Why don't more herbivores have sharp horns?

December 14, 2006 2:04 PM

Why don't more herbivores have sharp horns? Some of them do, like the Cape Buffalo and the Kudu, but most have horns that do not seem very useful for defense. Given that the cost in keratin is the same for sharp and dull horns, and that sharp horns don't seem to cause intra-species injuries, why wouldn't all herbivores have razor-sharp horns to defend themselves?
posted by turbojav to pets & animals (40 comments total)

Most animal horns are not very sharp because they are usually used to "fight" other members of the same species, and inter-species "fights" are rarely FIGHTS. You dig?
posted by Cosine at 2:07 PM on December 14, 2006

I don't think horns would be very good defense for tiny herbivores like bunnies and squirrels. What would a teensy squirrel horn do against a creature three or more times its size? In their case, it would be a waste, all they need is speed and reflexes.
posted by tastybrains at 2:11 PM on December 14, 2006

Most herbivores run away from predators, rather than trying to fight them. Those that choose to fight predators would have a selective disadvantage to their fellows that ran away and left them to it. It is only in rare cases that a herbivore is big and strong enough to fight (buffalo, elephant, rhinoceros) and as you have observed those animals have sharp horns and tusks. There is little point in an animal the size of a rabbit having sharp horns if its predators are likey to be larger and stronger - they should just run away.

Animals wiith dull horns more frequently use them for intra-species conflict (male dominance stuff) or to display to the ladies. I do not know the game theory in the former case, but maybe it is advantageous for this conflict to be non-lethal. In any case, much of the headgear worn by animals is not used for defence against predators.
posted by nowonmai at 2:14 PM on December 14, 2006 [1 favorite]

Random speculation here ... let's pose the question slightly differently: If two members of the species differed only in horn-sharpness, would the one with sharp horns have any advantage? That could lead to selection pressure.

I'm guessing that sharpness alone isn't enough. The sharp horn has to be on the end of a long horn, to give the weapon some reach advantage at close quarters, and the animal has to be big enough and have a strong enough neck to be able to jab with the horn. Tangling with a large predator at close range is a pretty risky proposition, so the animal has to be robust, fast or large enough to either endure or outrun those sharp claws.

If the animal doesn't have these attributes, the horn deters mainly through ramming (which uses the mass of the entire body instead of relying on muscles in the neck) or herding-related behavior (tight groups, horns toward the outside). I'd guess that safety in numbers is a big enough factor that it removes the extra selection pressure for sharp horns. Consider the wildebeest (Gnu) which has fairly short horns, used mainly as a last resort. They're sharp enough for a good jab to brush the lion or hyena away, but it mainly tries to outrun its enemies and stay in the herd.
posted by Araucaria at 2:20 PM on December 14, 2006

The premise of this question seems to presume that evolution will lead to optimal expenditure of various "costs." But that's not how evolution works. We cannot say that the genes of current organisms are optimized, but merely that they have survived, while other sets of genes have not.

The reasons for the survival of these genes can be due to selective advantage or historical accident. But trying to decide how one of these is the right answer will only result in just-so stories.
posted by grouse at 2:21 PM on December 14, 2006

Tastybrains, I think the thrust of the question is more "why don't all horned herbivores have sharp horns?"

Anyway, this Wikipedia article suggests that horns have more uses than simple defense—foraging, cooling, courtship, etc. It could be that among horned animals that uses their horns primarily for non-defensive purposes, sharp horns would be inconvenient and even dangerous.

In human terms: why use a razor-edged sword if you only need to cut yourself a slice of a cake?
posted by Iridic at 2:22 PM on December 14, 2006

trying to decide how one of these is the right answer will only result in just-so stories.


Clearly, only the belief in an intelligent designer can adequately explain such a question.
posted by norm at 2:26 PM on December 14, 2006

Yeah, a-and how come there aren't venomous birds?

I love threads like this one.
posted by jet_silver at 2:27 PM on December 14, 2006

I think a very sharp horn would be more fragile than a slightly dull one
posted by Iron Rat at 2:34 PM on December 14, 2006

jet_silver writes "a-and"

Have you been reading Against the Day?
posted by OmieWise at 2:40 PM on December 14, 2006

Huh. Why aren't there venomous birds?
posted by Iridic at 2:41 PM on December 14, 2006

Well, it's poisonous, not venomous, but there is the pitohui.
posted by rtha at 2:49 PM on December 14, 2006

i gotcher venemous bird right here: http://en.wikipedia.org/wiki/Pitohui
posted by sonofslim at 2:50 PM on December 14, 2006

damn!
posted by sonofslim at 2:50 PM on December 14, 2006

OmieWise: V.
posted by jet_silver at 2:51 PM on December 14, 2006

Honest, I didn't mean to threadjack: it's just that one day I was being glad that hummingbirds - very bold birds - weren't venomous too, and that set me thinking about sparrows with the venom of a lionfish and what the suburbs would be like then.
posted by jet_silver at 3:00 PM on December 14, 2006

Clearly, only the belief in an intelligent designer can adequately explain such a question.

