Antlers and horns are armament and signals

By Jeff Mitton

Antlers or horns adorn many of the species emblematic of the West: bighorn sheep, mountain goats, mule deer, elk, moose and pronghorn.

In general, antlers and horns serve similar functions: defense against predators, defense of mating territories or harems and to signal information about health to potential mates. But beyond these similarities are some interesting differences between horns and antlers.

Beyond the similarities are some interesting differences between horns and antlers.Antlers are paired, branched or forked structures that appear on the skull in spring as cartilage buds covered with hairy skin called velvet. They grow all summer but in the fall the cartilage ossifies or turns to bone. The blood stops flowing to the velvet, causing it to wither, shred and drop from the antlers, exposing dry, dead bone. During winter or early spring the antlers drop from the skull and the whole process begins again.

Horns are unbranched structures with a living bone core covered with keratin, a protein in our hair and fingernails. Horns are not shed, but grow continuously over the years. Damage to a horn is permanent, for horns cannot be repaired or regrown. Species usually have one pair of horns, but some breeds of domesticated sheep, such as Navajo-Churro, have two pairs.

Antlers are grown by male deer and their relatives, including mule deer, white-tailed deer, elk and moose. Adult females of these species do not have antlers.

In eastern Boulder, the ranges of mule deer and white-tailed deer overlap and there you can see that not all antlers are the same. Antlers of mule deer are forked, meaning that at each bifurcation or split the bones are of equal size. White-tailed deer have branched antlers, meaning that each antler has one main beam and several to many smaller tines or branches. The antlers of moose are palmate, meaning they are shaped like an open hand.

Cows, buffalo, sheep and goats have horns. Bighorn males grow large, coiling horns that are used in mating displays, during which two males run at each other and slam their horns together in brain-jarring collisions. The horns of a big ram can be almost a meter long and can weigh more than the rest of the skeleton. Females have much smaller spikes that do not coil. Both genders of buffalo, goats and cows have horns.

The horns of cows and buffalo are essentially smooth, but the horns of bighorn sheep and mountain goats have annual rings that can be used to estimate their age.

In mating systems in which the female chooses a mate after assessing the health and vigor of suitors, it is imperative that she focuses upon one or more characters that cannot be faked — she needs a reliable metric to choose the best genes for her offspring.

Female white-tailed deer choose mates by watching sparring contests among males and by assessing the size and symmetry of antlers. For a male, the annual contest of growing antlers cannot be faked because it takes so much energy to grow all that velvet and bone. A middle-aged healthy male can grow large antlers, but senescent males and males with diseases and/or heavy parasite loads don't have the necessary energy. They show up in mating season with small, asymmetric antlers and frequently fail to attract a mate.

Do pronghorn antelope have antlers or horns?

Remember that antlers are forked or branched, are grown anew each year and the bone dies as the velvet sloughs off. Horns have a core of living bone covered by a sheath of keratin and they continue to grow larger each year. Male pronghorns have branched projections or excrescences with a core of living bone and a sheath of keratin that is shed and then regrown each year. Female pronghorns have smaller excrescences — some are branched but some females seem to have simple spikes.

If we were forced to choose one or the other, it would be best to say that pronghorns have "horns" but pronghorns don't strictly conform to either of the definitions above.

Horns on our local species are paired and bilaterally symmetric, but Jackson's chameleon and the various species of rhinos have horns on the midline of the skull.

Coincident with the appearance of this column, the University of Colorado Museum of Natural History will open a new display of Antlers and Horns in the Bio Lounge.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

February 2015