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.