A major issue for many earth species is finding a mate when you need one. This problem is particularly acute for species that live in sparse environments (e.g., deep sea benthic zone) and those that are naturally antisocial (e.g., many spiders, slugs, etc.). This is solved by some species using one of two strategies:

  1. The female stores sperm from any encounter until she has eggs ready
  2. The (much smaller) male permanently attaches to the female and serves as her sperm supply when needed

I want to combine these strategies

When a male and female meet, and assuming the female judges the male acceptable, they mate in a manner appropriate to the species but instead of providing a supply of sperm, the male detaches a gonad and transfers it to the female. This free-floating male gonad then implants in the female’s body. Over time, the female may mate with multiple males. Each time she gains another male gonad for her “collection”.

When the conditions are right, the female matures an egg (I’m using singular here for convenience but for many/most species it will be more than one). Her body also sends a hormonal signal telling each of the male gonads in her collection to prepare sperm. The egg and sperm are released into the uterus together; one sperm gets lucky, and the rest is obvious.


One problem is that each male will have an incentive to make sure its sperm is the one that connects. This will lead to various strategies to exclude the competition.

The male gonad could release some sort of hormone or other chemical that prevents subsequent gonads from implanting. The female would have to evolve a mechanism to identify the offending gonad (chemical concentration sensing?) and destroy it (immune response?). This would be most effective if the gonads are housed outside of the uterus and release sperm into the equivalent of the epididymis.

Alternatively, the male gonad could attempt to grow so large no other gonads fit. But this seems easily dealt with either by resource restriction or, again, an immune response.

A male could also transfer multiple gonads but gonads are a lot larger and more complex than sperm so I’m thinking that would be too costly a strategy to make any sense.


Could this plausibly work?

I can’t see any other obvious problems or evolutionary pressures that would prevent this from arising or push for its elimination but feel free to point out anything I’m missing.

  • $\begingroup$ I can’t search for it at the moment, but I’m pretty certain there are insect species that follow a similar plan. Possibly ants? $\endgroup$
    – Joe Bloggs
    Sep 10, 2021 at 16:27
  • $\begingroup$ @JoeBloggs not ants, they use a strategy much similar to no. 1 listed in the question. $\endgroup$ Sep 10, 2021 at 16:39
  • $\begingroup$ If these animals have some sort of immune system then I don't see how what is essentially an organ transplant could possibly work without high-tech genetic tests to determine compatibility. What an immune system does is identify foreign, that is, not-myself, cells and attack them. $\endgroup$
    – AlexP
    Sep 10, 2021 at 17:28
  • $\begingroup$ @AlexP I don't see how the presence of an immune system inherently precludes this concept. An immune system has more sophisticated behavior than simply attacking every foreign cell. Some ant species store foreign cells (sperm) for a decade or more without triggering an immune response. I'm proposing that these creatures store something a bit larger. $\endgroup$
    – legio1
    Sep 11, 2021 at 20:48

2 Answers 2


Gonads need specific hormones to develop the appropriate gametes.

A male gonad stuck in a female body will not achieve that: one of the two sides will prevail. The male gonad will need a (minimal) body to provide the needed hormones. Which is basically your case 2.

  • $\begingroup$ This is not the answer I wanted but I can see the point. I'd need to package the transferred gonad with some sort of hormone supply system (and protection from the female's hormones), basically a very sophisticated spermatophore which might stretch the limits of "plausible". $\endgroup$
    – legio1
    Sep 11, 2021 at 20:58

Plausible. Yes

Example: Several anglerfish fish species do essentially what the question asks. The difference the males don't continue independent life, only fertilize one females eggs.

Many of the males have just enough functionality to fuse with a female. Upon fusing the male ceases to be independent, and essentially becomes a sperm providing organ. Providing sperm whenever the female has the resources to spawn. Multiple males may attach to one female depending on the species.


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