Why would an non-Eusocial animal have a caste of sterile individuals?

Formicidae and Apoidea have sterile workers,but those outnumber the fertile drones and princesses or queens.

In another animal that doesn't have queens or drones but just motile(male) and oogame(female) why would evolution support and want the existence of a precise caste of sterile individuals that does not outnumber the fertile ones like in eusocial colonies?

In humans and many other mammals males outnumber females cause they die more easily and have a way shorter lifespan, and this seems to work for the most part.

How would the males,females and sterile be distributed in this situation?

  • $\begingroup$ Did you mean does outnumber the fertile members? The eusocial model has dozens or hundreds of sterile individuals for each fertile individual. Also: human females outnumber males by a slight margin. $\endgroup$
    – rek
    Commented Aug 18, 2016 at 15:22
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    $\begingroup$ I think this question probably begs a strict definition of what you consider to be "eusocial". I chimpanzees, for example, produced sterile individuals to help spread the work of raising children and defending the group among more individuals without causing sexual conflict, would they count as eusocial organisms? $\endgroup$
    – ckersch
    Commented Aug 18, 2016 at 15:26
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    $\begingroup$ the birth ratio in humans is 108 males to 100 females , which is a great difference but females outlive males, that's why it appears that there are more females. $\endgroup$
    – Charon
    Commented Aug 18, 2016 at 15:26
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    $\begingroup$ This basically already happens, even in primates. I think your question can be answered by this wikipedia article: https://en.wikipedia.org/wiki/Cooperative_breeding. In real-life examples individuals aren't sterile, they just forego the chance to breed and instead help their relatives raise children. The article goes in to the costs and benefits and how such a system is theorized to have evolved. You could imagine that continued success of the system could lead to fully sterile castes. $\endgroup$ Commented Aug 18, 2016 at 15:47

3 Answers 3


Let's think about this from a human perspective.

After a certain age, all women are infertile. While not as striking an event as menopause, men's fertility declines precipitously as well. If these people were still evolutionarily "useless", one would think evolution would put in a kill switch so they weren't a drain on resources, but that is demonstrably not the case. So, what use, evolutionarily, is a post-child creation human?

  1. Raising the next generation. This does not necessarily have to be your direct descendant, but genetically adjacent individuals. Remember: if your nibling survives and reproduces, that's 25% of your genetics surviving.

  2. Supporting your siblings. Remember how your niblings were 25% your genetics? Well, your siblings are 50% your genetics! That's as good as your own kids, statistically speaking. This is particularly effective if there's a large disparity in ages (a sterile adult raising a young sibling).


  1. Adoption. Okay, they aren't your genetic material in any direct sense, but any solution about castes is going to be about what's best for a community, not just your family. It could be advantageous to have some specialized in having many children, and then some of the children can be raised by the infertile caste. And, speaking of specialization...

  2. Community specialization. You've hit on this with your note about there being more men but they die more. In an environment extremely hazardous to your species as a whole, it could be advantageous to have a caste that sacrifices fertility in order to focus its energies elsewhere (defense, exploration, labor, etc.).

Functionally, these last two are what eusocial insects do, and is basically the definition of eusocial. While this only occurs in insects (I believe), it wouldn't be unreasonable for a different species to develop a similar system, even if the balance of "breeders" and "workers" is different than, say, ants.

