Homosexuality in mammals has evolutionary utility | Science

2023-10-03 16:59:42

Science has documented homosexual behavior in some 1,500 species of all orders of the animal kingdom, from insects to primates, including spiders, dolphins and nematodes. This universality may have baffled Charles Darwin. In fact, sexual relations with peers of the same sex is considered a Darwinian paradox: from the point of view of the theory of evolution, it represents a cost to the biological effectiveness of the individual as it does not have a direct effect on the ultimate goal. , the perpetuation of the species. But there may be other indirect effects that explain not only its survival, but also give it a prominent role in the evolution of a good part of living beings. This is what is suggested by a scientific work that shows how homosexual behavior in mammals, although a minority, is more frequent in the more social species.

Some of the theories about why animals have sex with those of the same sex are reminiscent of those maintained until not long ago to explain human homosexuality. Among them are those who maintain that these are cases of identity confusion, or caused by the sexual frustration of not being able to mate with the opposite sex. A third relies on what is observed when life is caged. The reduced availability and control exercised over animals living in captivity in institutions such as zoos or laboratories would lead to homosexual practices. However, there are scientists who maintain that homosexuality may be an adaptive advantage.

An investigation carried out by Spanish researchers and published in the scientific journal Nature Communications has reviewed what science knows about homosexuality among mammals. Their results show that of the more than 4,300 species of the Mammalia class, homosexual behavior has been documented in 261 (4%). The range of these behaviors is very wide: it ranges from copulation to having a stable partner of the same sex, through genital contact or simple courtship. The percentage goes up as you go up the taxonomy (the way biologists organize the tree of life). Thus, above the species, in half of the mammal families there are homosexual relationships, a figure that rises to 63% on the next scale, that of order. They have also verified that there are no major gender differences: males have sex with males in 199 species, while females have sex with each other in 163. In more than half of these species, specimens of both genders have some homosexual behavior. .

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Within this tree of life, homosexual behaviors tend to be more frequent in some of its branches than in others. The orders where it is most common are those of carnivores, ungulates, marsupials, such as kangaroos and wallabies, rodents and, above all, primates. This last order alone, that of humans, includes 51 species where homosexuality is relatively common.

The work goes beyond quantifying homosexual behavior and tries to find out if it contributes something to evolutionary progress rather than being evolutionary nonsense. To do so, the researchers relied on the phylogenetic approach that, in the words of the University of Valencia researcher and co-author of the study, Miguel Verdú, “uses the trace left by evolution in all species to study how and when it originated. a certain behavior.” In their analysis, the authors find that homosexuality has emerged independently in different lineages. “By mapping sexual behavior between individuals of the same sex in the species of the mammalian phylogeny, we have been able to know how many times it has evolved, whether it has evolved recently or not and if it is associated with other behaviors that can also be mapped in the phylogeny” , adds Verdú.

Two females rub their genitals in LuiKotale, Democratic Republic of the Congo.Zanna Clay / Video: LIZA MOSCOVICE

What they have seen is that homosexual behavior does not appear randomly distributed among different species. This implies, according to the researcher at the Experimental Station of Arid Zones (EEZA), a CSIC institute and first author of the study, José María Gómez, “that this behavior appears concentrated in groups of evolutionarily related species, for example, in several species of rodents or in various species of ungulates or in many primates.”

With this non-random distribution and phylogenetic approach, the researchers were able to examine two hypotheses about the survival of animal homosexuality. On the one hand, they wanted to see if these behaviors fulfilled any social function. On the other hand, they investigated its relationship with aggression between adults, adulticide. Their results deny that this is a Darwinian paradox. Thus, the study has detected a significant association between the occurrence of sexual behavior between individuals of the same sex and the establishment and maintenance of social ties. Therefore, social species are the ones most likely to exhibit these types of interactions.

“Sexual behavior between individuals of the same sex is an adaptation that plays an important role.”

José María Gómez, researcher at the Arid Zones Experimental Station (EEZA-CSIC)

“Our study suggests that sexual behavior between individuals of the same sex exhibited by non-human mammals, rather than an aberrant or maladaptive behavior, is an adaptation that plays an important role in the maintenance of social relationships in both sexes,” explains Gómez in a press release. In some cases, such as the rhesus macaques of Puerto Rico, males have more sexual relations with each other than with females and do so to reinforce their coalitions.

The researchers also confirmed, although partially, their other hypothesis, by observing a correlation between adulticide and homosexuality: in species where aggression within the same sex is common, such as in those with hierarchical groups, such as baboons or baboons. But this connection only occurs in the case of males. Regarding this difference, the researcher at the University of Granada and also co-author of the study, Adela González, says: “We think that this occurs because the main factor that correlates with sexual behavior between individuals of the same sex in females is the development of social bonds instead of the avoidance of intrasexual conflicts.” The archetypal example of the former would be the bonobas, who rub their genitals as a means of strengthening their bonds.

The evolution of homosexual behavior in males and its relationship with the previous existence of adulticide among them is one of the results of the work most highlighted by the scientist, expert in animal cognition, at the University of Veterinary Medicine in Vienna (Austria), Antonio J. Osuna. . “This is extremely interesting, as it suggests that homosexual behavior in males could have evolved as a way to mitigate intrasexual aggression (between members of the same sex), but only between males,” he says in statements to SMC Spain. For Osuna, not related to this study, other types of non-lethal violence should be investigated further to see if sex between females plays a role there. Osuna ends by stating: “The study of homosexual behavior has been very limited for reasons that we all know, both in humans and in other animal species, and these reasons are purely social. “It is very interesting to discover that our common ancestors with other great apes already showed, with great probability, these behaviors, and that they are as natural as any other.”

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