Same sex sexual-behavior_rdale1_freshman_writing_bioDocument Transcript
Biol 4800 Renee Dale. Same-sex sexual behavior in animals Same-sex behavior is pre valent in many species although direct benefitsare not immediately apparent. Many hypotheses regarding the evolutionaryselective force in same-sex behavior have been formulated, some adaptive andsome non-adaptive. There are many hypotheses as to the possible benefits tosame-sex sexual behavior in animals, including adaptive explanations (socialglue, intersexual conflict, practice for heterosexual interactions,overdominance, and kin selection) and non-adaptive ones (mistaken identity,prison effect, and evolutionary byproduct). I will be focusing on fourhypotheses: social glue, intersexual conflict, prison effect, and kin selection.Social glue The majority of the data supports the social glue hypothesis. It is thehypothesis which is supported by the three species which engage mostprevalently in same-sex sexual behaviors – bonobos, bottlenose dolphins andJapanese macaques. The social glue hypothesis states that same-sex sexualinteractions maintain the peace between individuals within a socialenvironment by easing tension and increasing opportunities for reconciliation. Bottlenose dolphins have one of the highest rates of same-sex behaviors.In dolphins males are dominant, and have higher rates of same-sex activity,especially as juveniles. Bottlenose dolphins do not reproduce for the first 10-15
years of their adult life, they often engage in same-sex activites during this timeperiod . Male-male pairs appear to strengthen “alliances between smallgroups of males” . These alliances can be complex and consist of two to 14males. Same-sex pair bonds often continue after heterosexual mating [1,2].This data supports the social glue hypothesis in bottlenose dolphins bymaintaining the peace. Females are dominant to males in bonobos, and sexual encounters areused in both sexes to ease tension when it occurs . In an experimentinvolving a tree with 500-5000 small fruits available and a tree with 10-100large fruits available, the rate of same-sex activity was significantly higher forthe fruit patch with bigger and a smaller amount of fruit . This supports thesocial glue hypothesis, as the highly peaceful nature of bonobos and the highrate of same-sex activity, which often includes same-sex pair bonds, is oftencorrelated. This hypothesis was proposed for Japanese macaques .In macaquesfemales are dominant to males, and same-sex sexual encounters account forabout 30% of all sexual behavior. Often, after an engagement, the dominantfemale would attack the subordinate one, which led the researchers to questionthe hypothesis’ application to macaques. Since females mated irrespectiveranking of the other female, and it was just as likely that a high ranking femalewould mate with a low ranking one, it did not seem that they were mating foralliances. Because of this, it was concluded that “homosexual behaviour was
not designed by natural selection as a sociosexual adaptation for allianceformation” , but that rather in macaques the behaviors are sexual.Intersexual conflict The intersexual conflict hypothesis proposes that an individual canincrease his own reproductive success via same-sex interactions, or reinforcehierarchies. In same-sex interactions, sexual behavior is exhibited withdetriment to one individual; same-sex sexual activity may be involved inreinforcing hierarchy when a subordinate male is mistaken or acting as afemale. In viviparous Goodeid fish, subordinate males gain fitness by displayingthe female characteristic of the dark pregnancy spot. If one male loses a fight toanother male, he assumes a subordinate status, and begins to show the darkspot almost instantly. He then appears to be a female, is courted by males andmay subsequently sneak copulations with females. They also gain defense bydominant males and avoid further attacks . Goodeid fish support theintersexual conflict hypothesis by reinforcing hierarchy via same -sex activitybetween the subordinate and dominant males. Dung flies mount other males to deny them the opportunity to mate byblocking their ability to mount a female as she emerges from hiding .Females prefer large males, and a small male cannot mount a large male inthis manner, further decreasing a small male’s chances of copulating.
In cockroaches, males who males increase their own chances ofcopulating with females . Females mount the males, thus mounting preventsthe male on bottom from copulating. The male on top also engages in“pseudofemale” behaviors, that is, all the female’s courtship behaviors, whichinclude mounting and ‘feeding’. The male, however, performs a much moreviolent version of feeding, which effectively amounts to attacking his rival whenhe least expects it. These data support the intersexual conflict hypothesis asone male’s fitness is increased, whereas the other male is harmed.Prison effect The prison effect hypothesis proposes that the frequency of same-sexsexual behavior will increase in populations where the majority of members arethe same gender (usually artificial). This is thought to be because same sexenvironments create sexual stress that increase same-sex sexual behaviors,especially in non-social animals where it is not normally observed. Same-sexsexual behavior “preserves sexual function.. and maintain(s) reproductivefitness and interest in sexual activity”. This was observed in koalas, whichare normally solitary. When caged in a same-sex environment (43 females in azoo compound), they all engaged in same-sex activity not normally observed inthe wild . This supports the prison effect hypothesis, as koalas are notobserved engaging in same-sex behavior except under sexual stress. Switches in choice between same and opposite sex in damselflies can beinduced and reversed by changing the social contexts . With the
manipulation of sex ratios, the rate of male-male mate choice increased ordecreased, according to the number of females available; however, even whenthere were a high number of females available, some males (17%) stillattempted to mate with males. It is thought, therefore, that Ischnura elegansengages naturally in same-sex sexual behavior . This only partially supportsthe prison effect hypothesis, as Ischnura elegans engage in a certain amount ofsame-sex sexual activity in nature.Kin selection The kin selection hypothesis suggests that individuals can gain fitness byproviding “resources to siblings, thereby increasing their inclusive fitness” .Those engaging in same-sex sexual activity forego heterosexual mating to gainindirect fitness. This hypothesis is also applied to the helper-breeder system inbirds, where individuals gain fitness indirectly by helping a relative’s offspring.This idea implies that “non-breeders… are heterosexually inactive”. In AcornWoodpeckers, both breeders and helpers engage in same-sex activity, while inTasmanian Native Hens and Dusky Moorhens same-sex activity occurs in onlya small percentage of either. Homosexuality only occurs in 8% of the 222species with the breeder-helper system . The kin selection hypothesis isconsidered unlikely in many cases because of the small percentage of speciesthat engage in same-sex behavior.
Conclusion The social glue hypothesis, in most cases, the best supported hypothesisfor same-sex sexual behavior. Bonobos and dolphins have the highest rate ofsame-sex encounters, and are two of the most peaceful species, which is theprimary prediction of the social glue hypothesis. However, it is apparent thatthere is no one answer to explain how same-sex sexual behaviors persistdespite natural selection. There likely are multiple mechanisms for differentspecies as to why it is beneficial because of variability between species,especially the differences in sociality.
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