The recent and controversial proposals for anti-LGBT laws in several central and northern European countries have generated a great social and political debate regarding the nature of homosexual behavior. This causes many people on the street to ask the following questions: Is there a diversity of sexual behaviors in non-human animals beyond male-female interactions? Is homosexual behavior common in the animal kingdom? The answer is clearly YES.
Homosexual interactions, or same-sex sexual behavior (CSMS), have been described in more than 1,500 animal species. Sex between two females or two males is common in many types of mammals, birds, reptiles, amphibians, insects, mollusks, and nematodes. For example, they occur in various kinds of ducks and penguins, in many species of beetles, in lions, bison, deer, fallow deer, and other ungulates, or in primates such as langurs, macaques, bonobos, and mountain gorillas.
Evolutionary biologists studying behavior have wondered, for their part, about the function and context of these behaviors, in search of clues that help to explain the great diversity in animal sexual behavior. A recurring question has been: why do animals engage in homosexual interactions when they do not increase reproductive success? This question, however, does not have a single answer. What seems clear is that, in many cases, homosexual interactions can have a wide variety of benefits. This has even led to CSMS being recently proposed as the ancestral sexual behavior at the origin of all animals.
Homosexual behavior is associated with a wide variety of benefits, especially social, in many animals.
Homosexual behavior is associated with a wide variety of benefits, especially social, in many animals. Among those in which same-sex interactions have been documented, probably those with the highest proportion of these interactions are bottlenose dolphins. Male bottlenose dolphins form small groups that use mounts and genital contact to reinforce alliances between group members. Additionally, this behavior is also thought to provide them with practice that may be helpful in future encounters with females. An example closer to us is that of bonobos, chimpanzee-like primates. Bonobos are highly social and live in large groups, and as is common in any society, conflicts between group members are quite frequent. In the case of bonobos, this social tension is resolved by engaging in homosexual behavior, especially among females, in order to facilitate reconciliation between members.
In other cases, such as many insects, CSMS is more difficult to explain and it has been suggested on some occasions that it is due to errors in identifying the sex. For example, in many species the characteristics that serve to discriminate between the sexes (smells, shapes or colors) do not vary much between males and females, and it is thought that, on certain occasions, the strategy of males in situations of high sexual competence is try to mate indiscriminately.
Interestingly, although CSMS are very common in nonhuman animals, long-term orientation or preference for individuals of the same sex occurs with low frequency. Some examples come from males of certain penguins in captivity, which form lifelong mating bonds, and from a species of alpine sheep, whose males only mount females when they behave like males. In humans, however, although CSMS are also very common (homosexual experiences, bisexual individuals), the lifetime preference for individuals of the same sex is quite prevalent. More behavioral ecology studies are needed in a wide range of animal groups to advance our understanding of the causes of this variation between species.
In addition to the existing evidence, there are reasons to think that same-sex behaviors in nonhuman animals are more frequent than we think. For example, most scientific research in the fields of sexual selection, or behavioral ecology, is directed at understanding what are the characteristics of individuals or behaviors that affect reproductive success in male-female interactions, but few are dedicated to study interactions between individuals of the same sex. On the other hand, in many species, males and females are similar at a basic morphological level, which makes it difficult to distinguish whether the sexual interactions we observe are hetero or homosexual. More research is needed on the origin and evolution of sexual behaviors in animals. We also need more data on whether animals other than ourselves display homosexual preferences and orientations for life.
What is beyond doubt is that homosexual interactions serve several functions that may differ between species and that these interactions are common, natural, and diverse in their manifestations.