No, you'd need to also provide a reason why the intelligent designer would choose specifically not to make more herbivores with sharp horns, just like you'd need to provide a reason why he didn't make unicorns.
posted by grouse at 3:05 PM on December 14, 2006

This came up in my college biology class. A basic tenant: evolutionary specialization is geared mostly towards gathering of resources and the ability to process those resources moreso than anything else. The evolution of teeth most likely didn't arise for protection; rather, teeth evolved for the need to better gather & process resources. Therefore you have to think about it this way: What advantage in terms of gathering and processing nutrients do flatter teeth grant over the sharp horns for herbivores.
posted by jmd82 at 3:47 PM on December 14, 2006

On Post:
Re: venom: A bird's need to use venom isn't as necessary as land creatures such as snakes and spiders. Birds already have the escape advantage to be able to fly away- snakes and spiders don't have that ability. Species can't have every evolutionary advantage born under the sun- there's a huge energy implication for every trait that drove a lot of evolutionary adaptations. For example, higher species didn't evolve a closed circulation system because it was cool sounding or even protected us against our predators. Rather, it allowed a very controlled dispersion of nutrients to apendages which was not pheasible in our evolutionary ancestors (such as an open circulatory system where blood just kinda...moves around without too much control). This adaptation and consequently higher energy cost also allowed us to hunt with more efficiency and thus bring in more energy. The key here is a species will only evolve a trait AND propagate if it's energetically feasible. If a bird evolved venom at the price of a higher energy upkeep, what use is it if they can't sustain those energy requirements? Or if that extra energy requirement could be better spent, oh I don't know, being able to fly faster?
posted by jmd82 at 3:55 PM on December 14, 2006

just like you'd need to provide a reason why he didn't make unicorns.

I think you meant to say was:
just like you'd need to provide a reason why he let unicorns become extinct.

Stupid sexy sharp horns did them no good.
posted by Sprocket at 4:11 PM on December 14, 2006

I'm guessing it has something to do with the way different horns are formed. Animals with non sharp horms like deer have a very different horn producing mechanism than bovines or rhinos. It may not be possible for a deer to "build" a pointy horn, especially within the limits of the way they grown them under velvet and the fact they shed annually.
posted by fshgrl at 4:33 PM on December 14, 2006

Original Poster here. My question is why SOME herbivores have sharp horns, Texas Longhorn cattle for example, and others don't. I understand that a set of antlers would be unwieldy on a rabbit. I understand that intraspecies fighting is more dangerous with sharp horns, nevertheless some species thrive with sharp horns.

I don't think sharp horns are significantly more fragile. Somewhat more fragile, yes, but check out a Cape Buffalo. It has robust horns with sharp ends.

I'm aware that most prey animals survive by running away, but as was pointed out, if one herbivore in a herd has daggers on it's head and another has nubbins, predators will be more wary.
posted by turbojav at 4:51 PM on December 14, 2006

OP here again. As for deer not being able to "build" sharp horns, check out bull elk horns.
posted by turbojav at 4:53 PM on December 14, 2006

My question is why SOME herbivores have sharp horns, Texas Longhorn cattle for example, and others don't.

Because Texas Longhorn cattle have genes that lead to the development of sharp horns, and some other herbivores don't.
posted by grouse at 5:01 PM on December 14, 2006

Horned herbivores have the horns for display and as cosine said "fighting" they have some nice sharp hooves for defense.

Note: never approach a downed deer from the hoof side unless you are sure it is dead.
posted by Pollomacho at 5:04 PM on December 14, 2006

OP here. Cape Buffalo horns are pretty similar, not much differentiation in display going on, not like with deer. But let's say horns are for display and as a cosine for fighting. How come some species are sharper than others (or have the genes, whatever) why aren't all sharp?
posted by turbojav at 5:49 PM on December 14, 2006

I would think the "cost in keratin" for sharp horns would be higher, as they'd need to be harder to keep their point.

