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A hypothetical species is divided into males and females that exhibit pronounced sexual dimorphism. The males are further divided into multiple morphs (e.g. α-male, β-male, γ-male, etc). This polymorphism is extreme enough that a viewer unfamiliar with this species could mistake all these morphs for different species. If the species were to possess human-like intelligence, they might treat these morphs as separate genders (e.g. aqir, qntal, guqin, etc).

What selection pressures would favor such polymorphism?

EDIT: Reproduction is a simple matter of a male inseminating a female. At some point selection pressure caused the males to diverge into multiple morphs. Differences between the morphs are as extreme as those between dog breeds. The details are deliberately vague so that this question could be applied to plants, fish, mammals, etc.

EDIT: I do not mean to imply that morphs are necessarily hereditary or limited to a specific number.

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    $\begingroup$ You may want to read this bio.research.ucsc.edu/~barrylab/classes/animal_behavior/… $\endgroup$ – John Feb 2 '17 at 14:36
  • $\begingroup$ A thought I had. The creatures probably would have their own terms for these morphs/castes, and their language would evolve around these. Humans try to find an extension of a word or term we're familiar with to apply to this, but they would have this thing as a starting point, and more likely would describe other things in the terms of morphs. $\endgroup$ – J. Doe Feb 2 '17 at 15:31
  • $\begingroup$ You'll need to provide way more details about how your morphs look and how their reproductive cycle works for us to provide some answers. With the question on this format, every answer will be too generic. $\endgroup$ – T. Sar Feb 2 '17 at 16:27
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There exists a somewhat analogous situation to this in lizards, with 3 males that have different appearance, though they are not so drastically divergent as to be unidentifiable.

In this case they system works, and stays stable, because they are playing the evolutionary equivalent of rock, paper, scissors. Each male 'beats' one other males morphology when it comes to competing for mates, but they are equally beaten by the third male. Every generation one of the male morphology will have had more success then the others in reproducing, lets say your α-male, leading to many of α-male existing in the current generation. However, since another type of male, lets say β-male, is better at securing mates when competing with the far more common α-male these β-male will secure more mates in the current generation, leading to more β-male in the subsequent generation. That generation, now full of β-males, will in turn see the γ-male successfully out compete for mates with the common β-males. This finally sets the α-male up to make comeback in the subsequent generation since they can easily out compete with γ-male.

In lizards these males could be looked at as standard, harem, and sneaky males, based off of their mating strategies. The 'standard' males are exactly that, normal sized males that will compete for rights to mate with each female as it finds as most lizards do. Harem males are larger then standard males, and can easily best standard males in a fight. These males collect a large number of females together into a harem and guards them. When standard males are common the Harem males can easily beat the weaker harem males in fights, allowing them to secure large harems with minimal competition and thus secure numerous matings, effectively out competing Standard males.

Sneaky males are in turn the kryptonite factor of Harem males. These sneaky males are weaker then both other types of males, but they are fast and have a different mating strategy. Rather then fighting the much stronger Harem males they allow the Harem males to collect females into one place and then sneak into the harem to mate with the females while the Harem males are distracted defending their harem. When Harem males are common these males spend most of their time competing with each other to try to secure large harems, leading to increased injury and death amongst harem males and lots of females in one place for the sneaky males to mate with during the distraction. This results in the sneaky males producing the most young in the subsequent generation.

Unfortunately for sneaky males they can not defeat the other two males in a competition. Once sneaky males dominate they will not be able to find harems to sneak into, and instead will face standard males who can defeat and drive away sneaky males to secure mates, but never collect a large enough harem to allow a sneaky male to secure a mate while they are distracted.

A few different species of lizards demonstrate this pattern, and it has proven evolutionary stable. Males of every morphology secure mates in every generation, but the fact that each male 'beats' another male in competition prevents any one type of male from winning the evolutionary arms race, the more common any one gets the better off their counter male morphology becomes.

You could do something similar with your males. The only catch is that you want your males to be significantly different, while the above males have a less drastic morphological difference. My suggestion would be to make the males niche be more then just mating strategy. Each male is evolved for a specific niche of habitat or prey as well, developing more drastic changes in morphology to fit that niche; but still keeping the concept that one one morphology grows more dominant another will benefit from it.

One example I can come up with is that they prey on different levels of the food chain. α-male are larger, and eat a larger carnivorous species. β-male are smaller and eat a species that is preyed upon by whatever α-males eat. Thus when α-male are common there is more food available for β-male's because the β-male don't have as much competition for resources now that α-males are eating the other major predator of β-male preferred prey. In turn perhaps γ-male evolutionary niche depends on the prey species of β-male being less common. Maybe γ-males prefer some resource or living condition that the prey of β-male also prefer so that they can flourish once β-male remove their competition.

perhaps γ-males in turn build damns that change the nature of river above and below the river, creating habitat changes that make the species α-male prey on more common to complete the rock-paper-scissor dynamic.

