In context of humans, males produce sperms, store them (for few weeks? then regeneration?) and ejaculate them "on demand" but females can not do that. Females have an almost fixed pattern of production and release of ova leading to menstruation most of the times (menarche to menopause).

I know release of ovum is accompanied by some additional "preparations" for possible conception.

Why could nature/evolution have not "designed" the reproductive system in females similar to that of males eliminating the need for menstruation?

Production and release of ova during pregnancy could have been regulated (like milk in mammary glands) by hormones to prevent multiple pregnancies.

I guess it has something to do with the time taken to "prepare" the body for possible conception and, production time and survival time limits of ova and sperms.


1 Answer 1


Each individual optimizes the probability of offspring carrying their genes. Anatomical and behavioral optimization between sexes contribute to distinct strategies used between different sex.

I guess it has something to do with the time taken to "prepare" the body for possible conception and, production time and survival time limits of ova & sperms.

True. This is the hypothesis of any mating cost.

We need to understand two concepts. First, every investment has a cost (of offspring being dead). Second, every individual wants its genes to survive and propagate.

Consider a case of a mammal mating. We currently understand that the embryo will stay inside the womb for longer times inside the female. That means, what are the best chances a female can raise an offspring?

  • make numerous ovums that has equal chance, or
  • sacrifice numerous ovum contents, but one to form a single stable ovum?

The latter option would be better, while the former option allows the mammal to rear as much as possible, if the offspring die before birth say 50%, that will cost a whole lot of survival opportunity for the female. The latter is much clearer, we can get an almost good guarantee the offspring will at least be born alive.

Now, comes to a unique part. Since females can only have extraordinarily little amounts of ovums, would accepting weak sperms do the math above? Of course not. That's why some female mammals observe sexual selection, either by copulatory structures or behaviors that could indicate weak and strong males.

However, the need of females to preserve their genes hinders the males' effort to preserve their own. How is that? By selecting males, there would be less chance to fertilize the ovum. Remember, males need topreserve their genes. Which option will give the best chances, considering the females' investment above?

  • just one sperm, just one mating
  • copious amount of sperm, multiple mating

Even a single sperm would not be guaranteed entry to the ovum, why should consider the former? The latter would be better. Since females will always raise their offspring, mating multiply and with much more sperm content will increase the chances of the females' offspring will be the males'.

This is just a surface of the evolutionary basics that drive the mating behavior. In fact, both contribution and behavior will drive each other, resulting in complex behaviors such as Lek mating, or even paternal care.

I suggest you to read books about evolution and animal behavior such as Futuyma and Dugatkin. If you want to understand quickly, read Unit III of Futuyma. But try to read it from A to Z, both are great introductory books on the logic and hypothesis of evolution and behavior. After reading those books, you can read more research papers about mating cases you find intriguing. (If you want the quickest takeaway, see the TL; DR)

Good luck!


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