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I am assuming that monkey like animals are more likely to develop some form of horn or antler than apes(feel free to correct me), the question is what would be the most plausible function of the horns, defense or mating, and what type of environment would select for this as a favorable trait?

As for why monkeys over apes, the opposable thumb and greater intelligence seem like they would be able to fix most problems, and evolution of this type wouldn't happen, also monkeys seem simpler, and more likely to go either way on the brain v brawn route.

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Horns are a hindrance to an arboreal species, like a monkey. They are heavy and make jumping harder. They get tangled in branches. So are generally a nuisance if you are a tree climbing monkey.

Most animals with horns (or antlers) are relatively large: The horns are mostly used during the mating season. The males use their horns to fight each other. The most powerful males can establish a harem of females. Horns are also used for defence by nearly all horned animals. Cows, even though they don't need to battle each other for a mate, will use their horns to protect their young from predators.

Given this, it is most likely that horns would evolve among the larger land dwelling simians: Gorillas already have exaggerated canine teeth that are used in display and for fighting. The early Hominim Paranthropus aethiopicus had a thick skull and a prominent bone crest running down the middle of its skull. If horns were to evolve, it would be among the African great apes, Gorilla, Pan, Australopithecus and Homo, and their kin, and would be used for display, fighting over mates, and defence against predators.

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    $\begingroup$ thanks, two solid point's, actually answers some questions on a similar question I asked. However plenty of small animals develop horns. en.wikipedia.org/wiki/Horned_gopher en.wikipedia.org/wiki/Dik-dik $\endgroup$ – Eloc Apr 8 '17 at 21:05
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    $\begingroup$ I've seen them when researching this answers. The gopher is something of an exception. Horned rodents are rare. There is some doubt about what the horns are for. Chameleons also have horns. Monkeys might have chameleon sized horns. The dik-dik is evolved from larger ancestors, it didn't evolve horns itself. Given the evidence I'll stick with the general assertion: horns are most likely among larger, land-dwelling animals. $\endgroup$ – James K Apr 8 '17 at 22:11
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Horns and antlers have evolved independently in many diverse species of animals, from many different types of tissue, so it shouldn't be too difficult to fit this hypothetical creature into your ecosystem.

Most species with horns or antlers use them to attract mates. Having larger headgear indicates greater strength and virility (you've gotta be pretty strong to carry around up to 40 pounds of weight on your head). Antlers and horns are also used in direct competitions of strength between males to attract females.

Sexual selection doesn't require any particular environmental conditions to occur, so you can have some fun with your own particular evolutionary history. Perhaps your simians could use them to hook onto and hang from tree branches as a further evolutionary pressure. If your simians are tool users, this would have the extra benefit of freeing up their hands.

Horns can also be used for defense, but this is usually only seen in larger horned species. Most monkeys and apes will opt to run or hide rather than fight.

For a more complete answer, could you elaborate on your assumption that monkeys would be more likely to evolve horns than apes? I don't follow your reasoning there.

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  • $\begingroup$ Welcome to WorldBuilding! Interesting answer. If you have a moment please take the tour and visit the help center to learn more about the site. If something is unclear you should comment under the Post and ask for clarification. You can comment once you reach 50 reputation. Otherwise the OP could provide the additional information and nvalidate your answer, which is something we don't want. Have fun! $\endgroup$ – Sec SE - clear Monica's name Apr 8 '17 at 20:38
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    $\begingroup$ the opposable thumb and greater intelligence seem like they would be able to fix most problems, and evolution of that type wouldn't happen, but monkeys seem simpler, more likely to either way on the brain v brawn route $\endgroup$ – Eloc Apr 8 '17 at 20:46
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    $\begingroup$ @Eloc You really should edit your question to include any additional information requested. You can of course use comments to notify people of the edit if that is warranted, but the information should go in the post itself (in this case, the question). I made the edit for you this time; please do so yourself in the future. Thanks! $\endgroup$ – a CVn Apr 8 '17 at 21:48
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    $\begingroup$ @Eloc No problem. Oh, and when editing, please be careful to not invalidate existing answers. If an edit turns out to be needed that actually invalidates answers already given, it's often better to ask a new question and highlight the differences (to avoid it being closed as a duplicate). Providing additional information is almost always fine; changing the premise of the question is almost never fine. $\endgroup$ – a CVn Apr 8 '17 at 21:56
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The simians (infraorder Simiiformes) are the "higher primates" familiar to most people: the Old World monkeys and apes, including humans, (together being the catarrhines), and the New World monkeys or platyrrhines. Simians - Wikiquote

link

So if humans can form horns, monkeys probably can, too. Strength would be a factor.