  • $\begingroup$ Makes me want to write a story involving a race of aliens whose females die after menopause. It'd be an interesting culture... $\endgroup$
    – SethWhite
    Commented Aug 18, 2016 at 19:17
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    $\begingroup$ I find your logic about the kill switch highly suspect. Evolution does not target 'useless' features (otherwise we would have no appendix). Even if the geriatric consumed large resources and deprived other members of the same, there's no way for them to exert selection pressure on the future generation to specifically avoid becoming old - possibly the ensuing generations would simply evolve to consume less, develop new features to gather more resources, etc. $\endgroup$ Commented Aug 18, 2016 at 20:58
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    $\begingroup$ First of all, scientists are more and more finding out that the appendix is a "safe house" for beneficial bacteria and quite important. More importantly, we have to think about whole families. If the Smiths' grandparents collapse after becoming genetically irrelevant, and the Jacksons' grandparents don't, and then there's a food shortage, the Smiths, as a family, are much more likely to survive the Jacksons, as a family. Anyway, it's one thing to have small vestigial aspects that don't cost much, but things that cost a lot but aren't useful will get weeded out. $\endgroup$
    – Azuaron
    Commented Aug 18, 2016 at 21:08
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    $\begingroup$ The thing to remember about menopause is that (a) until it happens, the woman can have babies and (b) babies take a LONG time to become old enough to live without parents. So if a Stone Age woman has a baby at 40, she needs to live until 55 to raise that kid to 15. If she dropped dead at 42 when she hit the menopause, the kid would also die. (42 is a likely menopausal age for stone age subsistence diet - it is much higher in modern western women. Average 51 in the UK IIRC). $\endgroup$
    – DrBob
    Commented Aug 22, 2016 at 17:32

Hyper-aggressive males

In this scenario, the alpha male doesn't just lead and defend a harem, he will fight any adult male he encounters to the death. This starts a feedback loop where only the strongest, most aggressive males produce successful offspring and the young males need to flee the group before they reach adolescence and start producing the hormones that set off the alpha male.

Who doesn't need to flee? Male offspring that produce very little or no male hormones. These trigger neither fighting nor mating responses in the alpha male and can live their lives safely in the group, gathering food, guarding the group and generally performing the more risky tasks. Unfortunately, they don't produce viable sperm or have mating drive.

In this case, sterile individuals take the place of males, but there can't be too many or the male genes will not spread. The ratio of male : female : sterile might be 1 : 2 : 1.

Radiation-proof specimens

This scenario plays out in a (post-apocalyptic) world with many irradiated areas. a relatively small number of individuals are born with very high tolerance for radiation and the ability to detect it. Unfortunately, this comes at the cost of their fertility.

They protect their siblings/family/group from radiation by testing food and water. Some even eat/drink and regurgitate food before the rest of the group eat/drink it. Only 1 or 2 per group are needed to make it successful, so the ratio could be 1 in 10 of either gender. Perhaps it's determined by radiation exposure in the womb that turns on certain genes, leading to a variable rate.

It's just a phase

This scenario may not count as individuals may be fertile before or after their sterile worker phase. Individuals that experience stress while growing up due to food shortages or other factors caused by overpopulation do not develop sexually and instead become focused on work/production. Only a prolonged period of abundance will trigger development into fertile state.

This would be helpful in a species that is not very mobile or isolated due to geography (islands maybe?). The populations that don't have this feature will overpopulate their environment and collectively die off, while the ones with the sterile phase will stabilize until the situation improves, old individuals have died off or a new location is discovered to migrate to.


I think you may have been slightly mixing cause and effect.

In your example insects, every egg the queen lays has equal potential to become a worker, drone, or future queen. It all depends on how the larva is fed. Likewise, in the one eusocial mammal, the naked mole rat, pheromones in the queen's urine cause other females to become infertile. Sterile versus fertile is not an in-born trait, but rather a developmental one.

In some mammals, the dominant members do their best to suppress the sexuality of the subordinates. This can be by chasing rivals or attacking others who attempt to mate. So while not technically sterile, the subordinates are just as unable to pass on their genes. They have little choice except to live on the edge of the herd or the pack, just following along. This isn't a parallel to worker ants or mole rats, as the non-breeders don't serve the alphas. However, these subordinates have the chance the next season to move up the ladder and become breeders.

So now the question becomes: Would there be any advantage to the species if there existed a way to make subordinates permanently sterile?

My answer is no. Here is a mathematical proof by contradiction:

Assume to the contrary that $X$ is a non-eusocial species with a permanent sterile caste. As whole $X$ must gain some advantage from this. Members of the sterile caste consume food and other resources, so they must contribute in some way to the overall health of other $X$. The only possible such contribution is to share gathered resources with the breeding members or their offspring. Thus species $X$ is eusocial. We have a logical contradition. QED

  • $\begingroup$ "X must gain some advantage from this" -- I think it only needs to be nearly-neutral, but that's one for the experts. $\endgroup$ Commented Aug 18, 2016 at 18:33

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