Gay Animals and the Science of Sexuality

Posted by Jess Haley on July 4, 2019 in Animals, Behavioral Neuroscience, Culture, Neuroscience, Neuroscience in pop culture | Leave a comment

Earlier this year, a New York Times headline struck my attention: The Gay Penguins of Australia. The story details the lives of Sphen and Magic, two male Gentoo penguins at the Sea Life Sydney Aquarium in Australia. Sphen is 6 years old and rather quiet. Magic likes to chase after toys and is 3 years Sphen’s junior. In most regards, these two penguins are just like the rest of their colony – except that they’re a gay couple!

Last year, Sphen and Magic began to show their affections to each other by bowing. Their relationship progressed when they started presenting each other with carefully selected pebbles, a sign of admiration. Then came singing duets. But what really sealed their relationship was when they constructed a pebble nest together, a behavior typical of Gentoo penguin couples preparing for an egg.

Aquarium caretakers were impressed with their diligent nest keeping and when another couple started neglecting their egg – a common occurrence among young penguins – the caretakers decided to let Sphen and Magic try their flippers at parenting. These two penguins turned out to be great parents and when their chick hatched, it was named Sphengic. The couple took care of baby Sphengic – feeding it, singing to it, and keeping it warm. Sphengic is now a fully mature female and Sphen and Magic remain a committed couple.

So how rare are same-sex relationships in the animal kingdom?

It turns out that Sphen and Magic are not the first gay penguins to gain media fame. Two male chinstrap penguins, Roy and Silo, famously raised a chick named Tango at the New York Central Park Zoo in 1999. Later, Tango reportedly formed a same-sex relationship herself with another female penguin. Roy and Silo’s story was made into a children’s book, And Tango Makes Three. Many other cases of same-sex partnerships have been documented in a variety of penguin species around the world. However, this behavior is not limited to humans and penguins. Female macaque monkeys, male red flour beetles, female albatross, and male sheep, to name just a few, also engage in mating behaviors and/or long-term relationships with same-sex partners.

Like many behaviors such as eating, intelligence, and sociality, we question whether or not the underlying mechanisms are innate or learned – nature or nurture? – and we turn to animals to help us understand the underlying biological principles that govern these behaviors. However, unlike many of these behaviors, homosexuality is a highly polarizing topic of political and religious debate. At the core of this argument is whether or not sexuality is a choice, which logically begs the question as to what are the biological, social, and situational underpinnings of sexual partner preference.

What do we know about the science of sexuality?

Much controversy has surrounded scientific studies of sexual orientation in humans. One famous case is that of Dr. Simon LeVay, a gay neuroanatomist who worked at the Salk Institute in La Jolla, California. LeVay published a series of experiments which showed that a particular region of the anterior hypothalamus – the part of the brain responsible for releasing sex hormones – was significantly larger in homosexual men’s brains than in those of heterosexual men [1-3]. However, the majority of hypothalamic tissue used in these studies was acquired from men who had died from AIDS which led to criticism regarding the reliability of experimental design and conclusions.

Given the difficulty of studying anatomical and physiological differences between homosexual and heterosexual humans, animal models of sexual orientation present an important opportunity for research on this topic. Scientists have reported up to 1,500 animal species that are known to practice same-sex coupling [4]. The occurrence of homosexuality in nature is often underappreciated and requires further study.

Interestingly, around 100 different species of birds have been reported to display homosexual behaviors including the Gentoo penguin, Laysan albatross, Western Gull, Australian black swan, and graylag geese with incidences of up to 15% [5]. In populations of Laysan albatross on Oahu, Hawaii, researchers have noted that roughly one third of nests are pairs of unrelated females. These females behave similarly to male-female pairs which can be seen grooming each other and raising their young.

Same-sex albatross couples have not been reported to attempt copulation. Rather, the female-female pairing seems to be directed at improving the species’ evolutionary success when there is a shortage of males as two female albatrosses are more likely to successfully raise young than a single parent. These same-sex pairs often stay together for multiple years, with the longest reported period of 19 years. This suggests that once paired, two females will stay monogamously coupled. However, these females will copulate with male albatross that are paired to another partner. Thus, both females in a female-female pairing will often both lay eggs though only one of them typically hatches.

