Would lack of sexual dimorphism negatively impact a brutish humanoid species

Question

In the world I am making I am wanting to portray the orcs as a race of testosterone filled and fairy hating frat bros, where the only difference between a man and a woman is the thing between their legs.

Description of the orcs

They're distant evolutionary ancestors mainly consist of neanderthals, which can be seen in their head shape, abundance of hair, and strong physique. Their heights typically range between 7' and 8' with an athletic looking body. An orc pregnancy lasts 6 months with it being noticeable at the 4 month mark. Nearly their entire culture revolves around killing and warding off fairies such as elves, goblins, and trolls. Their clothing typical consists of a mixture of iron armor and limbs from fairies they killed. In order to find a mate they get in a fist fight with their potential mate and first one to concede or faint has to take raise the kids they may have. They democratically elect their leaders but their debates typically consists of blood brawls.

The Question

Would the orcs population or reproduction be negatively impacted due to their lack of sexual dimorphism?

Answer 1

Sexual dimorphism may affect the reproductive success of the species. However, it is not a simple relationship where no dimorphism leads to lower success and higher dimorphism leads to higher success.

There are various theories explaining dimorphism. For example, it is theorised that in monogamous species females tend to be larger: Larger females have a reproductive advantage because larger bodies can store more resources for producing offspring, hence, increased fertility. Larger size in males is shown to be related to male-male competition, intrasexual combat or territoriality (for example, here).

Please note that sexual dimorphism is a very complex phenomenon. And its presence or absence are not determined by 1-2 simple criteria, especially in intelligent species. For example, patterns of sexual dimorphism in human faces vary across cultures and are influenced by a huge number of different factors: Aesthetical preferences, evolutionary pressures, habitat, lifestyle, diet, etc.

Lack of sexual dimorphism may be adaptive and promote reproductive success. For example, a recent study of Malagasy mammals suggests that unusual climatic unpredictability on Madagascar have ultimately reduced SSD (sexual size dimorphism) in lemurs after dispersing to Madagascar. It is speculated that resource unpredictability resulted in an increase in the body size of females due to adaptations that either maximize energy intake or minimize energy expenditure.

As these examples show, it is not possible to state with 100% certainty that the orcs population or reproduction will be negatively impacted due to their lack of sexual dimorphism. It may or may not be affected. You can choose whatever fits your narrative better.

The described culture, though, does not seem to favour sexual size dimorphism since you are suggesting mate-mate combat, not male-male combat. In other words, female orcs should be strong enough to fight male orcs.

Some other factors that could reduce sex dimorphism are an egalitarian society, co-parenting (or raising children as a community), monogamy, seasonality and high variance in resource availability, and cultural preferences for same-size mates.

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TL;DR

No, lack of sexual dimorphism does not necessarily result in a negative impact on the reproduction of orcs.

Answer 2

The lack of sexual dimorphism per se is not a showstopper for reproduction, not all species have it. However, a female with high testosterone level might be negatively impacted in her capability to reproduce, at least with a human-like biology.

On the other hand, if your orcs follow the same mating logic of flatworms, there is no issue

if we look at this objectively at simpler lifeforms, the answer is clear: being the male is the better way to go because you still get to continue your lineage with considerably less effort.

Flatworms are hermaphroditic and can take on either parental role, based on who does the inseminating. Unable to have a logical conversation about the matter, the flatworms solve the problem in the only way they know how: by trying to stab the other with their penis and the first to inseminate wins. Now, the idea of love or intimacy during copulation is hands down a complete rarity among animals, but it’s not exactly the epic battle that it is with flatworms.

When they approach one another and are ready to mate, it is instantly game on. They sit halfway up and get their double-headed penis ready to strike. This also leaves them exposed to getting hit themselves. Penis fencing can last an hour and result in the flatworms being stabbed multiple times. Ultimately, one deposits sperm into the other and emerges victorious, completing the paternal obligation. The flatworm that lost immediately begins to search for food to make up the resources required for making the eggs.

The behavior could also well fit the brutish attitude of the orcs that you describe.

Answer 3

No.

The lack of sexual dimorphism or its presence or a high degree of it has nothing go do with the degree of sucess of reproduction of a species or how fertile they are. My reasoning is if that was true then animals with little or no sexual dimorphism would have low reproduction rate and most of those species would be infertile and thdy would have gone extinct. Dogs, cats, horses and starfish lack sexual dimorphism and are very fertile( the latter can achieve asexual reproduction by fission).

And there are some researchs saying that women like feminine physical features on men so I do not think that sexual dinorphism is that necessary for sexual attraction and the sucessfulness of reproduction.

Answer 4

No gender-separated beauty ideals

Their beauty ideal will not depend on gender, in the first place. Men and women look very similar and there is one beauty ideal. An orc can fall in love with any other orc.

Bisexual anarchy

Opening: "Nearly their entire culture revolves around killing and warding off fairies such as elves, goblins, and trolls." (....) " They democratically elect their leaders but their debates typically consists of blood brawls."

Both genders are macho.. Your warriors are blood-thirsty, and they yell blood brawls.

If these orcs are as uncivilized as you seem to design them.. and they would have little gender role templates to adhere to... why assume strict, heterosexual behaviours ? the orcs may copulate with whatever attractive orc individual they encounter, male or female !

I would consider assuming promiscuous, bisexual behaviour accepted among orcs. Having children will be a matter of statistics. It would depend on the popularity of sex of course, but as partners don't know in advance, you'd probably get about half the count, when you would compare it to a gender-separated culture like ours, which is filled with traditional taboos around the subject. These orcs solve the issue, by producing lots of children when their gender happens to differ.

Answer 5

It could be catastrophic

From your description I gather that they are violent and will get in fights as a normal form of socialization. Problem is, its not a good idea to punch a pregnant women. This is a greater problem is humanoids since the vital organs plus uterus is right on the punching area, while most mammals have it somewhat protected by being under the rest of the body. Granted that the orc gals can make the men get out of the brawl just as blooded as they do, but a fetus is bound to be more fragile than a full grown orc woman. And since the pregnancy can only be noted when its 2/3 finished, the chances of an abortion seem to be higher than you would want.

This is more of a problem for sapiens since our babies are oversized and tend to greatly restrict the mothers in the last few weeks of pregnancy. Making the baby get out of there early would make it safer (I don't believe in the ability of testosterone filled and fairy hating frat bros to NOT get into a bar fight 6 months into gestation), but it would mean I tiny helpless orc baby (even for baby standards), and I don't expect their survivability to be great either.

Plus, no sexual dimorphism increases the chances or you beating the crap out of a cutie just to find out you got yourself into a sausage fest. And vice-versa.

But it doesn't have to be

To be fair you can work around it by different dress codes to men and women, plus a "don't hit girls you brat" cultural up-bring, but I don't think this is what you are going for. The reason sexual dimorphism (either physical or behavioral) is so common in mammal its because it makes a lot of sense. It's just each sex becoming better at doing what they need to do, which often are not exactly the same thing.

Actual solution

Instead I would make the female orcs large and a little less aggressive. Being larger would help to give birth to a baby that is more like a 3-5 year old human, making him way more independent from mommy and daddy macho-men. Also it means men would not pick fight with women as much and the women would pick less fights overall. Larger might be wider in this case, if the female is wider, this mean that there's more biomass to protect the fetus.

Obs: I am no doctor and might be overestimating the risk, but I also expect orcs to punch harder then regular testosterone filled frat bros.