研究顯示，許多哺乳動物進化出同性行為以減少衝突
但研究人員警告說，這項工作無法為人類的性取向提供太多線索。
一對雄性倭黑猩猩。 大約 2500 萬年前，猿類從其他靈長類動物中分化出來，從那時起，它們的同性性行為比率就比狐猴等更古老的靈長類動物譜系高得多。 圖片來源：P. Wegner/imageBROKER, via Alamy
卡爾·齊默/ 2023 年 10 月 3 日 / 紐約時報
從蟋蟀、海膽到寬吻海豚和倭黑猩猩，科學家在 1,500 多種動物物種中觀察到同性成員之間的性接觸。
一些研究人員提出，這種行為自動物王國誕生以來就已經存在。 但一項針對數千種哺乳動物的新研究的作者描繪了一幅不同的圖景，他們認為同性性行為是在哺乳動物開始生活在社會群體中時進化的。 研究人員提出，雖然這種行為不會產生後代來繼承動物的基因，但它可以提供其他演化優勢，例如緩和衝突。
「這可能有助於建立和維持積極的社會關係，」西班牙阿爾梅里亞乾旱區實驗站的進化生物學家、這項新研究的作者何塞·戈麥斯說。
但戈麥斯博士警告說，週二發表在《自然通訊》雜誌上的這項研究無法為人類的性取向提供太多線索。 他說：「我們在分析中使用的同性性行為類型與在人類中觀察到的同性性行為類型非常不同，以至於我們的研究無法為其今天的表現提供解釋」。
先前對同性性行為的研究通常涉及對單一物種或一小群物種的仔細觀察。 戈麥斯博士和他的同事們轉而尋找導致某些物種而非其他物種行為的大進化模式。
研究人員調查了 6,649 種現存哺乳動物，這些哺乳動物的祖先起源於大約 2.5 億年前的爬行動物祖先。 他們查閱了科學文獻，發現其中哪些曾經進行過同性性行為──定義為從求愛、交配到形成長期關係的任何行為。
研究人員最終列出了 261 個物種（約佔所有哺乳動物物種的 4%）表現出這些同性行為的清單。
分析顯示，男性和女性被觀察到進行同性性行為的可能性大致相同。 在某些物種中，只有一種性別會這樣做。 但在其他動物中，包括獵豹和白尾鹿，雄性和雌性都會發生同性性行為。
研究人員隨後調查了哺乳動物中這種行為是如何發生的。 他們觀察一棵演化樹，發現參與演化的物種分散在樹枝上，這顯示這種行為在每個譜系中獨立出現。
「根據目前可用的數據，它似乎已經進化了多次，」戈麥斯博士說。
研究人員得出的結論是，現存哺乳動物主要群體（例如靈長類動物或貓）的最早成員可能沒有進行同性性行為。 隨著新譜系的演化，其中一些開始表現出這種行為。
例如，猿類在大約 2500 萬年前就從其他靈長類動物中分化出來。 從那時起，它們進化出了比狐猴等靈長類老分支物種更高的同性性行為率。
戈麥斯博士和他的同事隨後尋找這些同性分支的共同特徵。 對演化樹的統計分析顯示，它們往往是群居物種，而不是獨居物種。
加拿大萊斯布里奇大學的靈長類動物學家保羅‧瓦西（Paul Vasey）並未參與這項研究，他表示，許多研究同性性行為的研究人員推測，社會群體的演化有利於這種行為。 但他們關注的是單一物種，而不是整個生命之樹。
「對於任何熟悉文獻的人來說，我認為看到同性性行為與社交性相關並不令人感到驚訝，」瓦西博士說。 「很高興看到作者使用的方法支持這一結論」。
生活在社會群體中為哺乳動物提供了許多好處，例如更好地保護其免受捕食者的侵害。 但它也帶來了新的挑戰。 例如，哺乳動物社會可能會形成等級制度，其中排名靠前的動物使排名較低的動物與暴力保持一致。 但這些衝突可能會導致群體破裂，這對每個個體都不利。
戈麥斯博士說，同性性行為可能是哺乳動物管理不穩定的社交世界的方式之一。 這可能是哺乳動物建立聯繫和聯盟、打架後和解或將攻擊性轉化為求愛的一種方式。
但是沒有參與這項新研究的德國萊比錫的馬克斯·普朗克進化研究所的演化生物學家迪特爾·盧卡斯對這個結論表示懷疑。 「總的來說，這項研究的結果並沒有讓我相信同性行為的發生有單一的解釋，」他說。
他的懷疑部分來自於科學家研究所依據的數據。 觀察野生動物的挑戰可能意味著某些物種的同性行為被忽視。 盧卡斯博士說：「如果個體白天在空曠的地方活動，那麼觀察這種行為是否發生會容易得多」。
明尼蘇達大學的演化生物學家瑪琳·祖克（Marlene Zuk）並沒有參與這項研究，她讚揚研究人員將研究重點放在哺乳動物身上，而不是整個動物界。 「我們正試圖擺脫一種解釋來統治所有的解釋，」她說。
今年四月，祖克博士和她實驗室的博士後研究員喬恩·理查森基於對蟋蟀的實驗，對同性行為提出了不同的解釋。 他們表明，雄性蟋蟀有時會發出求偶歌曲，並嘗試與其他雄性蟋蟀和幼蟋蟀交配。
由於蟋蟀並不生活在社會群體中，這無法解釋祖克博士和理查森博士所記錄的行為。 相反，蟋蟀和許多其他物種可能會進行同性性行為，作為利用盡可能多的交配機會的策略的一部分。
祖克博士將該策略比喻為煙霧偵測器。 「你需要一個足夠靈敏的煙霧偵測器來偵測所有火災，」她說。「如果它確實做到了這一點，那麼當你烤麵包時，它有時就會誤響」。