Also, I think there could be a significant downside to sharp antlers, where an animal uses them to defend its territory from rivals, as in some cases those rivals are its own offspring.
posted by teg at 5:50 PM on December 14, 2006

NB: The pitohui is not venomous - it's poisonous. If you bite it, you get sick: that's poisonous. Venomous is when it bites you, and you get sick.
/soapbox

I wonder what the evolutionary "cost" of being poisonous vs venomous is?
posted by rtha at 5:53 PM on December 14, 2006

How come some species are sharper than others (or have the genes, whatever) why aren't all sharp?

To clarify my posts, there doesn't necessary need to be a reason for a evolutionary trait to occur. Evolution's not some power sitting up there thinking, "Hmmmm, should I give this deer these kind of antlers, and this elk these guys other antlers?" What matters is if a genetic mutation is passed on to its offspring.

In your case of dull vs. short horns, there doesn't need to be a reason for the difference other than a mutation occurring. I don't know my species, so just use these as examples. Say an elk has a genetic profile to develop sharp antlers. Now, say there a genetic mutation to cause dull antlers. If the new offspring with dull antlers survives in the wild and doesn't even use the antlers, your new species has this different trait if for no other reason than it survived and happened to have this different mutation.
posted by jmd82 at 5:58 PM on December 14, 2006

OP here - OK then why is the sharp horn mutation selected for in some species and not in others? With minor variation ALL Texas Longhorn cattle have long sharp horns. All moose have dull flat horns. Both are pursued by predators.

Defending territory from closely-related offspring can be done through nonlethal means, such as elk maintaining territory through "bugling".
posted by turbojav at 6:37 PM on December 14, 2006

It's all fun and games until somebody loses an eye.
posted by medusa at 6:53 PM on December 14, 2006

This is not necessarily an answerable question. You ask "why" these things don't happen like there's a rational reason - nature made a rational choice in selecting certain mutations, or it made an irrational choice. It's not like that. Mutations happen, and while they might provide an edge, it's not necessarily enough of an edge to prevent that particular more sharp-horned goat from being eaten, or falling off a cliff, so that potentially beneficial mutation doesn't get passed on. Evolution is extremely complex, and the best features don't necessarily make it. Also, sometimes you need lots of intermediate steps to get to something that then obviously serves a useful purpose - and the process can get interrupted in the interim before an adaptation really becomes an advantage. The point is that evolution isn't game theory, and you shouldn't think of it in rational-actor terms.
posted by Dasein at 9:38 PM on December 14, 2006

Because evolution doesn't have a purpose in mind. Exactly what Dasein said.
posted by number9dream at 10:10 PM on December 14, 2006

You mean the jackalope isn't real????
posted by The Deej at 11:03 PM on December 14, 2006

OP here. If the sharp-horned mutation isn't beneficial the why is it ubiquitous in some species? It's being selected for. Intermediate steps and a lack of purpose in evolution would suggest a mixed population, sharp/dull.
posted by turbojav at 11:54 PM on December 14, 2006

If the sharp-horned mutation isn't beneficial the why is it ubiquitous in some species? It's being selected for.

Remember that one could say the same thing about the allele for dull-horned mutations. It is a mistake to think that the adaptations you view as more complex make the species more evolved. Not every change is adaptive.

Intermediate steps and a lack of purpose in evolution would suggest a mixed population, sharp/dull.

Evolution does not have a "purpose." I think by this you mean that the sharp-horn allele has a selective advantage, as you said earlier. But you don't really know with the data that people answering here have.

I don't know whether the ancestral horned herbivore had sharp or dull horns, or a mixture. I don't even know that there was an ancestral horned herbivore—the adaptation may have arisen multiple times in the same way that bats, birds, and butterflies have wings but their common ancestors don't.

If the ancestral horned herbivore had dull horns, then part of it is a question of the allele for sharp horns ever arising, which is a question of a chance random mutation, which may not have happened in the ancestors of the dull-horned herbivores. It's then still a question of random chance whether the gene becomes widespread in the population. Even if I were stronger and smarter than everyone on the planet due to a freak mutation, that doesn't mean that I wouldn't die before I could reproduce due to some unfortunate event, or that my offspring would be able to reproduce widely.

Now assume that it passes all these trials of randomness, and that the allele is present in a large enough percentage of the population to make it more likely that natural selection is acting on it. Then at some point there could have been a selective sweep that resulted in the fixation of the allele. It's possible that the sharp-horn allele provided an advantage in survival that caused this. But it's also possible that this advantage only existed at the particular time that the selective sweep took place, and that in the diverse environments that the much larger population is now, the event would not repeat itself. It's also possible that the selective sweep might be related to some other trait and the fixation of the sharp-horn allele (which could be neutral or even deleterious) is due to genetic hitchhiking. Or there might not have been a selective sweep at all—the allele may have been fixed by genetic drift when the population was much smaller. That is to say that it could be due to random chance (yet again).

