Is there an evolutionary explanation that shows why the reproductive costs are mostly on the female sex? And therefore, why do males can potentially have more offspring?

Does that happen to create more competition in sexual selection?


2 Answers 2


I may have misunderstood your question, but there is no theory why usually reproductive costs are on the female, as this is a fact. Females have usually larger (and fewer) reproductive cells (gametes), they frequently have to carry the embryo until birth, and sometimes they have to attend to the offspring during the first stages of their lives. To put it very simple, females are stuck with their offspring from the moment of conception until they are born - sometimes even longer - while males generally can mate with many females in a short period of time, potentially having much more offspring.

A consequence of this is that, usually, females are choosy (they want good mates), whereas males just try to mate as much as possible. Female choosiness generates sexual competition, as females prefer to mate with males having certain characteristics that hint to a superior genotypes/phenotype that may be passed to their prole (e.g. beautiful plumage, social status, size of the wallet). Sometimes the costs of attending offspring after birth are higher than the costs associated with reproduction alone. In some of these cases, we may observe unusual behaviors, e.g. male-only parental care and choosy males.

This is a very broad and complex topic and a short answer here cannot cover all of it. John Maynard Smith developed the game theory approach most commonly used to investigate the consequences of different pay-offs for the two sexes and to explore how these can influence the behavior of the parents. Moreover, this topic is also covered in many behavioral biology textbooks. I have had very positive experiences with Krebs & Davis Behavioural Ecology (ISBN-13: 978-0865427310), which I would recommend.

Hope to have clarified some of your doubts.



  • $\begingroup$ Thank you very much for this answer! Unfortunately I don't have a very strong education in biology so I apologise for being unclear. In the question I was specifically referring to pregnancy and calories investment in giving birth(and maternal care as well). I guess the main evolutionary advantage that enabled sexual reproduction to evolve in this way lies in the competitiveness it generates among males (another reason may be reduced costs and easier reproduction). I will definitely check out those books! Thanks! $\endgroup$ Commented Aug 25, 2020 at 13:08
  • $\begingroup$ I would say that sexual reproduction has the big advantage of DNA recombination. Then, how sexual reproduction is actually carried out, it varies a lot even just inside the animal kingdom. One sex needs to give more resources to the embryo just from a cellular physiological point if view. Largest Investments cause competition in the other sex, which does not necessarily means females choose the best partner. For instance, an optimal choice is conditional to the environment and specific situation. I would thus be cautious interpreting sexual selection as evolutionary advantageous in general. $\endgroup$
    – E.B.
    Commented Sep 4, 2020 at 6:21

Within a two-sex species, there are roughly three possible evolutionary strategies for reproductive investment: Sex A and Sex B invest equally. Sex A invests more than Sex B. Sex B invests more than Sex A.

The last two are practically equivalent if you don't project your assumptions about differences between the sexes.

So the only alternative to different reproductive investments is equal reproductive investments. That option is not stable since sex A can invest 10% less at only 5% (assuming a linear relationship between investment and reproductive success, which is not accurate) reduced viability since the partner is not investing more yet. This means sex A can use the saved energy to reproduce with 10% more partners. This is a constant evolutionary pressure, so eventually one sex will invest a lot less than the other.

The other sex will probably not invest 5% less too, since that drops offspring viability by another 5% meaning they gain 10% more partners at the cost of 10% less offspring. There is no advantage here, so there is no evolutionary pressure. Alternatively, they could invest 5% more leading to 5% more reproductive success. This also gives no advantage.

The only reason for any sex to increase investment is if the reproductive success gets so low that it it is worth a larger investment to keep reproducing. There is no reason to assume the investment will primarily come from the smaller investing sex, so that won't change the difference in investments.


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