雄鸟下蛋，蟋蟀搭伙求偶：自然界的性别彩蛋究竟有多少？｜科学60秒

自然界的性别多样性

笑翠鸟｜图片来源：Nick Sun

长期以来，人们一直认为，大自然中的动物（包括人类自身）可以被严格地分为两种性别。不过，现实情况可能并非如此简单。

一项近期发表于《生物学快报》（Biology Letters）的研究发现，鸟类中存在大量“性别逆转”现象（个体解剖学生理性别与其遗传物质显示的性别不符），比例可以高达 3% ~ 6%。你可能很难想象，在澳大利亚的茂密森林中，一只遗传性别为雄性的笑翠鸟（Dacelo novaeguineae）能够长出完整的雌性生殖器官，还产下了一枚鸟蛋。

自然界中的性别多样性特征无处不在，但传统生物学往往忽视了这一点。研究人员通常倾向于关注“典型”的、理想化的模式样本，而将变异留在无人注意的角落，将其标记为不符合客观规律的异常现象，甚至完全忽略它们。

殊不知，这些隐秘且鲜有人知的行为，正是动物多样的替代繁殖策略和复杂社会行为的生动例子。它们可能代表着一种成功的适应性演化策略，也为人类打破二元性别界限提供了依据。

在美国约翰·杰伊刑事司法学院（John Jay College of Criminal Justice）的生物学教授内森·伦茨（Nathan Lents）的最新著作，《性演化：5 亿年的性、性别和交配如何塑造现代关系》（The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships）中，这样打破常识的案例还有很多，我们或许可以借此一窥与传统二元性别论截然不同的生物学真相。

内森的主要研究方向是古人类以及古猿的基因组演化，同时也在约翰·杰伊学院教授有关性与性别生物学的课程。他在与学生探讨相关话题和科学研究时，一个有关动物性别多样性的问题逐渐浮出了水面：这些在自然界中客观存在的多样性明明有其社会和生态作用，但在学界，研究人员为何总是对“非典型”的动物性别或与性相关的不同生存策略视而不见？

有些变异与任何一种性别分类都无关，但研究人员往往会把这些信息放进脚注里、塞进庞杂的数据图表中，而在真正进行分析时，只关注主流的二元性别。这么做不仅忽视了性别多样性的存在，更忽略了其对于生物的社会生活和生态生活中的作用。

为了回答这个问题，内森开始在近十几年间着重收集这些真实且有趣的例子，并将其编撰整理成书。他希望让公众了解到，性与性别的多样性是非常自然、非常正常、也非常合理的现象，几乎所有的社会性动物在性行为和繁殖方式上都会表现出多样性。

一个个体可能在多个层面被归类为具有雄性化的行为，同时另一些又明确地属于雌性化类别（在人类群体中也是如此）。当这样的案例足够多时，人们就会意识到，二元性别框架可能并不是看待生物性别的最佳方式。

以人类为例，红细胞计数、基础代谢率、上半身力量、体毛分布或身体脂肪分布等特征就具有连续性，且男性和女性的平均值之间存在大量的重叠，这种重叠程度远超男性和女性的特异性程度。内森认为，性别及相关的特征表现可能也是一个两端分别为男性化与女性化的连续谱，符合不同特征的个体数量各不相同。

一旦我们跳出非此即彼的二元性别思维框架，真正进入书中描绘的世界，就会发现，以往规定的二元类型，其在各自性别内部都呈现出巨大的多样性，这种多样性涉及方方面面，比如遗传学层面的染色体、解剖学与动物生理结构特征，还包括性取向、性别化的行为，以及多种与求偶、交配相关的社会性行为……而在二元性别之间，这些行为还有很多重叠。

内森的观点并不新颖。动物界性多样性的探索历史最早可以追溯到美国生态学家与演化生物学家琼·拉夫加登（Joan Roughgarden）的研究，她已经在这一领域工作了至少25年，这一颠覆性的学科进展在很大程度上也要归功于她。

2004年，科学家们还在努力将他们的研究数据归入那些井井有条的分类时，琼·拉夫加登已经出版了一本名为《演化的彩虹》（Evolution’s Rainbow）的书，提醒世人：“忘掉这些分类，观察动物们自然的行为，摒弃试图让它们符合主流范式（二元性别）的一部分，而是让它们以自己的方式获得成功。这将帮助你获得更多样的理解。”

琼在当年还提出了一个观念：不只有一种类型的雄性，也不只有一种类型的雌性；雄性和雌性在演化中获得成功的方式也并非只有一种。多样化的性别策略甚至在本质上就是一种成功的演化策略。