As a side note, we know you're the OP, so you don't need to keep repeating it.
posted by grouse at 2:32 AM on December 15, 2006

Sorry for saying I am the OP all the time, thanks for the tip.

Grouse's answer was the closest to an in-depth analysis I think.

But - Re: genetic drift. Random chance on a small population that then expands, all members now having the allele. Possible in a small number of species but the fact that the sharp-horned allele is found in many types of herbivores makes it unlikely that they all had small populations with the same type of random mutation that all got fixed through drift.

Re: Genetic hitchhiking. I am struggling to think of a common adaptive mutation exclusive to sharp-horned herbivores. There is a lot of variation between a black rhino and an eland.

Re: Selective sweep. What is that selective sweep and why did it affect only certain types of herbivores.

I know that not every change is adaptive, but if a change is not adaptive can it be ubiquitous in a species? If it's not adaptive wouldn't mutations on this change occur?
posted by turbojav at 3:03 PM on December 15, 2006

the sharp-horned allele is found in many types of herbivores makes it unlikely that they all had small populations with the same type of random mutation that all got fixed through drift.

Maybe, but there might be multiple mutations that would result in sharp horns—it wouldn't have to be the same one each time. So you might have multiple alleles that will result in sharp horns, each at different loci.

But I imagine many of these herbivores are in the same clades, so the sharp-horn allele wouldn't have to have arisen as many times as it seems. Additionally, the sharp-horn allele could have been in the ancestral species, and it's the dull-horn allele that arose through genetic drift.

Or, your originally assumption (as I interpreted it) that either trait can adaptive in different situations might be true. Or it might be true some of the time and not others. It is only wrong to start with this assumption before it is proven. If you started with the assumption that this was caused by genetic drift I would have told you that it was also possible that it was due to natural selection.

Some other points:

I am struggling to think of a common adaptive mutation exclusive to sharp-horned herbivores.

It doesn't matter whether you can think of one or even if one can be found. This genetic hitchhiking would have occurred in the past, and the advantage may have been one that is no longer relevant.

Humans are among the best-studied species at the genomic level, and while we can identify areas subject to selective sweep (and therefore genetic hitchhiking) we can't say what the advantage that caused this sweep was yet. There's no chance that it will be done for the group of animals you are talking about soon.

if a change is not adaptive can it be ubiquitous in a species?

Certainly yes. See genetic drift above.

Additionally, an allele may have been adaptive when fixation occurred, but is no longer adaptive. The forces of natural selection are much more powerful on smaller populations (such as these species when they were first isolated) than on larger ones (such as any species you are likely to be talking about).

Furthermore, evolution might take a path across the fitness landscape that results in a local maximum of fitness, rather than a global maximum.

If it's not adaptive wouldn't mutations on this change occur?

Mutations always occur. For a gene under purifying selection, the mutation is less likely to be fixed. A fixed mutation is called a substitution.

A deleterious allele is less likely to be under purifying selection than an advantageous allele, so nonsynonymous substitutions are more likely, yes. But remember that most of these substitutions are unlikely to rescue the deleterious allele and reverse the effect of the original substitution.
posted by grouse at 5:55 PM on December 16, 2006

Grouse, thank you, you have changed my view of evolution. I knew that"Not every change is adaptive" on an individual level: an antelope born day-glo orange might not live long enough to reproduce - but you've taken the concept to a macro level: all antelope might have a non-advantageous trait that arose independently of purifying selection.

I think people tend to view evolution as a highly efficient system, which ruthlessly eliminates all non-advantageous mutations, when the way you describe it, many traits might be described as random or anachronistic

I'm curious to know if sharp-horned herbivores have a higher fitness landscape. Could it be that we may be in one of those intermediate steps of evolution where sharp horns, whatever their provenance, might be slowly being selected for (or against)? Perhaps one could say this about any trait at any time. Thanks for your answers.
posted by turbojav at 10:37 PM on December 17, 2006

Pleased to help. This stuff can be a bit more complicated than it seems at first glance, and I don't think it is taught very well.

I think people tend to view evolution as a highly efficient system, which ruthlessly eliminates all non-advantageous mutations

Yeah, it depends on just how disadvantageous the mutation is. The more deleterious it is, the less likely it is to survive. As an extreme example, a lot of mutations or combinations of alleles in mammals will result in miscarriages. There is zero chance these will survive.

On the other side, there are mutations that probably do not change the phenotype in any measurable way. These are completely under the control of non-selective forces of evolution, like genetic drift.

But many interesting things are between. The question is how close to either side is a particular allele?
posted by grouse at 2:25 AM on December 18, 2006