Why do we have two sexes?

  • from single-celled organisms to eight sexes and more, all constellations exist
  • requirements of the germ cells to be mobile but also transport enough nutrients might make task splitting necessary
Overview of topics:
Why not more than two genders?
Why not just one gender?
What speaks for exactly two sexes?

The question of why we actually have two sexes in all possible options is less clarified than one might think1.

Why not more than two genders?

Among the unicellular organisms there are variants with eight and more mating types. They can only reproduce under complicated combination rules. Here we quickly find a reason why such a reproductive strategy has not become established for larger beings – it is simply too complicated. As a result, we would not produce enough offspring and other creatures would displace us.

Why not just one sex?

Single-celled creatures reproduce asexually by splitting up. In humans, this is the case when two identical twins arise from a fertilized egg cell. How practical is that! Everyone can decide for themselves when it is time and simply divide into two. What is missing is the exchange of genetic material, but nature has long known a solution for this as well. Slipper animals, for example, lie next to each other and simply exchange genetic material. This exchange serves, so to speak, to repair the material. Harmful mutants are eliminated. Such a process would even be possible alone by initiating a division process and thus doubling the cell nucleus and then remelting it (autogamy).

Let’s look at another case: hermaphrodites. These are creatures that accept both sexes. We know anemone fish from the Disney movie „Finding Nemo“. All Nemos are male when they reach sexual maturity. They live in different large groups of one female and small males. The female is the largest fish in the group. When it dies, the strongest male becomes the new female. This process takes one week2. One can imagine why this reproductive strategy has not become accepted for terrestrial, larger animals. It is difficult enough to specialize on one sex. If everyone has to develop towards two sexes, this is much more difficult. In addition, it could be complicated to agree on who will have which sex if the rules are not as clear as those of anemone fish.

What speaks for exactly two sexes?

As tempting as the splitting and exchange of genetic material in slipper animals sounds, this process is no longer sufficient for complicated organisms that need to have a high adaptivity. So the first important point is this:

A higher variability of the genetic material simplifies the adaptation to changing environmental conditions.

A second essential point is suspected in the requirements on germ cells. On the one hand, they have to be versatile and very mobile in order to be able to search and find each other. At the other hand, they also should have sufficient nutrients reserved. However, this „luggage“ would be at the expense of both number and mobility. A compromise would be that the germ cells transport just as many nutrients that they are still mobile and versatile enough. Nature has opted for the disruptive option and separated the tasks.

The functions of the germ cells have separated into:
→ sperm cells are present in high numbers. They are fast, but only of short vitality
→ egg cells are less mobile, but store enough nutrients for the division steps3

» next topic:

Parentel investment

The separation of egg and sperm cells causes that we have two sexes: female and male. Now let´s see what other consequences come from that separation. » Read more!

parental investment
Parental investment


  1. Bischof-Köhler 2006 S. 108
  2. wikipedia.org Abruf 09.07.2019
  3. Bischof-Köhler 2006 S. 110

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