What is an antler?
|Three red deer in velvet.|
|A fallow deer shedding velvet|
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|
|A caribou stag with large antlers|
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 nyala with his spiraling horns|
|An ankole-watusi bull|
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 of the opaki are very much like those of the giraffe, though there are some small differences.
|This black rhino also has a different sort of horn|
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 antelope are quite strange|
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.