Same-sex sexual partnerships: Is it part of evolution?

Whether or not same-sex behaviour is an important factor in evolution remains to be seen. "Given its persistence in species in many different animal groups, including humans, viewing it as an evolutionary force in its own right promises to provide a much richer understanding of the evolution of reproductive behaviour," Bailey says.

“Not long ago, the news was full of reports of two male Humboldt penguins at a zoo in Germany that adopted an egg, hatched it and reared it together.” - NewScientist, 5 December, 2009 p 49

For News makers this is one more evidence that there is same-sex liaisons in animal kingdom, but that is not all the point that one is trying to make. Evolutionary Biologists Marlene Zuk and Nathan baily from the University of California, Riverside, published a paper on the subject that include examples from dozens of species ranging from dung flies and woodpeckers to bison and macaques.

For argument sake, it would be interesting to question why same-sex behavior would evolve at all when it runs counter to evolutionary principles. Zuk and Bailey suggest that where it is common, same-sex sexual behavior is in fact an important driving force in evolution. Although both researchers believe that terms such as homosexual, gay, lesbian and transgender which are common terms used by mass media, and even some ethologists, should not be extended to animal kingdom – “ It’s not simply that they are burdened with the weight of social, moral and political implications, which can obscure objective scientific study,” says Bailey. “The problem is that while we can observe the sexual behavior in animals, we often have little inkling about what motivates it. Besides, as far as we know animals do not form sexual self-identities in the way humans do”. Hence they have preferred to use a more objective term – “same-sex sexual behavior” which they define as behaviors found in two animals of the same sex that you would find in opposite – sex pairs during courtship, copulation or parenting.

It might be made clear that, same – sex behavior is not synonymous with same-sex preferences, which have been observed in only a handful of animals. Hans van Gossum from University of Antwerp in Belgium and colleagues found that damselflies kept in all-male groups subsequently preferred to court other males rather than females, though the preference could be reversed simply by housing them with females (Biological Letters, vol 1, p 268)

Nor is it possible to infer any same-sex orientation from same-sex behavior, because orientation is established after observing partner preference over a long period of time. Among the male bighorn sheep, it has been observed that the males throughout their lives mount other males and female Laysan albatrosses…..

In 2008, Sara Lewis at Tufts University in Medford, Massachusetts and colleagues found out by peeping in the private lives of red flour beetles which scrounge in the pantry that they are able to locate males by homing in on airborne pheromones released by the male red flour beetles. What’s interesting is that even other males home in on males using the same route (Journal of evolutionary Biology, vol 22, p 60) The mounting male clambers on top of his quarry and extrudes his genitals, sometimes transferring sperm to the hind end of his partner. It seems that the results of the survey found that having been mounted by other males, the male red flour beetle then mated with a female. Few of the off springs of this union had come from males who had not copulated with the female, thus, adding to the survival of the fittest theory and not the sexual orientation theory.

Yet there is enough evidence in nature to prove that there is same-sex behavior in nature and Zuk and Bailey were intrigued that there might be common factors in these various theories. In Trends in Evolution and Ecology, vol 24, p 439, they explain that “first there are adaptive hypotheses, which provide an explanation for same – sex behavior that would boost the biological fitness of one or more of the individuals involved. For example, several species, including bottlenose dolphins seem to use same-sex behavior to promote social bonding. Others may have evolved them as a form of intra-sexual conflict. Indirect insemination, as in the flour beetle, provides a third possible adaptive advantage. Then there is the practice hypothesis, that individuals are honing their skills for mating which seems to hold true for male fruit flies at least.”

“Several other adaptive explanations have been invoked to explain same-sex behaviour in humans, including kin selection - helping to further the genes you share with close family members - and "over-dominance" - the idea that certain genes somehow increase fitness in individuals who possess a single copy of them but are associated with same-sex behaviour in people with two copies. Then there is "sexually antagonistic selection" - the idea that alleles promoting same-sex behaviour in men are favoured by selection because they increase the reproductive chances of their daughters.

There are also various non-adaptive explanations. Mistaken identity could indeed be one cause. Van Gossum's damselflies exemplify another idea, known as the prisoner effect, in which depriving individuals of interaction with the opposite sex prompts them to mate with members of their own sex. Then there is the evolutionary by-product hypothesis - selection for some other independent trait, such as high sexual responsiveness, might make individuals more likely to participate in same-sex sexual behaviour. It has also been suggested that same-sex behaviours appear when organisms are imperfectly adapted to their environment.”