卡爾·齊默撰寫「起源」專欄。 他是十四本書的作者，其中包括《生命的邊緣：尋找活著的意義》。
本文的一個版本發表於 2023 年 10 月 10 日紐約版第 3 頁 D 部分，標題為：研究檢視同性行為的作用。
Same-Sex Behavior Evolved in Many Mammals to Reduce Conflict, Study Suggests
But the researchers cautioned that the work could not shed much light on sexual orientation in humans.
A pair of male bonobos. Apes branched off from other primates about 25 million years ago and evolved a much higher rate of same-sex sexual behavior than older lineages of primates, such as lemurs, since then.Credit…P. Wegner/imageBROKER, via Alamy
Carl Zimmer / Oct. 3, 2023 / The New York Times
In more than 1,500 animal species, from crickets and sea urchins to bottlenose dolphins and bonobos, scientists have observed sexual encounters between members of the same sex.
Some researchers have proposed that this behavior has existed since the dawn of the animal kingdom. But the authors of a new study of thousands of mammalian species paint a different picture, arguing that same-sex sexual behavior evolved when mammals started living in social groups. Although the behavior does not produce offspring to carry on the animals’ genes, it could offer other evolutionary advantages, such as smoothing over conflicts, the researchers proposed.
“It may contribute to establishing and maintaining positive social relationships,” said José Gómez, an evolutionary biologist at the Experimental Station of Arid Zones in Almería, Spain, and an author of the new study.
But Dr. Gómez cautioned that the study, published on Tuesday in the journal Nature Communications, could not shed much light on sexual orientation in humans. “The type of same-sex sexual behavior we have used in our analysis is so different from that observed in humans that our study is unable to provide an explanation for its expression today,” he said.
Previous studies of same-sex sexual behavior have typically involved careful observations of a single species, or a small group of them. Dr. Gómez and his colleagues instead looked for the big evolutionary patterns that gave rise to the behavior in some species but not others.
