Week 9a: sex in nature – when weird is normal

So far in this blog, we’ve covered a lot of different topics–defining sex and gender, different types of hermaphroditism, transgenderism, and more. We’ve covered a lot of ground. But sex and gender are absolutely enormous topics, and we’ve just seen the tip of the iceberg.

For Week 9, I dove into a folder of papers titled “Miscellaneous.” I assumed I would pick a subtopic and focus on a few related papers, like social interactions or genitalia. I read abstracts about hyenas, snakes, chimps, and anglerfish. I learned about genitalia of more shapes and sizes than most people would ever imagine, bizarre social rituals, and looked at photos of fish that were the stuff of nightmares.

Much of our discussion about sex and gender has come down to a discussion of unusual or unexpected behaviors and identities. As members of human society, we often try to classify traits as “normal” or “weird.” As biologists, we extend this to classifying whether a behavior is adaptive or just a byproduct or random occurrence. But the more you delve into the topic of sex and gender in nature, the harder it is to categorize. Sex advice columnist Dan Savage frequently gets questions from people who want to know “am I normal?” His answer: “No! You’re not normal, but so what? . . . Everyone has something that disqualifies them from normal.”1 He says this about humans—but it is perhaps even more true when you broaden the scale to the entirety of living organisms2.There is something really amazing about learning about all of the different and varied sexual and gendered behaviors in the natural world. Are they weird? Sure. But are they normal? Definitely.

In the end of my search, I was so excited about so many different cool and wacky things I’d read that I picked three papers on three totally different topics. The papers I chose were noteworthy for being so weird–and so normal–at the exact same time. Here’s a sampling of what I learned and what we discussed.


Bonobo kiss – a weird thing to do in the big wide world

Paper 1: Bonobos have friends-with-benefits and make-up sex.

Some people might have you think that sex in nature is all about procreation. And for much of the animal kingdom, this is true. Most animals have sex only when the female is fertile. This is called “reproductive sexuality.” But plenty of animals don’t follow this rule–chimpanzees and bonobos have lots of sex when procreation isn’t a possibility. So do dolphins, and 15% of bird genera.

Richard W. Wrangham studied the sex lives of chimpanzees and bonobos in depth in order to understand why3. He finds that there are many possible reasons that apes have non-conceptive sex: 1) it may confuse males so that they aren’t sure which offspring are theirs, 2) practice, 3) exchange for favors. Bonobos have an additional kind of sex—what Wrangham calls “communication sex.”

It’s this last type of sex that I found the most interesting. Wrangham posits that communication sex is used to build social relationships (primarily alliances between females), prevent aggression, and repair social relationships after an outbreak of aggression. Communication sex is defined to include a wide range of sexual behaviors, from intercourse to fellatio to cuddling and kissing.

So-called “communication sex,” which primarily occurs between females in bonobo populations, is very unusual in nature. Only a subset of the small group that has non-conceptive sex does it. But some of those behaviors are things that we humans accept as basic to our lived experience–kissing, cuddling, holding hands.

In the big wide world, changing sex and/or gender is common (as we saw in fish). Kissing is not.

   → Why it’s weird: most animals don’t do this.

   →Why it’s normal: humans do this.


Paper 2: You mean to tell me that we can study females, too??!!!!

The second paper I chose was a discussion of a revolutionary new trend in sexual selection research. The paper4 came out this year, 2016, and was discussing research that had developed only in the past decade. Was it transgenderism in animals? Nope. Homosexuality? Nope. Effects of climate change or pollution? Nope.

It was the idea that females might play an active role in mate choice.

For decades, biologists had studied the elaborately spiraled penises in ducks.

Only in 2007 did a researcher suggest that this was only half the picture: males and females were in a kind of “arms race” leading to the evolution of elaborate penises and vaginas. Researchers began to study the for twisted vaginas in ducks, convoluted labial folds in whales and dolphins, and the thickened vaginal lining in bats.

Why is this idea so new still in 2016? One practical reason is that female genitalia are harder to see and harder to map because they tend to be internal structures. But also: Scientists are accustomed to looking for patterns. They love a good rule. But biology has few rules. It is not physics or chemistry. We do not bother with vacuums. Still, we try. We have learned that simple explanations are better than complex ones, so when someone comes up with a good idea, we get fixated on it. In sex and gender research, we became fixated on a model where males compete for females, driving the evolution of mating dances and peacock feathers.

   →Why it’s weird: we are accustomed to thinking of female genitalia as simple.

   →Why it’s normal: females are (generally) half the population. When you start to take a look, it seems really dumb that we never studied it before.


Paper 3: The Terrifying Case of the Parasitic Male Anglerfish

The final paper5 I chose because it was just so bizarre. Anglerfish are weird to begin with, emblematic of the unknowns of the deep sea. We don’t know a lot about them. This paper is a natural history paper, and it describes more so than it explains. The ‘what’ is the first step to understanding the ‘why.’ And this is the case with many papers we have read this quarter. They document uncommon phenomena observed in nature, putting it out there that “this happens!”, forcing us to look closer, see what we didn’t see before, and think about a ‘why.’

Here’s the deal with the anglerfish. They have extreme sexual dimorphism (sexes have different bodies): males are dwarfed—adults 6-10 mm, competing for title of world’s smallest vertebrates—but females are up to 60 times the length of the males and half a million times as heavy!

So how do the giant female and tiny male mate? Males start out as free swimming but attach themselves permanently or temporarily to bodies of giant females, using pincer-like teeth to grasp onto their mate. In some species, the attachment is followed by a fusion of tissues and connection of the circulatory systems of the male and female. This makes the male permanently dependent on the female. This is called “sexual parasitism.”

At first read, sexual parasitism seems really strange. But when you think about it a little more, this is just an extreme case of something we’ve seen before: simultaneous hermaphroditism. In cases where the male fuses with the female, the fish become a kind of self-fertilizing hermaphrodite.

   →Why it’s weird: this is an unusual sexual behavior in an unusual group of fish.

   →Why it’s normal: in the end, we can consider this behavior an unusual form of hermaphroditism, which is common in fish.


One example of sexual parasitism in anglerfish

It’s a big, weird world out there. Much of studying ecology and evolution is looking for patterns to make sense of that weirdness, but embracing that weirdness is also essential. Is it weird? Is it normal? The answer is probably YES.




  1. From an interview that can be found here
  2. If you really want to see how the sex advice framework can be extended to nature, look no further than evolutionary biologist Olivia Judson’s book, Dr. Tatiana’s Sex Advice to All Creation.
  3. Wrangham, R. W. (1993). The evolution of sexuality in chimpanzees and bonobos. Human Nature4(1), 47-79. PDF
  4. Pennisi, E. (2016). Female organs revealed as weapons in sexual arms race.Science351(6270), 214-215. PDF
  5. Pietsch, T. W. (2005). Dimorphism, parasitism, and sex revisited: modes of reproduction among deep-sea ceratioid anglerfishes (Teleostei: Lophiiformes). Ichthyological Research52(3), 207-236. PDF

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