If you make it so that these males have evolved to benefit from another type of male being common in multiple ways, benefiting in mating strategy as well as niche or presence of their prey species, you may be able to create a niche that is unique enough for each male to justify radically different morphological, but it would take a bit of thought to ensure the niches are unique enough to have each male look completely different from the other male while still having them keep the rock-paper-scissor dynamic and sharing the same rough habitat location (they are still competing for same females, so they must all live close to those females to compete for them)

I'd also mention that the female has to be able to produce the young of each male. if the males are different enough it becomes hard to believe that they are all capable of successful mating with the female. For that reason id suggest making all the males still have a fairly similar physiology. Perhaps the look different due to a comparatively superficial outward difference, most obviously their pelt/camouflage used to sneak up on their preferred prey, such that they can still be physically rather similar while still having very dynamic differences.

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  • $\begingroup$ excellent! I did not know this lizard setup existed. $\endgroup$ – Willk Feb 2 '17 at 20:19
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    $\begingroup$ It figures that the polymorphism would stabilize if the males were dependent on one another in a manner similar to eusocial insects. $\endgroup$ – Anonymous Feb 3 '17 at 16:02
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Sounds a lot like the Ruff. There are three different male morphs:

Three ruff male types. On the left side a faeder, in the middle a satellite and on the right site a territorial. Ruff and head tufts may have black, brown, orange and white colors. In satellites either ruff or tufts normally are pure white and they lack black feathers.
The faeder identity

It's genetically determined, and each one has a different mating strategy.

Most of the males (85%) are territorial. They stake out their claim on the lek (the mating arena) and wrestle with their neighbors. Satellites (15%) are not looking for their own territory. Instead, they join up with a territorial male. The females, you see, are looking for a show: the two males "give a show of courting and mounting each other". And then there's the faeder, which is very rare. It's funny you would mention gender identities, because faeders look female and hide their maleness during mating season:

Infrequently a female crouches for a faeder to solicit a copulation from him. If she crouches to be mated by a normal male, the faeder tries to slip between that male and the female and to mate her sneakily. And often, if the female thereafter crouches for another male, the faeder quickly crouches for that male too. Embarrassed by this double invitation, the male may mount the faeder rather than the female. The faeder in that way decreases the chance that his sperm will have to compete with that of another man.

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    $\begingroup$ Bisexuality as a successful mating strategy... Nature never ceases to amaze. $\endgroup$ – Joe Bloggs Feb 3 '17 at 17:00
  • $\begingroup$ @JoeBloggs Yes it is, but these Ruffs aren't bisexual. males are only attracted females mates after all, this is more crossdressing. Still it is quite effective, having been independently evolved by different groups of fish (which are by far the most common utilizers of this strategy), lizards, insects, and mammals, it's a trick that just keeps working. As for a species where bisexuality, not just pretending to be female, is actually an evolutionary successful mating strategy look at the bonobo. Every single bonobo is bisexual and benefit from being so, their also our closest relatives $\endgroup$ – dsollen Aug 11 '17 at 16:12
  • $\begingroup$ @dsollen: The faeder would be the bisexual one in this scenario, not that human terminology particularly holds up when discussing different species. $\endgroup$ – Joe Bloggs Aug 12 '17 at 9:58
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There are several examples of alternative mating strategies leading to different morphological traits among males in the same species. For example the horned beetle Onthophagus acuminatus has two different male morphs that pursue two different mating strategies. Healthy, well-fed males develop large horns which are used to fight off other males in order to secure mating partners. Males who developed malnourished forego growing horns because they won’t be able to compete with well-fed males and instead attempt to sneak around them. These beetles dig tunnels into dung balls and the horned male stands guard at the entrance of the tunnel, preventing other males from entering. The hornless males dig side tunnels that intersect with the main tunnel allowing them to slip past the male guarding the entrance and mate with the female. An example schematic.

In case you think think is just weird invertebrate stuff orangutans also actually have male morphs with similar reproductive strategies. After reaching sexual maturity some male orangutans will begin to develop flaps of skin on their cheeks. The cheek flanges are hallmarks of fully developed males, but some males will remain in a juvenile, unflanged state for many years. This “bimaturism” results in two morphs with different reproductive strategies. The flanged male is territorial, attracting females and warding off other males with loud calls. The unflanged male is nomadic and will follow females around attempting to mate with them.