幸运的是，包括内森在内的一批学者和作家现在已经认识到了这一点，他们从有性别的身体和生理学出发，得出了同样的结论，即二元性别划分会使人们在面对“反常的”特例时陷入困境：它不仅限制了人们的思维方式，也影响了人们进行科学研究的方式。

内森认为，这些具有严格定义的词汇和类别仅仅是人类创造出来的，并不代表现实情况。因此，我们必须保持开放的态度，在必要时对相关定义进行与时俱进的修订。

书中介绍了许多自然界中有关性别多样性的有趣案例，其中就包括夏威夷“沉默”的蟋蟀。

有些人小时有过养蟋蟀的经历，这些昆虫以其响亮的鸣叫而闻名。他们的叫声是一种吸引配偶来进行交配的性信号。多年来，许多研究者研究了蟋蟀发出的性信号及其信传递机制。

自 20 世纪 70 年代中期以来，昆虫学家就陆续发现了一些不会鸣叫的雄性蟋蟀。起初，科学家们还不愿相信这个事实，不愿意相信种群多样化可以允许哑巴蟋蟀的存在和繁衍。他们选择性地忽视了它们，认为它们是次优的，是错误的，是有缺陷的。

直到过去二十年间……[查看全文]

Nature's Sexual Spectrum Breaks the Binary

Rachel Feltman: For Scientific American’s Science Quickly, I’m Rachel Feltman.

The natural world is full of sexual diversity—traits that challenge binary definitions of male and female—but traditional biology has often overlooked it. Researchers tend to focus on “typical” specimens while relegating variations to footnotes or dismissing them entirely.

This approach has led scientists to miss fascinating examples of alternative reproductive strategies and complex social behaviors across the animal kingdom. What we’ve often labeled as anomalies might actually represent successful evolutionary adaptations that deserve serious study. And these creatures could help us understand how our own species breaks the binary, too.

Today we’re joined by Nathan Lents, a professor of biology at John Jay College of Criminal Justice. His new book is called The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships.

Thanks so much for coming on to chat with us today.

Nathan Lents: Oh, thanks for having me. I look forward to the conversation.

Feltman: So let’s start with your background: You know, what kind of research do you do, and how did it lead you to writing a book about sex?

Lents: Well, my research is actually in genome evolution, so I look at the human genome; I look at Neandertal genomes, other hominins; I look at ape genomes; and I look at how those genomes evolve over time, particularly over the last few million years. I’m interested in genome sequence evolution, basically. And that doesn’t have any direct bearing on [laughs] sex and gender, so this is a question I always get is: “How did I end up writing a book like this?”

Well, in my program at John Jay College I also teach courses in the biology of sex and gender, and I’ve been doing that for about 15 years. And this book really came out of that experience because I teach the course as a seminar, and the students would bring research articles, and I encourage them to, you know, find articles and—about sex and gender in, in humans and other animals, and we would discuss them.

And one thing kept coming up over and over and over again, and that was the idea of sex and gender diversity in animals—so different types of males and females and different strategies associated with sex—that the researchers themselves were kind of ignoring. They would relegate this information to footnotes or big charts of data, but they really only concentrated their analysis on what they considered to be the main type of male or the main type of female, sort of the archetypes. And biology does this a lot, where we focus our attention on, you know, the type specimen or the archetype, the ideal. And the problem with doing that is it—besides just ignoring the diversity that exists—it also fails to appreciate the role of that diversity in the social life and ecological life of the organisms.

And so after having gathered examples, you know, over a decade and a half, I realized, you know, there was a lot of information—this belonged in a book. And I also hope to educate the public about the fact that, actually, sex and gender diversity is quite natural, quite normal, quite expected in—basically all social animals will have a variety in the way that they approach sex and reproduction. And, and I think it was underappreciated, not just by the public but even by the scientists themselves.

Feltman: So your book makes a really compelling case for the existence of sexual diversity all over the animal kingdom, really breaking down the idea of there being a sexual binary in most places in the natural world. And I think the extent to which you make that case might be surprising to some of our listeners, so could you sort of unpack that main idea for us, of sexual diversity in the animal kingdom?

Lents: Sure, so when we talk about diversity of sex there’s a lot of ways you could think about it, right? So you could think about the body: sex to bodies, you know, male and female bodies. You could also think about behaviors, and that’s generally what my book covers. But it goes beyond that: you can look at chromosomes; you can look at sexuality; you can look at gendered behaviors.

So almost all of these aspects we think about behavior in these very binary terms, as either masculine or feminine, but what I show in the book is that, especially with behaviors, there’s actually a variety within both of those, and that variety tends to overlap quite a bit. So you can have animals that you might chart as masculine in several ways, but then some of their behaviors very clearly fall in the feminine category. And if you do this often enough, all throughout the body—and by the way, the same is true for humans—you start to come up with this idea that, you know what, maybe this isn’t the best way to think about it [laughs]: by trying to fit everything into these nice, neat categories, especially binary categories.

