Deer don't have horns just like cows don't have antlers.
A common mistake people make when talking about deer is saying that they have horns. The words "horn" and "antler" are used practically interchangeably for many species that sport such decorative head gear. But there is a very distinct difference between horns and antlers, and knowing the difference will not only make you more respected among biologists but will also give you some cool facts to throw into the conversation at parties. Well, maybe not the latter. Unless it's a party of biologists. Anyway, here's the difference.
Antlers are grown by males of the Cervidae family, which includes all species of deer, moose, and elk. They appear only on males, with the exception of caribou, and that's because they're used by males to compete with other males for mating rights with females. They are grown every spring and shed every winter. That's why a male sporting a big rack is really impressive: it takes a lot of energy to grow antlers, so being able to invest energy in large antlers while still staying healthy enough to compete shows females that this is a male with great genes.
Cervids have pedicels, bony structures that support the antlers as they grow. In the spring, testicular and pituitary hormones get the growing process started. Antlers are covered with velvet (such as in the antlers of the deer in the photo above) which carries blood and nutrients to the antlers during development. Animal Diversity Web explains, "As antlers near the end of the growing process, spongy bone in their outer edges is replaced by compact bone, while their centers become filled with coarse, spongy, lamellar bone and marrow spaces." When the antlers are done growing, the velvet dies and is shed as the animal scrapes its antlers on brush and trees -- an action which also stains, shines and sharpens the rack, and which looks really impressive to competing males and to nearby females. In winter, when the growth hormones quit pumping, the pedicel loses calcium which weakens the connection between the pedicel and the antler, and off come the antlers.
Horns, on the other hand, are found on members of the Bovidae family, which includes species as diverse as cows, sheep and goats to water buffalo, antelopes, and gazelles. Horns can appear on both males and females depending on the species, and the size and shape of horns vary wildly from species to species. Unlike antlers, horns are never branched, are never shed, and in many species horns never stop growing throughout an animal's life. A horned animal will always have its horns, unless of course they are broken off, and the horns will constantly grow which is important as they are worn down through use.
Horns have a bony core which is covered with a sheath of keratin, the same stuff that makes up our hair and fingernails. The bony core of a horn is not part of the skull but rather is fused to the skull with connective tissue. Similar to how cervids use their antlers, bovid males use their horns in fights and displays of strength during breeding season. In species where females also sport horns, they're usually smaller and built more as a defensive tool than an offensive weapon.
So, now you know the difference between antlers and horns. Antlers are found on cervids, are made of bone, are typically branched, and are shed every year. Horns are found on bovids, are made of a bony core with a keratin sheath, are not branched and are a permanent part of the animal. You're now ready to go corner someone at a party and share your newfound knowledge!