Male horns & antlers against female lack

In a number of mammalian species, the male has horns or antlers while the female has either none or much, much smaller. This is a puzzle to me, because for most mammalian species, the female faces the same enemies as the males. The answer that the males evolved longer horns or antlers as a sign of desirability from the female only could explain a difference in the length of the horns/antlers, but not their absence. So, why?

Staff: Mentor

In Maine, you may be restricted to "antlered deer" during hunting season. Some people used to call that "bucks only", but it's inaccurate because some does have antlers and it is impossible to check for gender while the deer is "on the hoof". Generally, antlers on does are short and unbranched, but still such does can be mistaken for spikehorn bucks.

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Only partly.
The difference in hormones between human sexes, which evolved for other purposes, had as a side effect the difference in facial hair growth which, as far as anyone can see, did not add any immediate survival advantage to the individual male. The sexual attraction of increased facial hair may have then evolved as the hair indicated the higher level of certain hormones desirable to the female, thus giving facial hair an evolutionary advantage it didn't have before, but only within the species. Horns and antlers, on the other hand, would appear to have had a direct evolutionary advantage with regard to other species, as well as later the sexual advantage within the species that it shares with the human mustache. That is, the primary reason for the mustache is not the same as the primary reason for horns and antlers. To restate: it is clear (with the mustache analogy) why the males and the females evolved different sizes of horns/antlers in those species where both of them share them (as in the Maine example of turbo), but my question concerns species in which the female lacks them. I was looking for an answer along the lines given by Wikipedia http://en.wikipedia.org/wiki/Horn_(anatomy), although not necessarily the same one, as I am not sure whether this answer is a good one:
"It has been theorized by researchers that taller species living in the open are more visible from longer distances and more likely to benefit from horns to defend themselves against predators. Female bovids that are not hidden from predators due to their large size or open Savannah like habitat are more likely to bear horns than small or camouflaged species."

In a number of mammalian species, the male has horns or antlers while the female has either none or much, much smaller. This is a puzzle to me, because for most mammalian species, the female faces the same enemies as the males. The answer that the males evolved longer horns or antlers as a sign of desirability from the female only could explain a difference in the length of the horns/antlers, but not their absence. So, why?

Animals often evolved sexual dimorphism because of mating conflict. I think that is the main answer to your question. The horns often develop more in males than in females because the males need it fighting over the females.

The most obvious reason for horns is for defense against other animals. Both males and females need horns to defend themselves against other species. As you point out, both of genders need to defend themselves against predators. However, the horns and antlers are also useful in fighting others of the same species.

Very often males fight over the females. The horns in that case are as much offensive as defensive weapons. The males with the bigger horns or antlers have a better chance of winning. Of course, other functions develop once the male has developed large horns. The horns may become a feature for intimidating rather than killing the male. Once the males develop the ability to grow large horns, the horns become a signpost for good health. However, the evolution of the larger horns was probably initiated when males started to fight each other over females.

Generally, the female mammal doesn't fight for mates. The male mammal often does. So maybe this needs an explanation.

A little selfish gene theory is useful, here. Think what the comparative risks are for the fighting gene. If the male gets himself killed fighting for a mate, then the fighting gene has wiped out only copies of itself in the male individual. However, suppose a female gets killed fighting for a mate. Then the copies of the gene in her body and the body of the children she is caring for are destroyed. So the genes for the fighting instinct greatly improve their chances of surviving into the next generation if their expression is suppressed in the female body.

The females don't have to fight each other (much). So they don't need such big horns. Better use the calcium to make milk rather than horn. You can improve the off springs chance of surviving almost as well by feeding it properly as by defending it.

Another reason may be that horns provided some risk toward the offspring. The mothers take care of the children. Yes, they need horns to defend themselves against predators. However, there is a small chance they could hurt their offspring with their horns. A rambunctious child could bump against the mothers horn and hurt itself.

I don't think this is important later in evolution once the sexual dimorphism is established. However, this may have been important early on in mammal evolution, while maternal care was still evolving.