In addition to many bird species, domestic sheep offer a unique model of same-sex sexual preference. Rams raised in all-male groups display one of four sexual behaviors: female-oriented, male-oriented, bisexual, and asexual. These variations in sexual partner preference occur spontaneously in a population with as many as 8% of rams preferring to attempt copulation with other males over females. In comparison, only 4.5% of adults in the U.S. identified as LGBT in a 2017 poll [6].  Due to the high incidence of male homosexuality in sheep and the homology between ovine and human brain structures, this species is the primary animal model for research designed to understand sexual preference. Researchers have revealed a correlation in the size of a small area in the anterior hypothalamus and sexual partner preference in male domestic sheep [7]. This area, named the ovine sexually dimorphic nucleus, was nearly twice as large in heterosexual rams than in homosexual ones, whereas this brain region was similarly sized in homosexual males and females. These data support a scientific theory known as the “organizational hypothesis” wherein high or low levels of exposure to sexual hormones before and around the time of birth affect the development of sexual behavior at puberty. In other words, this theory postulates that same-sex preference may result from exposure to a hormonal environment more similar to that of the opposite sex.

However, there remain alternative theories. One such alternative is an epigenetic basis for same-sex sexual partner preference [5]. Epigenetics does not refer to the inheritance of DNA itself. Rather, it refers to the study of changes that affect gene activity and expression. Epigenetic changes affecting the process of transforming DNA into proteins can be inherited, but can also occur throughout an individual’s lifetime due to external or environmental factors such as stress.

The epigenetic hypothesis is supported by the substantial heritability of homosexuality in contrast to its low concordance (~20%) between identical human twins [5]. Further, the prevalence of homosexuality is much higher than predicted in animal populations. If homosexuality were strictly heritable, it would be expected to be selected against by natural selection as same-sex partners cannot produce biological offspring that would carry on their genetic code. However, it is important to note that other evolutionary strategies may favor the occurrence of homosexuality in a population. Kin selection and group selection, for example, would favor the presence of adults that do not produce offspring but are able to assist in raising young, thereby increasing the overall fitness of the group. Regardless of the potential evolutionary advantages or disadvantages, more research is needed to understand the biological, environmental, and situational causes of homosexual behaviors in the animal kingdom to support these theories.

What’s the take-away?

Regardless of if you are human or penguin, gay or straight, maintaining committed relationships can be difficult. Like any couple, things aren’t always tail-waggingly-happy for our gay penguins, Magic and Sphen. As a young penguin, Magic often shirks his duties, leaving Sphen to take on the bulk of parenting. However, they remain a committed couple and Sphengic is thriving!

I am hopeful that more research in the near future will aid us in understanding what differentiates Sphen and Magic from the rest of their colony so that we may one day understand the genetic, epigenetic, situational, and social factors that underlie same-sex sexual preference in humans and non-human animals.

AUTHOR’S NOTE: Please note that sexual orientation is physically, emotionally, and politically nuanced. This article only touches upon a small facet of the whole picture and is meant to provide some understanding of what’s known about the ethological and biological underpinnings of sexuality. More importantly, this article serves as a primer for thinking about sexual orientation on a more universal scale in an effort to incite thoughtful conversation about human behavior and rights. It is my hope that an increased understanding of the science of sexuality will bolster changes in policy.

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References

1. LeVay S (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science 253(5023):1034-1037.
2. LeVay S, Hamer DH (1994). Evidence for a biological influence in male sexuality. Scientific American 270:44-49.
3. LeVay S (1993). The Sexual Brain. MIT Press. Cambridge, MA.
4. Driscoll E (2017). Bisexuality can benefit animals. Scientific American 26(2).
5. Rice WR, Friberg U, Gavrilets S (2013). Homosexuality via canalized sexual development: A testing protocol for a new epigenetic model. BioEssays 35: 764-770.
6.  “In U.S., Estimate of LGBT Population Rises to 4.5%”. Gallup.com. Retrieved 1 July 2019.
7. Roselli CE, Reddy RC, Kaufman KR (2011). The development of male-oriented behavior in rams. Frontiers of Neuroendocrinology 32(2):164-169.