Your requirement for 3 separate morphs reminds me of Paracerceis sculpta. This species has 3 distinct male morphs with 3 different mating strategies. The large α morph fights for females and creates harems of them. β morphs are smaller than the α morph, and instead mimic the appearance of a female. Using this disguise they infiltrate the harem to mate. The γ morph is even smaller than the β morph and uses its small size to mimic a juvenile and escape detection.

As your species gains in intelligence over evolutionary time it’s likely that these roles might fall apart. The example of the orangutans should illustrate that these strategies can still be effective even in fairly intelligent animals though. As your society develops perhaps the role of female choice, or of more equitable sharing of reproductive rights, helps keep all of the male morphs competitive.

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  • $\begingroup$ There's no specific number required. It might be as low as two or as high as five or even more. $\endgroup$ – Anonymous Feb 3 '17 at 16:01
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What you're describing sounds a lot like bees, ants, and other colony species, that use extreme heteromorphism to specialize into different roles. As for what selects for this, I'm afraid I don't know off hand.

If you have specific ideas of what each morph would be like, that would help to determine what kind of selection pressures would be involved, but if they're like ants I'd assume they all work together as cohesive units.

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  • $\begingroup$ The situation is complicated by the fact that the different male morphs would be in competition for females. Eusocial insects only display polymorphism in the non-reproductive members of the colony: they exist for the purposes of caring for the reproductive caste. $\endgroup$ – Anonymous Feb 2 '17 at 14:47
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    $\begingroup$ That does present an issue. As if different male morphs compete, there is the implication one group will edge ahead and that morph will become genetically dominant. If the males died after mating as in some species, and the females lived long enough, then they might be able to breed to keep each morph existent. But selecting for a competitive morph is... hard to guess at. $\endgroup$ – J. Doe Feb 2 '17 at 14:57
  • $\begingroup$ If you had a deadly and changing environment, then it's possible some morphs are dominant and successful through certain seasons, and necessary for the groups' survival, while other groups are necessary for other circumstances. So... at that point you basically have the Pikmin, and would need similarly diverse challenges they're suited for. $\endgroup$ – J. Doe Feb 2 '17 at 14:59
  • $\begingroup$ Sorry, forgot to tag you @Anonymous $\endgroup$ – J. Doe Feb 2 '17 at 14:59
  • $\begingroup$ That assumes that male offspring will always be the same morph as their fathers. If this is not the case then things get more complicated. This implies that males have the genes for all their morphs, but another gene or set of genes selects which they will develop as. $\endgroup$ – Anonymous Feb 2 '17 at 15:11
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The thing about being different, is that one of these Male Genders will objectively be better at securing the average female than the others, and will come to dominate. There needs to be a way to balance that out, or it will happen. One very good reason could be a kind of Horizontal Caste system.

With the Horizontal Caste system idea, each kind of Gender (alpha, beta, etc) is naturally very adept at certain kinds of tasks, which hare very different from all the others. Warrior Genders, Worker Genders, and whatever others you want. This leads to 2 results, Either one, the Caste system tips vertical, and we get Hindu Brahmans, who think they are better than everyone else, and treat the other genders badly, or two, they become very cooperative, and keep it mostly horizontal. They recognize that each one can't get things done without the others, and so they naturally group up to do so.

Lets throw out the Vertical Caste system, and say they went to the second option. This one is also the more likely option, Since the one Caste is truly unable to support itself, even if it had to. Different Castes specialize in what they do best, and work together to make the city run. Say Warriors are the Police, Fire Department, and Heavy lifters, while Workers are the coordinated and precision workers, who work with the Heavy lifers to get things in place. They might also be the ones who can work technology the best. Perhaps you also have Manager Caste who are great at planning and abstract thining, and the Leader caste, who are great at empathy and have lots of Charisma. The Leader and Manager can together work out a plan that is the best for everyone, then they have the Workers execute the plan, while the Warriors do the heavy lifting and protection.

Using this kind of system, the need for each gender will wipe out most segregation issues, and the lack of ability to do other things will cause them to group up and live with several individuals of the other Castes. Since each Caste can't do many things that may be critical for it to thrive, they will search out and make friends with the other castes to help them do those things. This will lead to tendencies for Romantic couples to be of different Castes, which will let diverse children be likely in most genetic scenarios.

The most obvious problem with this kind of system is that it can become unfair. Obviously, costs and workload are going to be different between the Castes, and some will view it as unfair. This can only be Mitigated. One good method to do this is to abuse their mental value of the tasks the others do. Managers are amazed at how Warriors can carry big things so easily, while Warriors are always amazed at how things keep working out if they do what the Managers ask of them, regardless of how strange those requests seem to the Warriors.

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