And when there’s so many exceptions and when there’s so many animals that don’t fit their bucket on at least some of those measures, you start to realize that the binary is really the problem, that these strict categories are not really upheld. Instead, what you see is a continuum, and there’s a continuum of masculinity and femininity, and how many individuals fall neatly into those buckets will be very different from trait to trait.

So if you look at, for example, in humans you might look at red blood cell count or basal metabolic rate or upper body strength or distribution of body hair or distribution of fat in the body, no matter what these features are that you look at you’ll see this continuum, and—with a lot of overlap between the masculine and feminine averages, we’ll say. And when you notice that the overlap is even larger than the degree of specificity that you see, per [se], you start to realize that the categories themselves really are too narrow to be helpful. But even worse than that, when biologists study the natural world with these categories in mind, they tend to collapse the diversity that they see into those buckets, to fit into those buckets neatly, even when sort of the natural variation really doesn’t fit into those buckets. And so what that leads to is, I think, a science that is less accurate, that is less informative than [one that emphasizes] appreciating all of the variety that exists there.

Now, my book concentrates on sexual behaviors, primarily, but there are other books and other people working in this space that concentrate on sexed bodies and physiology, and we all are coming to the same conclusion, which is that the binary is really failing us: it’s inhibiting our thinking; it’s inhibiting the way that we approach our science. And just because we have words and categories with strict definitions doesn’t mean that represents the reality. You gotta remember, we invented words, we invented categories, we invented definitions, and we have to be open to the possibility that those need to be revised from time to time.

And we’ve seen that in other areas of biology and other areas of science as well. And usually, you know, when we have these big paradigm shifts, it leads to a, you know, gradual embrace of a richer tradition. But unfortunately, with the area of sex and gender, it’s so tied up with gender stereotypes and a social order that we’ve constructed for ourselves that we all [feel] very committed to, you know? And when you feel committed to a certain social order there’s a resistance to change, and that resistance doesn’t come from the science; it comes from our psychology—it comes from our ways of thinking.

And that’s why not everything I say in my book will be accepted, even by other scientists. You’ll find scientists who disagree with a lot of what I’m saying, and what I’m hoping is not to be proven right and them to be proven wrong; what I’m hoping is to engage the conversation, to get more people thinking about this in an open-minded way, because that’s the only way that we’ll get to the truth, is to be open-minded, to really consider, you know, the full spectrum for what it is rather than what we wish it were, and then, you know, see where the science takes us.

Feltman: Yeah, well, and speaking of that, you know, potential pushback and the reticence to be open to this, you make the point in your book that these aren’t new ideas—you’re not coming up with the idea of sexual diversity [laughs] in the animal kingdom. Could you tell our listeners a little bit about the history of the exploration of sexual diversity?

Lents: Right, well, it’s a very good point because as I was collecting these examples, of course, I went to the literature; I said, “Surely, other people have seen this.” And that’s where I came across the work of Joan Roughgarden, and Joan Roughgarden has been working in this space, really, for at least 25 years, if not longer—and there are others as well who have been challenging our binary understanding of this, and I really encourage you to read widely if you’re interested in this because, you know, the story with primates is very different than the story with other kinds of mammals. Fish and birds, they’ve been evolving separately with their behaviors and their approach to sex for so long that a lot of times you can’t really compare among those different groups.

And so people have been working in these spaces, but I credit Joan Roughgarden for really identifying the notion that there’s not just one type of male out there; there’s not just one type of female; there’s not one way to be successful as a female or as a male—that, actually, variant strategies sometimes are successful just by virtue of being different. So I came across the book Evolution’s Rainbow, geez, almost about 16 years ago now is when I first read it, and it changed my whole perspective, and it really, in my mind, it took my blinders off, and then I went back to those same studies and I started to see them differently. And I could almost see the scientists themselves struggling to fit their data into these nice charted categories, when Joan Roughgarden was telling us all along: “Forget the categories and just watch the animals behave as they are, and drop your assumption that they’re trying to be like the dominant paradigm and just let them find success in their own way. And what that will lead you to is a richer understanding.”

Lents: One of my favorites is the crickets of Hawaii. So field crickets are, are well-known for their loud chirping, which is a sexual signal, and so a lot of people have studied their sexual signaling to understand how that works in a sexually reproducing species.

Well, one thing that happened recently, in [roughly] the last 20 years, is that an invasive parasitic fly began to infect the crickets on the Hawaiian Islands...[full transcript]

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