Episode 140 – Horns and Antlers

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Animals have a consistent habit of developing ornamentation on their heads for a variety of functions. In this episode, we discuss the diversity, terminology, and behavior associated with Horns and Antlers.

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A Whole Lot of Headgear

It’s very common for animals to have showy ornamentation on their skulls. Among mammals, the most diverse headgear is seen in artiodactyls. But the word “horn” is applied to a wide variety of features among a wide variety of animals, and these structures are used in a wide variety of ways. It’s a striking example of convergent evolution.

Top left: The horns of bovids, the group that includes bison, antelopes, and these water buffalo, among others, have a bony core surrounded by a keratin sheath. Image by Ikiwaner.
Top right: Antlers are found among cervids, including elk, moose, and this red deer. Antlers are branching bony structures that grow and fall off every year. Image by Bill Ebbesen, CC BY 3.0
Bottom left: The horn-like growths of giraffes and okapis are called ossicones. These are bony outgrowths of the skull covered in skin and hair. Image by D. Gordon E. Robertson, CC BY-SA 3.0
Bottom right: Pronghorns, which are found on antilocarpids (also known as … pronghorns) have a bony core with a branching keratin sheath which sheds each year. Image by Mongo, Public Domain.

By some definitions, the only “true” horns are those of bovids, while everything else is a “horn-like structure,” although in common speech, we use the word “horn” for a whole variety of structures, even outside of artiodactyls.

Top left: The horns of rhinos are made entirely of keratin. Image by Rodrigo J De Marco
Top right: The horns of Jackson’s chameleons are a bony core covered in a keratin sheath, strikingly similar to those of bovids. Image by Movingsaletoday
Bottom left: Many beetles, like this Hercules beetle, have outgrowths of the exoskeleton that we commonly call “horns.” Image by Didier Descouens, CC BY-SA 4.0
Bottom right: Rhinoceros vipers have “horns” that are simply long horn-like scales (funnily enough, also made of keratin). Image by Dawson, CC BY-SA 2.5

Headgear has a variety of functions. They are commonly used for display, showing off to mates, signalling fellows from across the plains, frightening off predators, or intimidating rivals. Of course, they can also be used more aggressively, to fight off anyone – predator or competitor or nuisance – who gets too close. But their most famous function is ritualized combat, wherein rivals, usually males, clash heads in battles for territory or mating rights.

Top: Clashes between bovids often involve butting heads and ramming their horns against each other. Image by Pradeep717, CC BY-SA 4.0
Bottom: Antler-battles are often wrestling matches, where competitors lock horns for a contest of pushing, pulling, and twisting. Image by Tambako the Jaguar, CC BY-ND 2.0

A History of Headgear

The evolution of horns, antlers, and horn-like structures is, unsurprisingly, very complex. Among the various ornamented animal groups, these structures have been evolved and lost many times. There are some interesting trends and patterns related to animal lifestyle and behavior. For example, bovids in open environments tend to have larger horns visible over long distances, while those in more closed forested environments often have shorter sharper horns that allow more maneuverability. And of course, large horns are common in animals with highly territorial behavior.

The evolution of antlers, particularly the habit of regrowing them every year, has been the subject of lots of hypothesizing. It might be that this highly energetic process is a strategy to ensure each year’s set of antlers is fresher, larger, and more ready for combat.

Just a small sample of headgear in the fossil record.
Top left: Ceratogaulus, the only known horned rodents. Image by Ryan Somma, CC BY-SA 2.0
Top right: Megaloceros, the “Irish elk,” proud owner of history’s most ridiculous antlers. Image by Momotarou2012, CC BY-SA 3.0
Bottom left: Syndyoceras, one member of the proceratid family, whose skulls were adorned with multiple pairs of horns. Image by James St. John, CC BY 2.0
Bottom right: The famous Triceratops, whose ceratopsian cousins featured all manner of horns. Image by Michael Gray, CC BY-SA 2.0
And that’s not even including all the rhino-like things of the past!

Learn More

Horns and Antlers, What’s the Difference?

A literature review of horns and horn-like structures (technical)
Evolution of ruminant headgear (technical)
Evolution and development of antlers (technical, paywall)

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