All headgears of hoofed mammals come from a common ancestor

Think about mammals and the animal kingdom’s most iconic headgears: the majestic antlers of a moose, the curved horns of a bull, or even the knobby ossicones of a giraffe.

They seem as different as the mammals that sport them. However, a new research is turning that notion on its head.

A fascinating study suggests that these diverse headgear styles all sprouted from a common ancestral root. In other words, they’re more like evolutionary cousins than completely separate inventions. This discovery is a major step in understanding how bone develops across all mammals.

Headgear of mammals

Researchers focused on the following headgears in animals:

Antlers

Animals such as deer, elk, and moose have antlers. These structures consist entirely of bone and are unique because they shed and regrow annually.

During the growth phase, a soft, vascular skin known as “velvet” covers the antlers, supplying nutrients and oxygen to the developing bone.

Once fully grown, the velvet is shed, revealing the hard, bony structure beneath. This cyclical process of shedding and regrowth allows the animals to produce larger and more complex antlers each year, which play a critical role in mating displays and combat during the breeding season.

Horns

Horns are found on animals such as cattle, sheep, goats, and their kin. Unlike antlers, horns are permanent structures that grow continuously throughout the animal’s life.

They consist of a bony core covered by a sheath of keratin, the same protein that makes up human fingernails and hair.

Horns serve various functions, including defense against predators, dominance displays, and intraspecies combat. The continuous growth of horns means that they can become quite large and elaborate over the course of an animal’s life.

Ossicones

Ossicones are bony protrusions found on giraffes and their relatives, such as the okapi. These structures are covered in skin and fur, giving them a distinctive appearance.

Unlike antlers, ossicones are not shed and regrown; they are permanent structures that form early in the animal’s life.

Ossicones play a role in thermoregulation and may also be used in combat between males during mating season. The presence of skin and fur over these bony knobs helps protect them from injury.

Pronghorns

Pronghorns are unique structures found exclusively on pronghorn antelope. While they are similar to horns, they differ in several key ways.

Pronghorns consist of a bony core covered by a keratin sheath, much like horns. However, the sheath of pronghorns is forked, giving it a distinctive appearance, and is shed and regrown annually.

This combination of features — being both forked and shed annually — sets pronghorns apart from other types of headgear found in ruminant mammals. Pronghorns are used primarily for defense and display during the breeding season.

Evolution of mammal headgears

Scientists have long puzzled over how these different headgear styles evolved. Did they develop independently in each animal group, or is there a deeper connection?

That’s where Zachary Calamari, a researcher from the American Museum of Natural History, comes in.

Calamari and his team delved into the genetics of headgear development. They focused on transcriptomes, which are essentially snapshots of the genes that are active in a particular tissue at a particular time.

By comparing the transcriptomes of developing horns in cattle to those of antlers in deer and skin in pigs (which lack headgear), they uncovered some intriguing patterns.

Common thread in mammals’ headgears

The research revealed that all ruminant headgear, including horns, antlers, ossicones, and pronghorns, likely evolved from paired bony outgrowths near the frontal bones of the skull.

These structures originate from the cranial neural crest, an embryonic cell layer essential for forming the face. This cell layer not only contributes to facial bones but also forms these diverse headgear types.

By studying gene expression patterns, scientists found that the same genetic mechanisms are at work in both horns and antlers, supporting a single evolutionary origin.

This discovery helps explain the shared characteristics and variations in ruminant headgear, providing deeper insights into the evolution and development of these unique structures across different species.

Understanding this common origin underscores the interconnectedness of evolutionary processes in shaping the diverse adaptations seen in ruminant mammals today.

“Our results provide more evidence that horns form from the cranial neural crest, an embryonic cell layer that forms the face, rather than from the cells that form the bones on the sides and back of the head. It is striking that these are the same cells that form antlers,” explains John Flynn, a curator at the museum.

Evolution loves a good remix

This study paints a picture of evolution as a master remix artist. Instead of inventing headgear from scratch each time, it seems that nature took a basic bony blueprint and tweaked it to create the dazzling array of head adornments we see today.

“In addition to gene expression patterns that support a single origin of horns and antlers, our results also show the regulation of gene expression patterns in these structures may differ from other bones. These results help us understand the evolutionary history of horns and antlers and could suggest that differences in other ruminant cranial appendages, like ossicones and pronghorns, are also elaborations on a shared ancestral cranial appendage,” Calamari explains.

So, the next time you see a moose with its impressive antlers or a giraffe with its quirky ossicones, remember that these seemingly distinct features share a surprisingly deep connection. It’s a reminder that even the most diverse forms in nature can often be traced back to a common origin.

The study is published in the journal Communications Biology.

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