Is it a Horn or an Antler?

One problem that a lot of people have is determining whether a mammal species has horns or antlers. Though most of the time there is a cursory knowledge on the subject, few can actually identify what specifically makes them different. Many ungulates (hoofed mammals) have one of these two types of cranial projections, and each type is really quite different and easy to identify. Let's begin:

What is an antler?

Three red deer in velvet.

A fallow deer shedding velvet

An antler is a branching structure that grows from a bone base known as the pedicle. The antler itself is bony, and its growth is made possible by the highly vascularized tissue on the exterior of the antler known as velvet. During mating season, the velvet will slough off. The male will work at removing all of the velvet by rubbing his antlers against trees, foliage, or whatever else might be handy. After the velvet is shed, the antlers will have a characteristic appearance like that of polished wood. This is when males become combative, displaying and clashing antlers to try and get the chance to mate with as many females as possible. Depending on the species and its range, the antlers will then be shed before the onset of winter or retained and shed at a less predictable time. However, in almost all cases antlers are shed annually.

When an antler is out of velvet, it would be possible for a portion to break off without causing harm to the animal. This is due to the fact that the velvet is where the nerves and blood supply are. However if a breakage occurs that takes part of the pedicle with it, this could result in permanent malformation of antlers that grow from that side in future seasons. It's also possible for the damage to be so bad as to no longer allow antler growth. This would be possible if the entire pedicle were torn off.

A sambar deer out of velvet

For those who prize antlers either on their own or as part of a taxidermied mount, size matters. The larger the antlers, the better and, in cases such as the whitetail deer, the more "points" or projections, the better. This is due to the fact that males with large, heavily branched antlers are the oldest and thus had to be strong, tough, and sometimes elusive to survive year after year. Growing antlers takes a lot of energy, and as such antler size will be greatly decreased in years where little food is available. Abnormalities in antler shape can be caused by a number of things, including injury to the pedicle, injury to velvet during growth, or poor diet. The largest antlers ever known were found on the now-extinct Irish elk, with antlers up to fourteen feet from tip to tip.

A caribou stag with large antlers

Antlers are only found in Family Cervidae, which is also commonly called the deer family. This includes not only your deer, but your moose, elk or wapiti, caribou, and other related species. In the vast majority of species with antlers, only males will  have antlers. The exception is the caribou, where both sexes have them. Those on females are significantly smaller and also retained after males shed theirs.

So, if the structure is branching and if it is shed at any point in time, it is most likely an antler. Though this is a good rule of thumb, there are antlers that are not branched, such as those seen in the pudú.

What is a horn?

An oryx, a species known in part for its long, straight horns

A hartebeest with his curved horns

A horn is significantly different in structure from an antler. A horn has a bone core that is covered in a sheath of keratin. They grow continuously and neither part is ever shed. They are also never branching, but can be curved in a wide variety of ways. Since the vascularization of a horn is internal in the bone core, it is extremely painful for an animal to break a horn. Such breakages can be very bloody and result in permanent damage, unlike in an antler where the break is only present until the antler is shed (unless the pedicle is damaged). If an ossicone--the tissue that is present before horn growth begins--is damaged, then this can result in abnormal horn growth or even a complete lack of a horn on that side. During my own experiences studying captive hartebeest, there were two individuals who were missing horns for this exact reason. One was missing only one horn, but the other was missing both.

A nyala with his spiraling horns

Also unlike antlers, it's quite common for both males and females of a species to have horns. In the species that do, females will usually have smaller horns. The difference in size may be in length or diameter, and sometimes may even be in shape. In these cases, the smaller, straighter horns of the female are great defensive stabbing weapons. Generally the species with horns on both sexes are large-bodied, with the smaller species more likely to have hornless females. Males still often compete with their horns, frequently butting heads in battles to try and find out who is the strongest and toughest. The winner of these scuffles are usually the ones that end up mating with the most females.

An ankole-watusi bull

The largest horns currently known are found on domestic cattle. While the longest horns on record are found on a Texas longhorn bull, the largest horns in circumference are on a lesser known breed known as the ankole-watusi. These cattle have extreme large horns that are actually helpful in keeping them cool in a similar way to the ears of an elephant. The blood vessels in the horns radiate heat, working to keep the animal's core body temperature down.

Like antlers, those who collect horns also usually want the biggest ones they can get. Again, this is due to the fact that the animals with the largest horns are the oldest. Depending on the animal's normal horn shape, elaborate spiraling or a shape that is unexpected may be favored.

Horns are only found in Family Bovidae, which includes such species as antelope, cattle, sheep, gazelle, goats, bison, duikers, and numerous other groups.

But wait, I've heard about other animals with horns!
It's true that there are a few more species out there that have what are known as horns. However, the structure of these horns are quite different from those I described above. There are three of these groups:

Giraffes (and okapi)

Giraffes have a different sort of horn

The horns on a giraffe are unusual in that they are covered in hair. They do share the boney core of true horns, but the skin and hair covering is truly unique. Other strange features us the placement of the horns over one of the sutures on the skull and the presence of the horns that are fused to the skull from birth. In species with true horns, the horns don't fuse to the skull until later in life.

The horns of the opaki are very much like those of the giraffe, though there are some small differences.

Rhinoceros

This black rhino also has a different sort of horn

As many of you may already know, the horn of a rhino is very different from what is more commonly called a horn. Their horn has no boney core and actually is more like a thick mass of hair than anything else.
 
The rhinos are also the only species I have spoken of that are outside of the Order Artiodactyla: the even-toed ungulates. They are actually in the Order Perissodactyla, the odd-toed ungulates, which is a significantly smaller group that also includes the horses, zebras, and tapirs.

Pronghorn

Pronghorn antelope are quite strange

Perhaps the most unusual are the horns seen on a pronghorn. While they are sometimes considered true horns, they have many characteristics that are very unusual for horns and are more like antlers. First is the trait that gives this species its common name: the branched or "pronged" horn. There is a small projection that comes off of the main horn. Also, while true horns have a permanent outer sheath, the keratin sheath on the pronghorn is actually shed annually! So, the pronghorn has what could be called a combination of a horn and an antler.

In addition, though unusual, you can also sometimes find animals with a number of horns or antlers other than two. One, three, or even four are sometimes seen, along with hornless individuals of a normally horned species. These differences appear to be more common in traditionally horned animals than those with antlers or the other types of horns I just mentioned. Though most often this is a strange exception, there is at least one species that regularly comes with extra horns: the four-horned antelope.

Sources are Animal Diversity Web, Mother Earth News, University of Missouri, PubMed, and Oklahoma State University. Images are from Wikimedia Commons under Creative Commons licenses or are copyright free: one, two, three, four, five, six, seven, eight, nine, ten. The hartebeest image is mine.