more likely to be ignored because they are too small, and thus do not provide enough nutrients for the predatory effort, or are too big, and will thus require an unreasonable degree of energetic investment to bring down. This is not to imply that the threat of injury isn’t factored into these behavioural calculations, but we just don’t routinely see predators avoiding horned, aggressive prey in modern ecosystems. On the contrary, both spotted hyenas and lions routinely attack African buffalo (lions especially), a species which is very well known for its horn-led predator defence (Kruuk 1972; Schaller 1972).

Clearly, the idea that horn-like structures serve as anti-predator devices is complicated by a lot of conflicting data. While no one doubts that these anatomies are sometimes used to deter predators, zoologists are engaged in a long-running scientific conversation about the extent and significance of their anti-predator role (e.g. Geist 1966; Estes 1991; Roberts 1996; Caro et al. 2003; Gerstenhaber and Knapp 2022). One especially important issue, which has implications for our discussion of dinosaurs, is whether the horn-like structures of female mammals exist primarily to deter predators (Bro-Jørgensen 2007; Stankowich and Caro 2009). While the formidable cranial weaponry of male mammals is often readily explainable through sexual selection (on which, see below), the function of the same anatomies in females is harder to fathom. Some argue that predator defence is their main purpose, which would imply a much wider role for this behaviour than has been documented in field studies. But others point to social selection as their principal adaptive driver (e.g. to dispute territories or mimic males) or simply regard them as non-functional, suggesting they only exist at all because of a genetic link to male horns. The latter must be the case for the females of some species, such as giraffes, which reportedly never seem to do much of anything aggressive with their heads.

I really like this image from Nikolay Spassov's (1979) paper on horned dinosaur evolution for its novel depiction of horned dinosaur combat. I especially like the interlocking of the frill spikes as we (or, at least, I) tend to forget about them possibly playing a role in physical competition. The horn-like structures of male mammals are shaped to match certain styles of intraspecific combat, and it's possible that ceratopsids were driven by similar evolutionary forces.

Thankfully, we can push most of these conflicting ideas and caveats aside to discuss the horn-like structures of male mammals. It is beyond doubt that intraspecific interactions have a far greater role in shaping these anatomies than interspecific ones, with the cranial ornaments of male giraffids, bovids, cervids and other taxa strongly influenced by sexual selection (Geist 1966; Bro-Jørgensen 2007; Knell et al. 2012). Their cranial structures are so strongly moulded by intraspecific adaptive pressures that they adopt sizes, shapes, textures and orientations that exclude them from effective predator defence (Estes 1991; Roberts 1996), instead becoming better suited to absorbing, catching and parrying the blows of rivals during physical intraspecific contests (Geist 1966; Packer 1983; Bro-Jørgensen 2007). They do not evolve these morphologies randomly, either, but change in response to specific fighting strategies and environmental circumstances. Horned female bovids may also engage in fights with other individuals (both male and female) of their species for resources, but their lessened behavioural emphasis on these bouts means their horns remain less developed than those of males — the significance of this is yet another area of discussion among zoologists.

So, having just thoroughly complicated this seemingly simple topic to a great extent, let’s bring this discussion back to dinosaurs and the assumption that horned dinosaurs wielded their horns like swords against dragon-esque theropods. While models of ceratopsids defending themselves with their horns are undoubtedly validated by the behaviour of some living species, a case can be made that we’ve overstated the importance of horns in predator defence among living animals and, by extension, dinosaurs. The message from the modern day is that horn-like structures can and might be used against predators, but that this behaviour is by no means ubiquitous. It may not even be that common, according to some researchers. It seems that intraspecific selection is more than sufficient to explain most horn-like structures among living species and that predatory influences, if present at all, are relatively minor for most species. We can’t know how much of this insight can be transferred over to dinosaurs, but if ceratopsid facial anatomy was being shaped by intraspecific factors (and we think it was; see above), then we have to entertain all that this brings. This means, in addition to the traditional view of horned dinosaurs being effective foils of predatory theropods, we have to consider some other possibilities suggested by their modern analogues. These could include, for instance, that only some horned dinosaurs actively fought predators; that their retaliations against attacks may have been ineffectual; and that some species may have rarely, and maybe never, turned their horns against other species. And this door swings another way: we have sufficient data from living animals to stop thinking that horned or spiked dinosaurs were the most formidable prey species and that “defenceless” dinosaurs like hadrosaurs and sauropods would be pushovers for their lack of obvious weaponry. Determining which fossil animals are “the most dangerous” from their raw anatomy overlooks the huge impact of non-fossilisable factors that contribute to anti-predator responses, such as temperament, prey awareness, physiology, intelligence, behavioural plasticity and so on. It’s a disappointing limitation of the fossil record that we can investigate what dinosaurs and other extinct animals were capable of, but we’ll never know what they were truly like. Questions about "the most dangerous dinosaur" and similar fall into that void.

Megasuperhypertheropod Tyrannosaurus encounters the unarmed sauropod Alamosaurus. "They said it was defenceless! Defenceless!"

So, in sum, the take-home here isn’t that anti-predator roles for ceratopsid horns are a non-starter, but that the behaviours of living animals complicate this seemingly simple hypothesis. If intraspecific evolutionary pressures on horns and related structures operate mostly independently of predatory pressures today, that has to be our model for Deep Time as well. This opinion comes loaded with caveats, of course, the biggest one being that we’re in a shifting landscape as goes determining the exact roles of horn-like structures in living species; as this changes, so might our ideas on extinct animals. And there’s a lot more we could discuss, too. There’s the aforementioned data about ceratopsid cranial functionality, there’s that healed, T. rex-bitten Triceratops horn described by Happ (2008) that is taken by some as evidence of defensive horn use (I’m not sure I agree; there’s no way of knowing the exact circumstances under which that horn was bitten), there’s the bigger picture of armed dinosaur co-evolution with different theropod clades… but we have to end here. I’ll conclude by borrowing a line from Farlow and Dodson (1975) who succinctly put the ceratopsid anti-predator hypothesis where it should be almost fifty years ago with the mere use of italics: “the evolution of ceratopsian cranial morphology probably reflects diversification through species-specific compromises among various selective pressures… and possibly predator resistance”.

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