Social Selection Theory

Same-sex behavior and multiple-sex phenotypes do not fit perfectly into Darwin’s theory of sexual selection. A more widely accepted theory for how these evolved is the “Social-selection theory” \cite{Roughgarden_2012}. Instead of individuals seeking out mates that will provide the most “fit” offspring, individuals of multiple sexes work with each other to provide resources that benefit the species as a whole. Mates often seek out others with high parental investment, even if it means mating with an individual considered weak  \cite{roughgarden2004}.
Same-sex behavior can also facilitate bonding between individuals, as seen with many marine mammals. Dolphins, sea lions, whales, and other marine mammals have been well documented engaging in same-sex behavior. These behaviors facilitate life-long bonds between members of the same sex, which benefits the species’ survival \cite{Orbach_2017}.
Fish display a wide variety of same-sex courtship that increases the reproductive success of individuals involved.  \citet{roughgarden2004} gives several examples of different fish species forming male partnerships to court females, build nests, and protect their eggs. Fish that participate in same-sex behavior as a form of species cooperation are selected to reproduce due to their social traits.
One particularly interesting example of a species displaying same-sex behavior as a social inclusionary trait is the all-female species of whiptail lizards that reside in Hawaii and parts of the southwestern United States. This species reproduces through parthenogenesis, a form of asexual reproduction that allows a female to produce a genetic clone without genetic material from a male \cite{Crews_1980}. Because sexual activity is so costly and unnecessary for this species to reproduce, it would be expected for individuals to refrain from any sexual behaviors. However, researchers \citet{Crews_1986} have observed the same form of male-female courtship that other species of whiptail lizards display between the females of this species. The all-female lizards may demonstrate same-sex behavior to bond and develop intraspecies cooperative relationships.
Many species of birds display same-sex courtship and pairings that can lead to the “adoption” of orphaned individuals. One of the more famous examples of this is penguins at zoos and aquariums that form same-sex relationships and adopt abandoned eggs \cite{l2019}. While the public viewed this as a shock and abnormal for a while, it is actually quite common among many bird species \cite{MacFarlane_2010}. In this example, same-sex behavior can be viewed as a social selection trait, as it benefits the survival and evolution of the species as a whole.
The prevalence of same-sex behavior in penguins at zoos and aquariums has opened up space for a new dialogue on same-sex behavior in animals. When these behaviors were first observed, visitors and guests may have seen it as abnormal or perhaps even a sideshow. But for the LGBTQIA+ community, this was the first representation of same-sex behavior that became popular in the media.