Once shunned as a subject unfit for science, same-sex behavior among animals—documented in more than 1,500 species—is generating an explosion of new research
A pair of bottlenose dolphins touch beaks and pectoral fins in Dolphin Cay on the Bahamas’ Paradise Island. (Photo by Stephen Frink/The Image Bank/Getty Images)
MAX WAS DISTRAUGHT. The 12-year-old chimpanzee had been threatened and chased by a dominant female at Zambia’s Chimfunshi Wildlife Orphanage. Now he was agitated: baring his teeth, screaming, turning from one friend to the next. “He was just not in a good place, bless him,” says Jake Brooker, the primatologist who observed the scene in 2019. Nearby chimps offered comfort to Max. But his distress persisted.
Then another adult male, 17-year-old David, approached from the side. His mouth was agape. His eyes were fixed on Max’s groin.
Brooker, then a Ph.D. student and now a postdoctoral research associate at England’s Durham University, aimed his video camera at the duo. He watched as David performed fellatio on Max, who in turn touched his companion’s chin and whimpered. The contact lasted just over three seconds, and the fact that it involved two males didn’t surprise Brooker. “I’ve never really seen a male chimp that is entirely heterosexual,” he says.
What did surprise him was the trust implicit in the act. Chimpanzees bite, and often attack, one another’s most vulnerable spots: ears, fingers, toes, genitals. “I guess there was some kind of understanding between the two of them that it wasn’t going to end in an aggressive way,” Brooker says.
The encounter was yet another data point in a growing understanding: Homosexual behavior is more common among nonhuman animals than scientists once acknowledged. By several estimates, researchers have observed such activity in more than 1,500 species. “These aren’t rare anomalies,” author Eliot Schrefer writes in Queer Ducks (and Other Animals), a young-adult book that drew national attention to the issue in 2022, including an appearance by Schrefer on The Daily Show. “An explosion of research over the past 20 years has shown significant amounts of same-sex sexual behavior throughout the animal kingdom,” he writes.
Scientists have long known that heterosexual behavior varies wildly among animals. Snowshoe hares breed promiscuously. Male elephant seals preside over harems of dozens of females. Female Galápagos hawks mate with multiple males, who take turns incubating the eggs and feeding the young.
Likewise, animals exhibit diverse same-sex behavior. Before splitting a honeycomb found together, female bonobos will rub genitals, which Durham University primatologist Zanna Clay says reduces tension and helps them share. Bonin flying foxes, a type of bat, curl up in same-sex clusters to keep warm during the mating season, and oral-genital stimulation is common in the male clusters. Male zebra finches form lifelong socially monogamous relationships that include courtship dances and sex, and that don’t break up when females become available. Female Laysan albatrosses in Hawai‘i share nests, mount each other sexually and raise their young together.
The recent upsurge in scientific exploration of same-sex behavior has gotten a boost from a new generation of researchers, some of them LGBTQ+. But these young scientists were hardly the first to notice.
Paul Vasey, a sex researcher at Canada’s University of Lethbridge, has spent three decades studying female Japanese macaques, or snow monkeys, who often enjoy short-term sexual “consortships” with one another. “There’s an urgency,” Vasey says of these interactions. “They need sex, and they need it now. They’re loud about it. And they’re pushy.” Vasey tested various possible social explanations for these same-sex couplings, including dominance and reconciliation after a conflict. “I found no evidence for any of them,” he says. Instead, he found a more straightforward explanation: “They’re getting immediate sexual gratification,” Vasey says.
Early in her career studying bottlenose dolphins, behavioral ecologist Janet Mann observed two young males she called Cookie and Smokey, whose mothers had died months apart. “They would hold pectoral fins, like hold hands, and swim together,” says Mann, a professor at Georgetown University. When they did travel away from each other, their reunions were filled with petting and rubbing. They also engaged in sexual mounting behavior.
The dolphins Mann studies, in Shark Bay at Australia’s westernmost tip, have a lot of sex. But little of it results in pregnancies. “The amount of homosexual behavior is so high,” she says. “In fact, we see very few heterosexual matings altogether. We know they happen because there are offspring. But we see very few.”
Mann has identified several reasons for this same-sex behavior, including dominance and practice among juveniles for adult mating. Sometimes, though, it’s just about relationship building. When an older male comes into a group of younger dolphins, he might flip on his side and allow a younger male to mount him. “It’s very friendly, and he doesn’t reciprocate,” she says. “It seems more like a bonding thing between an older and younger male.”
Mann first observed Cookie and Smokey in the late 1980s. They remain together today. Over the years, they have separated and reunited. They have both fathered offspring. They have endured shark attacks. “They’re pretty mangled looking now,” she says. “But that bond is still there.”
