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I'm designing an alien species and I want the males to have deer-like antlers. Now I also need this species to have a genetic predisposition (as in it is not just a cultural affectation) to be primarily monogamous (as their progeny require a lot of attention/effort much like human larvae).


Of course since antlers in the real world are used by stags to attract mates and "defend" (mostly symbolically as they aren't actually that useful to harm opposition) their harems from other males the two traits I want are at first glance incompatible.

There are potential solutions to this problem however. The first, probably most obvious answer is to find a utility other than that of male compitition for my creatures' antlers that is somehow exclusive to males. I really can't think of any possible usefulness that would only apply to males though.

The second resolution that I have considered would be that monogamy is a relatively recent evolutionary development in the species that I want to create and thus that the antlers are vestigial. This would of course mean however that larger antlers would gradually be selected against, quickly leading to stags without any antlers at all (there are conditions which can cause a stag to be born unable to ever grow antlers without even needing successive mutations in the real world). Approximately how long would it take for most males to not have antlers?


My main question is this: Would it be realistic for the majority of the males within a primarily monogamous species to have antlers?

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    $\begingroup$ Why not? Some small species of deer like Dik-dik are monogamous. If you accept monogamy for a season (as opposed to lifelong monogamy) your choice would be broad. $\endgroup$ – Alexander Jul 11 '17 at 22:03
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    $\begingroup$ "are at first glance incompatible" - why? I see no fundamental difference between attracting and defending harem vs attracting and defending one lady. $\endgroup$ – Mołot Jul 11 '17 at 22:05
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    $\begingroup$ Why would sexual selection work against antlers? Some evolutionary scientists theorize the male Beard in humans is an example of how sexual selection can work toward the development of a trait which doesn't contribute directly to survival or ease of reproduction. Maybe lady deer think dudes with antlers are sexy. $\endgroup$ – MozerShmozer Jul 11 '17 at 22:06
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    $\begingroup$ @Mołot I was always under the impression that monogamy inexorably led to a species to having low sexual dimorphism... Now I feel silly. $\endgroup$ – AngelPray Jul 11 '17 at 22:08
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    $\begingroup$ They are vestigial as you note, but also still attractive to the females. This kind of sexual preference selection can give rise to rapid runaway extreme traits. $\endgroup$ – JDługosz Jul 13 '17 at 0:17
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The antlers can be a signal of health, a strong immune system, etc, just like a peacock's tail. The peacock's tail is actually a drag on survivability (literally), it makes it more difficult for the peacock to escape predators (and this is observed in the wild). The same has been shown for bright colors in other male birds; it reduces camouflage compared to the female's drabber brown to black feathering: The latter evolved because color in a female is unnecessary (or vice versa).

For a peacock, we know females are attracted to larger tails and symmetric tails, and avoid males with missing feathers or lop-sided development; which may be a sign of disease or other fitness failures in escaping predators, which would reduce the likelihood of chick survival. (The females don't know that, of course, they are just genetically predisposed to attraction to large, symmetric, colorful tails).

For antlers, demand the same thing: large, symmetric, unbroken, and uniform in color and visible texture. No patchiness, no thinness. The male has to be healthy with plenty of calories to spare to carry such a rack. It does not have to be that they FIGHT with it; just like peacock's do not fight with their tails.

Even if they are an intelligent species like humans, it won't matter: Human men and women both are still very much predisposed to the physical characteristics of healthy individuals: Models are almost always very close to perfectly symmetrical in face and body; women are attracted to tall men with deeper voices, men are attracted to young fit women. Everybody prefers smooth skin that has no hint of disease. Your antler species will be the same, no matter how "intelligent" they may be, attraction is at a much more instinctive animal level in the brain than rational thought is likely to be.

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Skew the reproductive process to slightly favor male offspring, thus creating a relative shortage of females and thereby allowing the females to be selective in their choosing a mate.

Then make big antlers attractive to those females either as a symbol of virility or because some other positive trait (strength, intelligence, longevity,...) is proportionally linked to antler size.

This would allow the vestigial antlers to resist the pull of natural selection, despite their no longer having any survival-assisting functionality of their own.

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    $\begingroup$ For game-theory reasons, most species are expected to have around 50-50 sex ratios. Here's a slideshow that discusses this and outlines some things that are theoretically expected to shift the ratio: ocw.mit.edu/courses/economics/… $\endgroup$ – sumelic Jul 11 '17 at 22:19
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They look like antlers, but really, they're antennae.

If we can avoid the whole "fight for a harem" angle we eliminate a lot of unpleasant implications. You could, for example, say: The bone in your species' antlers is laced with fine traces of gold, making them highly sensitive to electromagnetic radiation. This provides a competitive advantage - e.g. either in finding food or avoiding predators - that females find valuable. Why do we only find them on males? Perhaps they have a downside as well. If antlers make you visible to predators, whatever sex is caring for the young may not benefit from them, while those who can stand and fight are better off.

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/relatively recent evolutionary development in the species/

Consider a circumstance where males and females (the latter with young) have different feeding methods / patterns. This is the case for giraffes, elephants and a range of other animals. It keeps males from competing with females and young for food.

Now your creatures. The antlers are likely one of a package of secondary sex characteristcs left over. Imagine the antlered males have different feeding habits that require the same hormonal environment that causes the antlers - maybe they need bigger more muscular jaws to crack the fruit, or their foraging requires upper body strength to upend rocks etc. Maybe the males with more male secondary sex characteristics are better killers and catch meat, enriching their own diet and that of their family. Antlerless males that look like females can of course move in and live like females and maybe this is happening with your species to some degree. That might be fine or it might disadvantage the male's own offspring because of increased use of historically "female" food resources / non-use of historically "male" food resources. The low hormone male will have reduced fitness and this trait will not spread.

If there remains a selective advantage (different food resources) for the males using their secondary sex characteristics to forage in the old way, the antlers could remain as a holdover. Selecting against antlers means selecting against the entire package.

Another possibility is that secondary sex characteristics, antlers and aggression usually used for intra-male mate competition might also be useful against predators. I am thinking of baboons. Male baboons will team up to drive off a leopard or similar predator. If having males with secondary sex characteristics mean you lose fewer infants to leopards, selection will keep them around.

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