Cutaneous horns, also known by the Latin name cornu cutaneum, are unusual keratinous skin tumors with the appearance of horns, or sometimes of wood or coral. Formally, this is a clinical diagnosis for a "conical projection above the surface of the skin."[1] They are usually small and localized, but can, in very rare cases be much larger. Although often benign, they can also be malignant or premalignant.]1]1

link

Woman with a horn and another growing

Perhaps they happened in error but were much admired and over time, horns developed.

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    $\begingroup$ en.wikipedia.org/wiki/Horn_(anatomy) these are the type of horns I'm referring to, i thought about using your example, but i wanted something more firmly in the dna than a disease $\endgroup$ – Eloc Apr 8 '17 at 20:50
  • $\begingroup$ I think you could start them off as a benign tumor and 'for some reason' there are a quite a few with this and again for some reason, they are admired and over time and mutation, they develop into 'what we see today' -- all of them have horns and have a use beyond admiration for them. $\endgroup$ – WRX Apr 8 '17 at 20:52
  • $\begingroup$ that's rather creative, but not quite to my own taste $\endgroup$ – Eloc Apr 8 '17 at 20:56
  • $\begingroup$ @ Eloc or mine, but it makes your idea plausible. $\endgroup$ – WRX Apr 8 '17 at 22:17
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Horns are formed to defend against predators. This requires mass and acceleration for impact. Monkeys are specialized in climbing, not running. You need a species that runs or has the mass to headbutt a predator. So you likely need:

  • Speed
  • Mass
  • An enemy
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Sexual Selection

As mentioned by creeon, it is much more likely that horns would develop for mating purposes. Horns used for defense require you, as a prey animal, to be within striking distance of a predator with presumably sharp claws and/or a large set of powerful jaws.

Even worse, the horns are on your head! To even use defensive horns, your neck must be that much closer to jaws that wish to close around your neck. Not to say it isn't plausible, but the most parsimonious answer would be that of sexual selection.

The evolution of numerous behavioural and physiological traits throughout the animal kingdom are due to sexual selection. Darwin even defined and described it in 'The Descent of Man, and Selection in Relation to Sex'. Not only that, sexual selection pressures are often powerful enough to produce features that are harmful to the individual's survival! The most flagrant example being the peacock's tail — an ostentatious but quite impressive display that is most definitely a hindrance when escaping from a predator. This type of extreme sexual selection is an example of a Fisherian runaway - exaggerated male ornamentation by persistent, directional female choice

Horns on a primate might be sexually selected for any or all of the following reasons:

  • wrestling aids to assert dominance and implicitly display fitness to potential mates. These would tend to be bigger and more intricate like a deer or other cervid's horns to bind your opponent, much like an ice hockey fight where the goal is to hold your opponent with your free hand to prevent them sliding away.

  • headbutting 'helmets' like the above pic to assert dominance and implicitly show fitness, much like rams or muskox. These would tend to be flatter and would likely be fused directly with the cranium to cushion the skull.

  • no wrestling involved, horns are merely an 'honest signal' (i.e. no presumption being made of conscious intention) of fitness or some other, more granular positive trait that a mate would want their offspring to have.

It is also not likely that monkeys over apes would develop horns - all primates are social, with varying degrees of gregariousness. The most intraspecies mate competition would lead to the strongest sexual selection pressure for horns, but both monkeys and apes show enough mate competition to get the selection pressure started.

further reading on the evolution of horns: The Evolution of Horn-Like Organs (V. Geist)

(would post more links but this is only my first answer on the site)

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