As a nature know-it-all kid, I loved talking to (more likely at) people about my favorite animals. One of those favorites was the rhinoceros, a dinosaur-like animal I loved to watch on nature shows. My go-to trivia fact was that rhino horns were not bones, but actually “made of hair”. Thanks to a recent reader request, we’ve now come full circle, and I’m delighted to return to this question for a more in-depth perspective. While seven-year-old me was correct that rhino horns are not bone, the story is a bit more complicated than them being made of hair. In this Science Deep Dive post, let’s dig into the science of rhinoceros horns and learn some cool rhino facts along the way.

What are rhino horns made of?

To get straight to it: rhinoceros horns are composed primarily of keratin, which is the same structural protein making up much of our hair and fingernails. However, they have a pretty complex structure by comparison. While the horns of many animals actually grow around a core of real bone, rhino horns form from keratin secreted by specialized cells around a “core” of calcium and the protein melanin. This core provides the structural support enabling the horn to maintain a given shape. It also allows the different parts of the horn to wear away at different rates, creating a sharpened point. Science communicators often liken this to the way a pencil gets sharpened, although the metaphor isn’t exactly perfect!

Scientists disproved the common claim that rhinoceros horns are made of bunched-up hair as early as the 1960’s. Technological advancements like scanning electron microscopy enabled more in-depth investigations, leading to the discovery that they consisted of hairlike filaments linked around a central core in what engineers would call a composite. Researchers have observed that this is very similar to horse hooves and bird beaks, rather than cow horns, for example.

Read more: What’s the difference between horns and antlers?

How do rhino horns grow?

A study in 2015 shed light on how these amazing horns grow. The skin around and underlying the horn is different than skin on the rest of the rhino’s body, and includes dermal papillae, which are extensions of deep-skin tissue that extend up into the outer skin (epidermis). Essentially, these papillae produce the keratin-based fibers that make up the horn, secreting the protein much like the nailbed under the skin of our fingers that forms fingernails. In this case, a more complex network of linked and interacting papillae in a circular pattern produce both the core and the surrounding fibers. Rhino horns can grow about 3/4 of an inch (2cm) per year.

Do rhino horns keep growing forever?

Yes! Just like our fingernails, rhinoceros horns are continually growing, and continually being worn away by regular use. Since rhinos have relatively long life spans (around 30-50 years), these horns can get quite long. A black rhino (Diceros bicornis) horn found in Kenya holds the record at over 4 feet (1.3m!) Wear and tear from rubbing against the ground, trees and vegetation, or other rhino horns keeps the length from getting too excessive.

What are rhino horns for?

Rhino horns are, first and foremost, a defensive weapon; rhinoceroses use these formidable horns to defend themselves against large predators like leopards or lions. Male rhinoceroses will also use their horns to spar and compete over mating opportunities, although the most prominent use of the horns is in defending calves. Rhino horns can also serve an important purpose in rhino feeding behavior, aiding them in breaking branches or rooting in the ground for browsing leaves or making a mud wallow.  

Do both male and female rhinoceroses have horns?

Yes, both male and female rhinoceroses have horns. This is typical of most mammals where females must care for and protect young, like reindeer and water buffalo. Female rhinoceroses are fiercely defensive of their young, and readily employ their horns against any perceived threat. Females typically have longer horns than males, probably because they have much more occasion to use them when keeping vulnerable calves safe.

Do all species of rhinoceros have horns?

All of the five extant (that is, not extinct) species of rhinoceros have horns, although they number of horns that they have, and their characteristics, are somewhat difference. Although folks ask me for these particulars often when this topic comes up, I have plenty of trouble keeping them straight. So for reference, I made a quick guide to rhino horns with information that I found across the web. Check it out!

Common Name	Latin Name	Native Range	Number of Horns	Horn Length
White Rhino	Ceratotherium simum	Eastern and Southern Africa	2	~2ft (60cm), up to 5 ft (1.6cm) or more
Black Rhino	Diceros bicornis	Central to Southern Africa	2	20 inches (50cm), up to 4 ft (1.2m) or more
Javan Rhino	Rhinoceros sondaicus	Ujung Kulon National Park, Java (No seriously, that’s it, nowhere else)	1	~8 inches (20cm), up to 11in (27cm)
Sumatran Rhino	Dicerorhinus sumatrensis	Sumatra and Borneo	2	6-10 inches (15-25cm)
Indian or Greater One-Horned Rhino	Rhinoceros unicornis	Northern India, Pakistan, Bangladesh, Nepal	1	10 inches (25cm) , up to 20 inches or more (57.2cm)

Are rhinoceros horns valuable?

Unfortunately, rhinoceros horns have gained impressive value under the false belief that they carry certain medicinal benefits. Despite being illegal to trade or possess under CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora), rhino horns are the subject of a massive black market in which the material is valued from between $15,000 to $100,000 per kilogram, exceeding the value of gold. This value creates good incentives for poachers, who illegally hunt rhinoceroses from protected areas in order to remove their horns.

This puts a lot of pressure on small rhino populations, all of whom are endangered to some degree. While there are only a few thousand each of the African rhino species, less than 100 Sumatran and Javan rhinos remain. These critical numbers mean that these species are at critical risk of being lost forever.

Read More: Why is it bad when a species goes extinct?

As you read above, although their structure is unique and interesting, rhinoceros horns have nothing in them that you wouldn’t get from chewing your nails and having a glass of milk. The wasteful pursuit of rhino horns under the superstitious belief that they provide health benefits is an ongoing tragedy. What makes things worse is that in market economics, assets gain value when they become rarer. As rhinoceros populations are further reduced by poaching, horns become even more valuable, putting even more poaching pressure on them. Conservation biologists call this an extinction vortex.

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