When she began her fieldwork, Mann says, little was known about bottlenose dolphins. Still, she did not detail her findings about their same-sex behavior until 2006, when she wrote a chapter for the anthology Homosexual Behaviour in Animals: An Evolutionary Perspective, which Vasey co-edited and was published by Cambridge University Press. She had worried earlier that publishing too soon could stymie her bid for tenure. “I didn’t care what people thought about my sexual orientation,” she says. (She is heterosexual.) “But I did care about being able to do the research I love and not having it derailed by people’s prejudices.”
Moral attitudes have long clouded scientific thinking about same-sex animal behavior. In 1906, naturalist Edmund Selous watched an all-male group of showy shorebirds called ruffs, including one inseparable pair, in the Netherlands. “They are constantly, so to say, mistaking one another for the female,” he later wrote. “Perverted sexuality is the real keynote.” A few years later, a researcher in Antarctica witnessed same-sex copulation among Adélie penguins but then coded his findings in Greek letters and didn’t publish them.
More recently, in 1986, butterfly scientist W.J. Tennent witnessed four male Mazarine blues vying for the attentions of another male in Morocco. He published his observations under the title “A note on the apparent lowering of moral standards in the Lepidoptera” and compared the courtship behavior to what he called the “horrific sexual offences” that humans commit.
Since then, attitudes have changed. Universities have grown more diverse, and researchers who identify as LGBTQ+ have become more visible. Today, research on animal homosexuality carries less stigma.
The current tight academic job market, counterintuitively, might have emboldened young scientists. “Our generation has faced so much precarity and uncertainty, in terms of even being able to follow a traditional academic path, that there’s a little bit more, ‘Well, we may as well do what we want to now,’” says Julia Monk, an ecologist at the University of California, Berkeley.
In the 2010s, when she was a Ph.D. student at Yale University, Monk read the academic literature on same-sex animal couplings. It seemed to her like every time researchers studied another species, they treated homosexuality as both a deviance and a discrete phenomenon that required a species-level explanation. “There’s been this all-consuming quest,” she says, to understand “what strange, unexpected benefit this behavior could present that would make it worth engaging in something so crazy.”
To Monk, who identifies as bisexual, this approach seemed like unsound science. “Any other behavior, if it was so widespread, we wouldn’t think about it as independently evolving in each different lineage,” she says. The simplest explanation, she adds, would be that the behavior surfaced earlier in the evolutionary process.
Maybe, Monk reasoned, the primordial default wasn’t heterosexual. Maybe early animals just had sex. After all, differentiating between males and females requires physical differences between sexes, along with advanced recognition skills, both of which evolved over time.
During a late night at the Yale lab, Monk mentioned this to a heterosexual colleague, biologist Max Lambert. Her alternative hypothesis intrigued him. Lambert, for his Ph.D., had studied frogs that changed sex as tadpoles. For decades, researchers had attributed this to environmental toxins. That’s not what Lambert’s data showed. “No matter what I threw at frogs, I couldn’t find a signal of pollution causing them to switch sex,” he says. “They were switching sex, but they were just doing it everywhere.”
For two weeks after their discussion, Lambert couldn’t sleep. “I was trying to figure out why Julia must be wrong,” he says. “But I couldn’t figure it out. This answer she presented made so much intuitive biological sense.” There doesn’t have to be a reason for animals to adopt same-sex behavior, he surmised—not if it was part of their repertoire from the start.
“Things exist, and they don’t go away in evolutionary time scales unless they’re really, really harmful,” says Lambert, who now works for the Washington Department of Fish & Wildlife. “They can just persist because they just do.”
The duo connected with three other early-career scientists. Over a series of meetings, the majority-queer team refined the hypothesis. The resulting article, published in 2019 by Nature Ecology & Evolution, proposed “an ancestral condition of indiscriminate sexual behaviors directed towards all sexes.” Animals only would have narrowed their sexual targets, the authors argued, after males and females evolved to have different sizes, shapes, colors, sounds and chemical signals.
The paper garnered praise but also some pushback. “I remain unconvinced,” says Vasey, who believes that Monk and Lambert’s argument “assumes an evolutionary similarity among same-sex sexual behavior across the animal kingdom.” Given the diverse expression of homosexuality from one species to the next, the Canadian sex researcher says he finds this improbable.
Vasey believes that papers such as Monk and Lambert’s show a “breathless enthusiasm for the existence of phenomena that are interpreted as evidence of sexual and gender diversity in nature. That sort of enthusiasm, I think, says more about what people want to see in nature, because it becomes a building block for how they see themselves.”
Monk says she’s not making an ideological case. “We don’t really want the takeaway from this to be: Same-sex behavior is natural; look at all these animals who do it, and thus it’s OK for humans,” she says. “But we need to understand how our cultural norms are constantly being projected onto our understanding of animals and the natural world. It’s important to grapple with how much of what we see and understand, even through the scientific process, is shaped by that culture that we’re steeped in.”
Barry Yeoman is a journalist in Durham, North Carolina.
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