FunNature & AnimalHow do you experience evolution with sex?

How do you experience evolution with sex?

In the animal kingdom sexual diversity is something widely generalized . There are not a few animal species that present behaviors that we would call altersexual , that is, that do not conform to the artificial heteronormativity , and the human species is as good an example as any other. There are a large number of animal species that exhibit homosexual and bisexual behavior, as well as transgender, intersex, or androgynous cases . Animals that change sex depending on age, temperature, environmental pressure, and even animals that pass themselves off, in appearance and behavior, as members of the opposite sex, in a kind of cross-dressing. Contrary to what many people would have us believe , sexual diversity, far from being something unnatural, is present in nature .

Up to 30% of female deer engage exclusively in homosexual-type relationships, with an additional 40% exhibiting openly bisexual behavior . Meanwhile, some of the males lack antlers and take on female-specific behavioral roles. On the one hand, this allows them to have better survival —as they do not participate in mating fights— and on the other hand, a greater probability of reproducing, by sharing space with them more permanently.

There are innumerable examples like this: pairs of male flamingos that take care of abandoned eggs and chicks; homosexual male elephants who, before copulation, engage in erotic and onanistic games with each other ; female penguins mating for life, and even situations of mènage a trois de swans, in which a bisexual male mates with both a male and a female to raise offspring together (with or without the female) . A curious behavior that, although we have known about it for more than 85 years, belongs to a species that is traditionally described as monogamous, even in current textbooks.

Some people more than others, but we all like to experiment with new experiences when it comes to sex . If I may anthropomorphize, we can say that there are a large number of extraordinary examples that could qualify as experiments in sexual evolution. Cannibalistic females in which the male must defend the offspring, parasitic males of the females, females that absorb the male’s body until it is reduced to a pair of testicles , male fish that give birth to their young alive… the catalog is extensive. But, speaking of sex, of all the animals that evolution has experimented with, the group of bed bugs, or cimicids , is perhaps the one that takes the prize for originality.

The general situation around which everything that follows swirls is the form of copulation, physically speaking. The males of this group of insects have a modified saber-shaped penis that they use to literally pierce the female’s body , and inseminate directly into her circulatory system, ignoring the fact that they still have a perfectly functional sexual apparatus . The sperm then find their way to the reproductive organs on their own, swimming free inside the female’s body.

But evolution has also developed a form of defense in females. In most species, a series of paragenital organs can be seen as a target, where the cuticle is thinner and, for the male, it is easier to penetrate without causing serious damage . In some cases, under these targets there is a series of ducts that approach or even empty into the female reproductive system .

But the matter does not end here. It turns out that males, when trying to reproduce, show no preference for copulating with a female or with another male. They are completely bisexual . This leads to a new evolutionary adaptation, this time to defend males from other males. In the particular case of the species Afrocimex constrictus , an extreme is reached such that the males, in fact, have the same external paragenital organs as the females .

However, it seems that in these cases, when copulating, males do so for less time with other males, and therefore cause less internal damage to them than to females. This leads to a new adaptation: they often develop the appearance of males , thereby lessening the damage caused by penetration. By having shorter copulations, the amount of semen inoculated is significantly lower, but this is offset by higher postcoital survival .

What happens to the sperm in the passive male is not yet clear. It was once hypothesized that he stored in his reproductive organs the sperm of the male that had penetrated him, and that he later transmitted it —instead of his own, or perhaps mixed with his—when he himself copulated. However, to date, no evidence has been found that it actually happens .


Bagemihl, B. 1999. Biological exuberance: animal homosexuality and natural diversity. St. Martin’s Press.
Bayon, A. 2021. Don’t be fooled. The Skeptic, 56, 20-25.
Dewar, J. M. 1936. Menage a trois in the Mute Swan. British Birds, 30(6), 178-179.
Reinhardt, K., Harney, E., et al. 2007. Female‐Limited Polymorphism in the Copulatory Organ of a Traumatically Inseminating Insect. The American Naturalist, 170(6), 931-935. DOI: 10.1086/522844
Siva-Jothy, M. T. 2006. Trauma, disease and collateral damage: conflict in cimicids. Philosophical Transactions of the Royal Society B: Biological Sciences, 361(1466), 269-275. DOI: 10.1098/rstb.2005.1789

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