The researchers surveyed the 6,649 species of living mammals that arose from reptilelike ancestors starting roughly 250 million years ago. Looking over the scientific literature, they noted which of them had been seen carrying out same-sex sexual behaviors — defined as anything from courtships and mating to forming long-term bonds.
The researchers ended up with a list of 261 species, or about 4 percent of all mammalian species, that exhibited these same-sex behaviors.
Males and females were about equally likely to be observed carrying out same-sex sexual behavior, the analysis showed. In some species, only one sex did. But in still others — including cheetahs and white-tailed deer — both males and females engaged in same-sex sexual behavior.
The researchers then investigated how the behavior arose in mammals. Looking at an evolutionary tree, they found that species engaging in it were scattered across the tree’s branches, suggesting that the behavior independently arose in each lineage.
“With the current data available, it seems that it has evolved multiple times,” Dr. Gómez said.
The researchers concluded that the earliest members of major groups of living mammals, such as primates or cats, probably didn’t engage in same-sex sexual behavior. As new lineages evolved, some of them started exhibiting the behavior.
Apes, for example, branched off from other primates about 25 million years ago. Since then, they evolved a much higher rate of same-sex sexual behavior than species on older branches of primates, such as lemurs.
Dr. Gómez and his colleagues then looked for traits that these same-sex branches had in common. A statistical analysis of the evolutionary tree revealed that they tended to be social species instead of solitary ones.
Paul Vasey, a primatologist at the University of Lethbridge in Canada who was not involved in the study, said that a number of researchers who have studied same-sex sexual behavior have hypothesized that the evolution of social groups had favored it. But they were looking at individual species, rather than across the tree of life.
“For anyone familiar with the literature, I don’t think it is a huge surprise to see that same-sex sexual behavior is related to sociality,” Dr. Vasey said. “It is nice to see this conclusion supported by the methods used by the authors.”
Living in a social group offers a lot of benefits to mammals, such as better protection from predators. But it also creates new challenges. Mammalian societies may form hierarchies, for example, in which top-ranking animals keep lower-ranking ones in line with violence. But these conflicts can cause a group to fracture, which is bad for everyone.
Dr. Gómez said that same-sex sexual behavior might be one of the ways that mammals can manage their unstable social worlds. It may be a way for mammals to form bonds and alliances, to reconcile after a fight or to divert aggression into courtship.
But Dieter Lukas, an evolutionary biologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who was not involved in the new study, was skeptical of this conclusion. “Taken together, the findings in this study have not convinced me that there is a single explanation for the occurrence of same-sex behavior,” he said.
His skepticism came in part from the data on which the scientists based their study. The challenges of observing animals in the wild may mean that same-sex behavior in some species goes overlooked. “It will be much easier to observe whether the behavior occurs if individuals are on open ground and active during daytime,” Dr. Lukas said.
Marlene Zuk, an evolutionary biologist at the University of Minnesota who was not involved in the study, commended the researchers for focusing their study on mammals alone, rather than the entire animal kingdom. “We’re trying to steer way from one explanation to rule them all,” she said.
In April, Dr. Zuk and Jon Richardson, a postdoctoral researcher in her lab, put forward a different explanation for same-sex behavior based on an experiment on crickets. They showed that male crickets will sometimes produce courtship songs and try to mate with other males and with juveniles.
Since the crickets don’t live in social groups, that can’t explain the behavior Dr. Zuk and Dr. Richardson documented. Instead, crickets and perhaps many other species may engage in same-sex sexual behavior as part of a strategy to take advantage of as many opportunities to mate as possible.
Dr. Zuk likened the strategy to a smoke detector. “You want a smoke detector that is sensitive enough to detect all fires,” she said. “And if it does that, occasionally it’s going to go off when you burn your toast.”
Carl Zimmer writes the “Origins” column. He is the author of fourteen books, including “Life’s Edge: The Search For What It Means To Be Alive.”
A version of this article appears in print on Oct. 10, 2023, Section D, Page 3 of the New York edition with the headline: Study Examines Role Of Same-Sex Behaviors.