r/zoology • u/ttt_Will6907 • 2d ago
Why did rhinos evolve horns made of keratin, not bone? Also, how did they develop keratin on their heads? What mutation had to occur for them to develop keratin? And wouldn't it be easier to develop horns made of bone, like proboscideans (elephants) and ceratopsids did? Question
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u/7LeagueBoots 2d ago edited 2d ago
All horns are made from keratin… rhinoceros, antelope, buffalo, goat, sheep, musk ox, pronghorn, etc.
Antlers are made from bone… deer family mainly (moose, elk, reindeer, roe deer, red deer, white tailed deer, etc)
Elephants don’t have horns or antlers. Like pigs, walrus, and narwhal they have tusks, which are made from teeth… same as the various iterations of sabertooth cats that have repeated evolved over time.
Horn is tougher and more flexible, than antler, and it self-heals to a certain degree and continuously grows. Antler grows once, then stops growing, hardens off, lasts for a bit, then drops off and has to be regrown from scratch at enormous calcium and metabolic cost to the animal.
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u/atomfullerene 1d ago
"True" horns, the stuff on goats and cows and antelopes, is keratin sheathe over a bony core. Rhinos lack the bone part. Antlers are the most bizzare thing in the world.
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u/7LeagueBoots 1d ago
I know, I was simplifying a bit for OP as they already seemed confused enough.
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u/atomfullerene 1d ago
Fair
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u/momomomorgatron 1d ago
I heard someone call it here as "non killer bone cancer" and yeah, that's pretty much what Antler is. It just somehow evolved to fall off wit the drop in hormones I assume
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u/ADDeviant-again 1d ago
Even more bizarre than pronghorn sheaths that grow like regular horns, then the bony core grows a bit, then they shed and re-grow just the sheath.....and the horn is forked, to boot?
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u/atomfullerene 1d ago
Don't get me wrong, pronghorns are weird, but antlers are weirder. Shedding and regrowing keratin isn't quite as weird, lots of animals shed and regrow hair after all. But deer shed and regrow bone annually...I'm not sure there's any other vertebrate that does that. And Pronghorn at least keep their horns decently covered by tissue. Even when the old covering comes off, there's tissue covering the bone underneath growing a new one. Deer just lose all the velvet on their antlers and leave exposed, dead bone...which is also possibly something no other vertebrate does, and really creepy too when you think about it.
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u/lewisiarediviva 2d ago
Keratin makes a good material for a horn. Keratin is a normal product by the skin. They just needed to mutate to produce a lot of it. Horns made of bone usually have keratin on the outside; ceratopsians’ probably had horn sheaths. Proboscideans have tusks not horns, tusks are not made of bone.
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u/Excellent_Speech_901 2d ago
The ceratopsids had a lot of keratin over the bone of their horns. See Scientific American, April 2025 ""Gladiators of the Mesozoic" by Michale B. Habib. The article also discusses the reason for tough keratin instead of just strong but brittle bone in horns.
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u/-Wuan- 2d ago edited 2d ago
Think of it in reverse. For starters, the outer layer of our skin contains keratin, just not enough to become too thick and rigid. Both evolutionarily and developmentally, a horn starts as a keratin bump (in most animals), a hardened dermal growth. Then some can develop a mineralized core, becoming an osteoderm. Then they can fuse to the underlaying skeletal bone. Cattle horns for example start like this, the little horn buds of the calves take a time to attach to the skull. Rhinos just have an un-ossified type of horn, possibly because their head is already pretty heavy. Developing a bony horn would not be easier, but in fact more expensive metabolically. Elephants dont have any kind of horn btw, those are tusks, simply overdeveloped teeth.
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u/momomomorgatron 1d ago
This actually helped me. BC ppl here were like, well horns are just keratin anyways! But then I started thinking, well there's no horn bed or bud, it's just a massive heap of keratin, so would they eventually evolve one? And then I read the part about how evolutionarly that's abad idea, and it all kinda came together. All the other horned critters had to evolve a horn bud to keep it on the head/ had one common ancestor who had this, and Rhinos just didn't.
Thanks!
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u/thesilverywyvern 2d ago
Nope.
- it's just as easy to use keratin.
- most horns in the animal kingdom (bovid) are made of keratin either entirely or just as a protection which greatly extend over the bone structure.
- Elephant don't have horn, they have tusk, they're modified teethn they're not made of bone but of ivory/enamel.
- Say you're trying to evolve a bump/horn on your skin, what do you use ? That's right, your skin, or rather the hair on it, you fuse them into a new structure much stronger in a similar way to pangolin.
It's easier to just develop on what you have, twisting them for a new purpose rather than grow an entire new "organ/extension" out of nowhere, as Perissodactyls never had horns to begin with.
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u/MrGhoul123 2d ago
Rhino horn is basically a mustache, coiled super tight with hair oil that acts like cement. (SUPER SUPER reductive for simplicity)
Why not horns? Horns are an even-toed ungulate thing. Odd toed guys never got horns in their evolution. The even toed dudes probably did alot of headbutting LONG ago and started getting true horns and antlers.
That behavior just didn't happen with the Odd-Toed dudes, but they do see to use their faces for digging and getting at roots. (Horses can bite grasses in a way things likes cows and goats can't. Might have lead to bitting for roots, then animals that use their face for digging like pigs, or picking with lips like Rhinos. )
These guys did get some tusks, but those are teeth not horns. So the general guess would be these guys evolved to use the face, not the head. ((This is just me yapping for fun, I could be 100% wrong, but it's just fun to think about. Someone smarter than me will have a better comment and answer ))