“Why don't horses have horns?” (You'd be Surprised!)

A Virgin with a Unicorn by Domenichino.

Growing up around horses and having a father that knows a lot about them, you tend to ask every question concerning about their behavior and their biology. My dad can answer any question that is horse-related, but there was one question my dad could not answered me.

“Why don't horses have horns?” I asked him one time. The only response I got from him was a man in deep thought, trying to remember if he ever had read something that could answer my question. But he failed to do so.

During their evolutionary history, horses were a very successful family of ungulates. They have managed to colonize a good portion of the world save for Oceania, the oceans and Antarctica. Yet none of the dozens or so species - living or dead - had developed cranial appendages or hornlike organs on their heads. One would have thought that at least some species might have experimented with it. Some would say horses favor speed over horns in their evolution. While horses and their relatives do rely on their speed and herd strength for their survival, so do a majority of pecorans, the lineage of ungulates were cranial headgear has diversified. So why didn’t they? Why didn't nature selected horses to become real-life unicorns? If you think about it the biological concept of a unicorn - without the magical elements - is a very plausible animal. After all cranial appendages are bifunctional organs that are used for combat and sexual-visual organs of display (Bubenik, 1990; Emlen, 2008).

It was not until a few years ago that I solve the mystery and what I find is surprising. But here is are three points to consider.

It's Very Costly
The cranial appendages are very costly in nature to have. In order for a successful condition of horns or antlers, a multitude of factors come into play: the right resources, the environment being suitable and the health of the animal (Emlen, 2008). If something disastrous were to happen to the animal (such as injury or malnourishment) then there would be a negative affect on the condition of the headgear. Indeed a textbook example would be the White-tailed Deer (Odocoileus virginianus) during hunting season. Hunters have reported seeing individuals with abnormal antler growth or injury, caused by starvation, poor genetics, and injury from other bucks (Rue III, 2000). That is why male pecorans often don’t do as much activity when compared to females. They mostly relax and try to bulk up. That is why osteophagia is common among pecorans (but not in equines), as they are obtaining important calcium for their cranial development (Hudson et. al, 2013; Cáceres et. al, 2013). There even seems to be a correlation with size, with larger pecorans practicing osteophagia more (Hudson et. al, 2013). This leads to the second point.

Hunters are all too familiar with the negative affects of poor health can cause in the develepoment of antlers in deer. Photo by Soos Osborn.

Sexual Selection and Fitness of Males
Sexual selection is a major driving force in the evolution of ungulates. The horn and antler diversity among members of Pecora is no exception to this. A classic example can be seen in Africa’s incredibly diverse antelope fauna. Females selected those males who are able to maintain fitness, maintain their dominance, and fulfill the energy requirement which the cranial appendages demand (Bubenik, 1990; Emlen, 2008). This resulted sexual dimorphism among males and females, often males being bulkier and larger than the females (Bubenik, 1990). Basically the ultimate payoff for such a costly organ is the right to breed and passing on your genes. Similar can be said for rhinos - the evolutionary relative of the horse -, as their horn-shape and bulk often varies among males and females (Bubenik, 1990). It should be noted that some females also have horns as a way to defend themselves and their calves from predators and harassment from males (Geist, 1998).

Sexual selection was the driving force for the impressive variety of horns among bovids. Drawing by Encyclopedia Brittanica.

None of this ever occur in the evolution of horses. For the case of the horse, physical stamina is what matters for them. In contrary to the popular notion of the stallion being the leader of the herd, all species of equines live in matriarchal societies lead by a matriarch. The stallion is best described as the bodyguard and protector of the herd, and his “award” is the right to breed with all the females in the matriarch’s herd. This leads to the third and final point, one I thought that was very interesting.

Biters and Headbutters

While they lack horns, they make up with their teeth. Unknown source.

The Russian ecologist Valerius Geist - known for his work on North American ungulates - had written what I considered to be the best source on cervid deer of the world Deer of the World: Their Evolution, Behaviour, and Ecology. In this book, he has made lots of interesting hypotheses on why large mammals exhibit certain characteristics. This include why pecorans had evolved cranial appendages but not equines. Geist had found an interesting relationship between teeth and headgears. Mammals that have powerful bite forces tend not evolve cranial organs on top of their head. In these mammals, extreme canine teeth (e.g baboons) and tusks (e.g. walruses, elephants) had evolved as combat organs instead. Cranial headgear mammals, like pecorans, on the other hand generally do not have powerful bite forces, especially with the reduction in upper teeth in horned ungulates (Geist, 1998). Instead they using their head instead as a weapon (Geist, 1998).

When looking at the mandibles of equines and comparing them to, say, a bovid skull, equine jawbones are more robust. Indeed stallions, knowing from personal experiences with working with horses, are among the most dangerous animals to work with. You would be lucky if they kick you instead of biting you, because when they bite, they bite. Hard. They can cause a lot damage, including breaking your skin and even crushing bone. As mention earlier in this post, the stallion needs to use every energy in his body to protect his harem of mares and their young and use any means necessary in combat. Often times there is a lot of biting involved. With all of this said, we can finally answer my childhood question and the reason is this - natural and selective pressures favored equines to be bitters still, and that any cranial headgear would be costly or redundant.

Equines fight dirty - lots of kicking and biting involved. Photo by Fred Holley.

