Week 08: Homosexuality in Nature

Have you ever wondered if, how, or why animals engage in homosexual (same sex) behavior? In nature, homosexual, or same-sex sexual behavior, can take many forms. Two common homosexual behaviors include courting and mounting. Courtship includes any behavior used to attract a mate and can involve ornate displays and posturing, while mounting is the actual act of an animal climbing on top of another in the act of copulation. While homosexual behavior is common in nature, we are still learning what traits are associated with homosexual behavior, what genetic factors might contribute to homosexuality, and what benefits individuals can gain from homosexual behavior. We discussed papers that examined homosexual behavior across wide taxa, observing individual behavior, early life and mating strategies, and genetic factors that contribute to homosexual behavior.

Traits associated with homosexuality in birds

The first paper discussed examined factors that possibly influence or maintain same-sex sexual behavior in many bird species. This is an attempt to answer a wider question in the literature: What adaptive value, if any, does interacting sexually with an individual of the same sex provide if the behavior does not directly contribute to an individual’s fitness? To answer how same-sex sexual behavior is maintained in many bird species, the authors compared different mating strategies and the developmental stage of offspring at hatching. Mating strategies range from socially monogamous, where two individuals mate for life and rely on each for other things like nest building and gathering food, to obligately polygamous, where individuals come together only to mate and mate with multiple partners. The state of development at hatching ranges from precocial chicks, advanced stage at hatching, able to feed itself in a short time, to altricial chicks, who require considerable care and feeding at hatching. To further increase the sensitivity of the study, the authors split same sex sexual behavior into two categories: mounting behavior, and courting behavior.

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Left: precocial chicken, right: altricial zebra finch [source: De Groef et al. 2013]

The authors found that male-male same-sex sexual behavior is more prevalent than female-female across species. This includes both mounting and courting. Male-male mounting and courting is increasingly observed as the degree of polygamy increases, while female-female same sex interactions are increasingly observed as the degree of monogamy increases. In regard to developmental state of chicks at hatching, male-male mounting is increased in altricial and precocial species, and decreased in species that are between complete altricial and complete precocial. Female-female courting and mounting is decreased in altricial species.

These data indicate that male and female same-sex sexual behavior is influenced by opposite factors.  These patterns may be due to differences in male and female mating strategies. This is especially apparent in polygamous species where males would want to mate with as many individuals as possible and females would prefer to only mate with the fittest males. The observations in this paper may also be influenced by a male bias in the data. The authors acknowledge that same-sex sexual behavior is more readily observed in males, as they are generally the sex that puts on a large display.  This would cause female same sex behavior to be underreported.  These data also assume that sexual behavior is the same whether it is directed towards the same or opposite sex, which is currently unknown. It would be interesting for a study to observe the differences in mating behavior directed to the same vs opposite sex within the same species. If mating behavior is different, this indicate that the data in the paper we read is incomplete. However, based on what the paper describes, there are factors that influence the overall expression of same-sex sexual behavior across bird species. It is important to note that these factors aren’t necessarily true for other taxa or unobserved bird species.

Genetics of homosexuality in fruit flies

We continued our discussion on homosexuality in nature talking about genetic manipulation in fruit flies. In Drosophila melanogaster, males court females with an elaborate but extremely stereotyped ritual, in which the male follows the female, sings a species specific song, licks the female’s genitalia, and curls his abdomen for copulation. Watch the movie in this link to see how it happens. Previous studies identified a gene called fruitless (fru) that was required for males to court females and properly perform the courtship ritual.  Certain loss of function alleles for fru cause males to court both males and females and perform a subpar courtship ritual. When these alleles are expressed in females, they are unaffected. This is due to the nature of the fruitless gene itself. Fru is alternatively spliced in males and females.  This means that even though the same gene is present in males and females, and the DNA sequence is the same between males and females, the final protein product is different. In males, fru gets made into a functional protein that influences male behavior, while fru in females does not get turned into a functional protein. This is why losing the function of fru in females doesn’t do anything.

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Mounting behavior; see video here

The authors hypothesized that this sex specific splicing of fru specifies male sexual orientation and directs a proper courtship ritual. To test this, the authors transgenically altered how fru would be spliced in males and females. The authors were able to confirm that how fru is spliced matters. Males that expressed the female form of fru did not court females at all, while males that expressed the male form courted females just like males who had not been altered. Surprisingly, the authors found that females expressing the male form of fruitless courted females. Even more surprising was that these female courting females were successful at courting females. Females expressing the male form of fru performed the male specific courtship dance comparable to males and even attempted copulation with other females. These findings are super cool.  They show that one gene can control a whole set of behaviors, even in animals that don’t normally express that gene.

Benefits of homosexuality in horned beetles

Yutaka Iguchi observed the interactions of female horned beetles around food.  Previous research focused primarily on fighting between male horned beetles for feeding sites. Male horned beetles have large horns that they use to fight and gain resources. Females also compete for resources, but instead of using their smaller horns to fight, they head butt each other to gain access to feeding sites.

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The study by Iguchi characterized female horned beetle interactions at feeding sites. To induce competition, a large female was paired with a small female. Both fighting and mounting behavior were observed in the majority of pairings. In the majority of cases, smaller females mounted larger females and only very rarely did fighting occur after mounting.

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Mounting behavior by smaller females occurred most frequently after a lost fight. Normally only the victor gains access to the food, but smaller females that mounted larger females after losing a fight were often given access to the food site. Iguchi then posits the idea that mounting behavior is an alternative tactic (alternative to fighting) to gain access to food. Here the same-sex interactions observed in horned beetles revolve around access to food, not necessarily an interest in mating with the same sex.

Summary

All together, we discussed same-sex behavior in beetles, birds, and flies. We ranged from gaining access to food to altering sex specific protein expression in order to gain a larger understanding of homosexuality in nature. A final thought, though these findings represent a wide array of animal life, they don’t necessarily hold true for all animals. In the case of the horned beetle, same-sex sexual behavior was used as a means to gain access to food. In birds both mating strategy and offspring developmental stage at hatching influence same sex sexual behavior prevalence. Finally, in flies, a single gene can cause males to court both males and females and females to court females. These findings are all different ways in which same-sex sexual behavior, or homosexuality, occurs in the animal kingdom.

-Zach

 

References

MacFarlane RG, SP Blomberg, G Kaplan, and LJ Rogers. 2006. Same-sex sexual behavior in birds: expression is related to social mating system and state of development at hatching. Behavioral Ecology 18:21-33. PDF

Demir E, and BJ Dickson. 2005. fruitless Splicing Specifies Male Courtship Behavior in Drosophila. Cell. 121:785-794. PDF

Iguchi Y. 2010. Intrasexual fighting and mounting by females of the horned beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae). European Journal of Entomology 107:61-64. PDF

 

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