The frequency same-sex relationships in the animal kingdom

Anthony Marrelli
Argus

Birds do it, bees probably do it – same-sex relationships. Biologists have started to consider evolutionary implications for the animals in question.
“It is clear that same-sex sexual behaviour extends far beyond the well-known examples that dominate both the scientific and popular literature such as bonobos, dolphins, penguins and fruit flies” Nathan Bailey, a researcher in the Department of Biology at UC Riverside, said.
There is a caveat, however. The review also reports same-sex behaviours are not the same across species, and that researchers may be calling qualitatively different phenomena by the same name. For example, male fruit flies may court other males because they are lacking a gene that enables them to discriminate between the sexes.
However, that is very different from male bottlenose dolphins, who engage in same-sex interactions to facilitate group bonding, or the female Laysan Albatross that can remain pair-bonded for life, and cooperatively rear young.  Same-sex behaviour in this species may not be aberrant, but instead can arise as an alternative strategy of reproduction.
Almost a third of Laysan albatross couples are female-female pairs, and they are more successful than unpaired females when it comes to rearing chicks.
Penguins have been known to form long-term same-sex bonds in which males will engage in sexual activity. Toads generally don’t discriminate between sexes, while marine snails all start out male and, when they mate with another male, one of them helpfully changes sex.
Dolphins will often touch their genitals together or one male might even mount another and penetrate its blowhole. Bonobos go the furthest in same-sex bonding with regular copulation among males.
Homosexual behaviours are flexibly deployed in a variety of circumstances: as alternative reproductive tactics, as cooperative breeding strategies, as facilitators of social bonding, or as mediators of intrasexual conflict.
Once this flexibility is established, it becomes a selective force that can drive selection on other aspects of physiology, life history, social behaviour, and even morphology.
New research on same-sex animal behaviour also finds that, although many studies are performed in the context of understanding the evolutionary origins of same-sex sexual behaviour, almost none have considered its evolutionary consequences.
Same-sex behaviours such as courtship, mounting, and parenting are traits that may have been shaped by natural selection.
“Our review of studies also suggests that these same-sex behaviours might act as selective forces in and of themselves,” Bailey said.  A selective force, which is a sudden or gradual stress placed on a population, affects the reproductive success of individuals in the population.
“When we think of selective forces, we tend to think of things like weather, temperature, or geographic features, but we can think of the social circumstances in a population of animals as a selective force, too,” Bailey stated.
Same-sex behaviour radically changes those social circumstances by removing some individuals from the pool of animals available for mating.
Bailey’s work noted that researchers in the field have made significant strides in the past two and a half decades studying the genetic and neural mechanisms that produce same-sex behaviours in individuals, and the ultimate reasons for their existence in populations.
However, like any other behaviour that doesn’t lead directly to reproduction, such as aggression or altruism, same-sex behaviour can have evolutionary consequences that are just now beginning to be considered.
For example, male-male copulations in locusts can be costly for the mounted male, and this cost may in turn increase selection pressure for males’ tendency to release a chemical called panacetylnitrile, which dissuades other males from mounting them.