However, these are only theories and already have strong opposition to some of these theories.

Even without further investigation of these hypotheses there is enough evidence to conclude that same-sex sexual behaviour has a wide variety of origins. Zuk and Bailey were also struck by the idea that evolutionary biologists have been missing an important piece of the puzzle. Regardless of why same-sex behaviour exists, if it is common enough, it is likely to affect social interactions within a population, change the behaviour of other individuals, and even nudge the evolution of other traits in a different direction. "Researchers have not studied the evolutionary consequences of same-sex behaviour, but we found some tantalising examples that suggest it might be worthwhile to do so," says Bailey.

Example 1: The desert locus. In the midst of this orgiastic chaos, males are sometimes mounted by other males, and so miss the opportunity to copulate with females or simply to feed. However, they can minimise the chances of this happening by releasing large amounts of a pheromone called phenylacetonitrile.

Example 2: The common toad. A male toad has to be persuasive to get a female to mate with him - in fact, he has to squeeze the eggs right out of her before he can fertilise them. Males accomplish this feat by embracing the object of their affection in a tight mating "hug" called amplexus. Sometimes, the male toad unable to distinguish the female from the male can go mount a male as well, prompting the male toad to pipe up with a special chirp which prompts the other male to 'get off me' .

Example 3: The Laysan albatross. “These large, graceful seabirds establish breeding colonies on islands in the Hawaiian archipelago, and recently it emerged that in the Oahu colony over 30 per cent of the nesting pairs consist of two females. Female-female pairings have been observed in other birds, such as California gulls and roseate terns, but never at quite such a high rate. What's more, Lindsay Young from the University of Hawaii found that many of the albatross female-female pairs remain faithful over several years. They engage in mutual preening and even occasionally copulation, and, like female-male pairs, each year they raise a single chick. Both females will have laid a fertilised egg and randomly shunted one aside (Biology Letters, vol 4, p 323).

Changing evolution
The fact that female same-sex bonds accounted for nearly a third of the breeding pairs in the Oahu colony makes for interesting population dynamics, according to Bailey and Zuk, and it prompts the question of what evolutionary consequences the colony might experience as a result. For instance, in colonies where females without a mate remain single, the male gains little by straying from his female partner. Even if he did fertilise the egg of a non-paired female it would not survive as it takes two adults to raise a single chick. In the Oahu colony, though, males that mate with females outside their long-term pair bond might gain an edge over those that do not. "So one evolutionary consequence to keep an eye out for in Laysan albatross populations that have high rates of female-female pairs is the evolution of males that spend more time copulating with females to whom they are not permanently bonded," says Bailey.

From the female perspective there are possible evolutionary consequences too. Consider the procedure for deciding which of the two eggs in a female-female partnership is incubated. It appears to be random: in a population with only opposite-sex pairs, females never need to distinguish their own eggs, so the ability to do so is unlikely to have evolved. But imagine if a genetic mutation arose in one member of a female-female pair that enabled her to distinguish her egg from that laid by her partner, says Bailey. "The mutation would probably spread through the population and tip the dynamics of female-female relationships more towards conflict rather than cooperation."

All this is hypothetical since same-sex behaviour has not been studied from this angle before. Nevertheless, there is no doubt that the prevalence of female-female pairs in the Oahu colony changes the costs and benefits of traits such as extra-pair copulations for males and egg recognition for females. What's more, Bailey points out that the evolutionary consequences might reverberate way beyond this colony. That is because the excess of females in Oahu is a consequence of females having migrated in from elsewhere. By adopting same-sex parenting behaviour, female Laysan albatrosses could escape colonies with dwindling resources and reproduce even when the sex ratio in their adoptive colony is biased against them.

Whether or not same-sex behaviour is an important factor in evolution remains to be seen. "Given its persistence in species in many different animal groups, including humans, viewing it as an evolutionary force in its own right promises to provide a much richer understanding of the evolution of reproductive behaviour," Bailey says.

Courtsey: Homosexual Selection by Kate Douglas, NewScientist December 5, 2009

http://www.newscientist.com/article/mg20427370.800-homosexual-selection-the-power-of-samesex-liaisons.html?DCMP=OTC-rss&nsref=love-sex