To sum up what we have, while horses lack the headgear their distant pecoran cousins have, stallions need the energy saved to protect the matriarch's herd from rogue males and predators. The amount of energy required for this is costly, if not more so than to have horns or antlers (Riechert, 1988). It would have been too much to have these organs for what stallions do. That is why they mostly kick, chase and bite at their predators and rival males instead. Similar can be said for other ungulates such as tapirs, camelids, swine, peccaries, chevrotains, musk deer and hippos. Like equines, they too rely on their teeth and powerful jaws to combat. These three points have finally answered my childhood question that my father could not have answered. Or does it? As Geist points out, there was one family of ungulates that had both used headgear and teeth for combat. 

Rhino-like "Horses"

Life restoration of Megacerops coloradensis. Artwork by Dmitry Bogdanov.

The fossil perissodactyl family Brontotheriidae were the among the first terrestrial mammals to have become truly massive during the Eocene. Indeed their large size is what made them such icons to the public, but what is more is in some of the species there appears to be conspicuous frontonasal horns (Bales, 1996; Mihlbachler, 2008). Unlike the horn of a rhino which is made of keratin the horns of brontotheres were made of solid bone. Of course this was common in the larger and more later species but all brontotheres have powerful jaw bones and large teeth. This has hypothesize that brontotheres were unique in that they were among the few mammals (if not, only) to have evolved both horns and teeth as combat weapons (Geist, 1998). But what makes them even more interesting is not so much of their overall size or the weapons they have - rather it is their evolutionarily relationship with today's equines.

Skull of Megacerops coloradensis. Note the jaw shape, the teeth and horn. Photo by Alan S.

Brontothere systematics is relatively well documented as at least 41 different species have been described (Froehlich, 1999; Mihlbachler, 2008 and Holbrook & Lapergola, 2011). An incredibly diverse family, the earliest brontotheres were small and vaguely pony-like. These early forms lacked horns, although as some brontotheres got larger some species evolved horns (Bale, 1996). While the larger species have a vague resemblance to rhinos these two families were not closely related at all. In fact the placement of brontotheres among the perissodactyls was not fully understood. It was commonly assumed that brontotheres were a basal lineage of perissodactyls that was only distantly related to the other families. Recent discoveries and new systematic work might have found where exactly brontotheres fit on the tree of life. According to these recent work, brontotheres might have been descendants from the same ancestral that also evolved into horses. In other words, Brontotheriidae and Equidae are sister taxa (Holbrook & Lapergola, 2011) and both are descended an ancestor that used teeth and biting for combat. In essence brontotheres were "horses" that had evolved to become bulkier. Larger mammals tend to conserve more energy than smaller mammals (Lundstedt & Burke, 1985), the brontotheres could have afford using the energy required for the growth and development of their horns as with modern horned or antlered ungulates. While brontotheres were evolving, the evolution of the horse and other equines favored physical stamina as their weapon.

With that being said, it depends on how you answer it and the reasoning behind your answer. The family Equidae never evolved horns or antlers as nature did not select them to be such creatures. Yet the family Brontotheriidae certainly did evolved horns. and given the closeness of these two families, one could make an argument that "horses" did - at some point in their evolution - evolved horns. Just that these "horses" looked more like rhinos and less like the mythical unicorns. Yet this might be the closet thing we will ever get in nature selecting "horses" becoming unicorns.

References

• Bales, G. S. (1996). Heterochrony in brontothere horn evolution: allometric interpretations and the effect of life history scaling. Paleobiology, 481-495.
• Bubenik, A. B. (1990). Epigenetical, morphological, physiological, and behavioral aspects of evolution of horns, pronghorns, and antlers. Horns, Pronghorns, and Antlers (pp. 3-113). Springer New York.
• Cáceres, I., Esteban-Nadal, M., Bennàsar, M., Monfort, M. D. M., Pesquero, M. D., & Fernández-Jalvo, Y. (2013). Osteophagia and dental wear in herbivores: actualistic data and archaeological evidence. Journal of Archaeological Science, 40(8), 3105-3116.
• Emlen, D. J. (2008). The evolution of animal weapons. Annual Review of Ecology, Evolution, and Systematics, 387-413.
• Froehlich, D. J. (1999). Phylogenetic systematics of basal perissodactyls. Journal of Vertebrate Paleontology, 19(1), 140-159.
• Geist, V. (1998). Deer of the World: Their Evolution, Behaviour, and Ecology. Stackpole Books, 5-8.
• Holbrook, L. T., & Lapergola, J. (2011). A new genus of perissodactyl (Mammalia) from the Bridgerian of Wyoming, with comments on basal perissodactyl phylogeny. Journal of Vertebrate Paleontology, 31(4), 895-901.
• Hutson, J. M., Burke, C. C., & Haynes, G. (2013). Osteophagia and bone modifications by giraffe and other large ungulates. Journal of Archaeological Science, 40(12), 4139-4149.
• Lindstedt, S. L., & Boyce, M. S. (1985). Seasonality, fasting endurance, and body size in mammals. American Naturalist, 873-878.
• Mihlbachler, M. C. (2008). Species taxonomy, phylogeny, and biogeography of the Brontotheriidae (Mammalia: Perissodactyla). Bulletin of the American Museum of Natural History, 1-475.
• Riechert, S. E. (1988). The energetic costs of fighting. American Zoologist, 28(3), 877-884.
• Rue III, L. L. (2000). Way of the Whitetail. Voyageur Press, 88-93.