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So I've got this species of nocturnal humanoids that reproduces asexually, or rather, they use a form of parthanogenesis for their reproduction and have only one sex.

I've got them figured out for the most part but I wanted their fertility to be tied to their fitness in ways that make it so that if an individual is physically fit or strong then they'd be infertile and unable to reproduce(but able to protect the group) and if they were unfit or weak then they'd be fertile and able to reproduce/make more of their kind(while relying on the infertile or strong ones of the group for protection, etc), if that's at all possible for their fitness/muscularity to have an affect on their fertility.

Can fertility be tied to an individual's strength?

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    $\begingroup$ intensive sport can cause fertility issue to women (mostly because of the intensive diet who go with). for men, it's mostly doping product abuse who trigger infertility (and "performance") issues, also in some sport to much heat near the scrotum.(mostly cycle). $\endgroup$
    – sab
    Commented Feb 18, 2022 at 15:54
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    $\begingroup$ Are we inventing a fictitious physiology for this plot? What is the exact problem? Can't you just say a hormone suppresses zygote production; the strong ones get it and the weak ones don't? If that can't happen, then that is a fit problem for this site. $\endgroup$
    – Vogon Poet
    Commented Feb 18, 2022 at 16:44
  • $\begingroup$ Access to reproductive rights and members of the other sex definitely can, very often is in nature, it's not fertility but it still means the strongest are the most likely to have the most offspring [reads question beyond title] 🙄 doesn't exactly work with parthenogenesis though 🤗 $\endgroup$
    – Pelinore
    Commented Feb 18, 2022 at 17:50
  • $\begingroup$ Fictional biology is fictional and ral biliogy is diverse enough to justify a hell lot of stuff as long as you describe it properly and stay coherent within your own world. E.g. if there is someone going against the norm there needs to be a reason other than "the plot compels him to be different" but aside from that just about anything goes. $\endgroup$
    – Terry Bogard
    Commented Feb 24, 2022 at 1:58

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You could invert the causality: It's not that exercising makes them infertile. Becoming fertile requires them to stop exercising.

Members of the species are born without a womb, but they can choose to grow one later in life. However, this process requires them to keep lying down for several month. When they finished growing their womb, then their muscles will have atrophied from the long rest. The process might also require that they put on a lot of body fat during that time. Perhaps changing their diet to consume a high caloric surplus is actually what triggers their metamorphosis.

Now there is of course the question why a "breeder" would not try to get back in shape after they grew their womb. The answer could be that the transformation to a breeder isn't permanent and will reverse as soon as they become too active and lose weight.

Why would they evolve like that? It could be a mechanism to prevent overpopulation. When there are not enough resources around that some people can afford to lay down and eat for several month, then there are not enough resources to sustain a higher population. So people should rather work to improve conditions before they breed more children.

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Sure, it's a thing in humans

Granted, it's such a common problem, but there's actually kind of a funny bit of trivia about male bodybuilders - they have a higher-than-average rate low testosterone and sperm count. And that's kind of weird, right? You would think that these 'manly men' would be the peak of 'masculinity'. Except it turns out that's not how hormones work.

Excessive hormone use has consequences. A lot of bodybuilders take supplements, which in turn suppresses the body's natural hormone production pathways and atrophies them. So in your fictional species, you could accomplish this by the means of a fruit which replicates the effect of hormone stimulants. (Such fruit is biologically possible.) Your creatures which want to become protectors eat the fruit, those that don't, do not.

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It could just be a cultural thing.

Their society is divided into a "worker" caste and a "breeder" caste. While they are biologically identical, their culture expects them to live different lifestyles.

The workers are expected to do all the work in society, and when there is still time left to exercise their combat skills. That means they get plenty of exercise, making them fit and strong. The rules of the society say that they are prohibited from breeding. They might be physically able to bear children, but it is considered scandalous for them to do so. A pregnant worker would endanger their unborn child by still working. But a worker who does not work is lazy, and laziness is the worst possible character trait for a worker. So either way, members of the worker caste who become pregnant are immoral people who deserve to be ostracized at best and punished at worst.

The breeders caste, on the other hand, is expected to live a sedentary lifestyle where they spend most of their time laying around and producing babies while the workers feed them. They are not expected to move, because moving would bear risk of injury, and injury could endanger their unborn babies. This would mean they get hardly any exercise at all which means they would be physically weak.

Because only one of those castes bears children, cast membership can obviously not be hereditary. So it would need to be chosen after they are born. If you want a plot where a member of the species has to choose whether they want to be strong or want to bear children, then there might be the option in their society to switch caste later in life.

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It is possible.

  1. Fragility. The fetus is easy to damage - until delivery / separation, the parent must exercise caution and do not perform any burdensome activities. After delivery / separation, the baby is weak and needs to be cared for / fed with the parent's secretions (milk) for a period of time. Any higher physical activity stops secretions (milk)

  2. The stages of adulthood. Juvenile individuals are highly active as they grow up and become reproductive, they become lethargic, less mobile and more focused on lifting their body weight to a level necessary for producing offspring. If you want, it can be cyclical - after giving birth you again have a hormonal storm, transition to an immature form and an aggressive period.

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Castes.

enter image description here

https://entomologytoday.org/2014/01/30/novel-genes-determine-different-castes-in-ant-colonies/

Social insects have castes: phenotypic specialization for the job they are supposed to do. Genetically these castes are the same. Not shown is the queen - the reproductive caste individual. Some social insects have more than one queen.

Your individuals are phenotypically specialized for the role they play in their society. Maybe it is preprogrammed like in insects. Or maybe in primates? Sara Hrdy proposes that the reason female primates live on past reproductive age is that they become a sort of defensive caste - they cannot have more babies and so are selflessly dedicated to protecting the offspring (and their descendants) that they already have. It is pretty plausible.

I can see a society where small prereproductive females act as caregivers for younger siblings and relatives. Then they enter reproductive age and have the babies. Then when they age out of reproduction they become large and formidable - and also smart, because they are old. I can see that society without even squinting.

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