She studies slug sex by the seashore: UCSC researcher works to unlock secrets of banana slug sex

Yams, cat food and lettuce sustain these banana slugs in Janet Leonard's lab on UC Santa Cruz's Coastal Campus.
(Kevin Painchaud / Lookout Santa Cruz)

Banana slugs are embedded in Santa Cruz culture, but few know about the creatures’ secretive, sultry sex lives — or the local banana slug “rancher” documenting what slugs do under cover of night. Janet Leonard, an ethologist at UCSC, has built a career on understanding the mysterious sexual world of hermaphrodites, with a 20-year focus on West Coast banana slugs. She’s part of a long line of puzzled slug researchers. As Henry Pilsbry and E.G. Vanatta wrote in 1896, “he who attempts the identification of a West Coast slug to-day is not only a bold man but also one probably doomed to a miserable failure.”

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As UC Santa Cruz students graduate this week, many will celebrate the silliness of Santa Cruz’s beloved banana slugs, the official school mascot since 1986. But few know about the slug rancher on UCSC’s own campus.

Janet Leonard works tucked away in a weathered, barnlike building along the coastal cliffs of UCSC’s Joseph M. Long Marine Laboratory. She’s a UCSC research associate with a Ph.D. from the University of Wisconsin-Madison, the author of more than 50 peer-reviewed publications and a leading expert in her field.

And yet, many in Santa Cruz, even those at UCSC, have never heard her name.

She doesn’t have a lab website, she doesn’t tweet about her papers, and she doesn’t make slug TikToks. But on one cloudy afternoon, she very kindly opened up her lab — which she calls a “banana slug ranch” — and shared her collections.

On the shelves of a dark, damp and surprisingly small closet in Leonard’s lab, about 40 slimy, shell-less gastropods of various shapes, sizes and life stages reside in apartments made of stacked plastic food storage containers. Leonard spends hours waiting for the slugs to mate.

Banana slugs — genus: Ariolimax — are simultaneous hermaphrodites, meaning every slug has both male and female reproductive organs, and every slug can lay eggs. They can self-fertilize, but more often, they cross-mate, which gets complicated.

Slugs have a “common genital aperture” which is, as Leonard explains, “the opening where the penis comes out, the penis goes in, the eggs come out.” That opening is on the right side of each slug’s head. Slugs have to align those openings before any action can occur.

With two penes (the accepted plural of penis) and two vaginas in play, and some gastropod gymnastics required to line up their genitals, banana slug sex is not a fast process. But Leonard is OK with that. She’s spent over 20 years patiently performing the tedious task of documenting banana slugs getting it on.

“One of the things I’ve demonstrated is that the libido of banana slugs leaves a lot to be desired,” Leonard said. “You oftentimes don’t get a copulation at all. And when you do, it can involve two hours or more of foreplay, and then they settle down to a two-hour copulation. It’s just not a rapid way to collect data.”

Leonard’s slugs hail from across the state — from Arcata to Felton to Big Sur. Leonard even has a permit to collect them in California’s national parks.

Three-month-old banana slugs in Janet Leonard's UC Santa Cruz research lab.
(Kevin Painchaud / Lookout Santa Cruz)

They come in more shades of yellow than a box of crayons: dandelion, goldenrod, sunglow, mustard, yellow-green, green-yellow, laser lemon, mellow yellow, and — yes — banana yellow, like the ones we see in Santa Cruz. Some mature slugs are thick and heavy, others are long and skinny.

At the “ranch,” pearl-shaped slug eggs in semi-opaque pastel pinks and yellows lie tucked beneath fresh arugula leaves, so hungry hatchlings have a meal ready when they emerge.

And hatchlings, with their tiny tentacles and translucent bodies, just might be the cutest dang mollusks on the planet.

But Leonard doesn’t admire banana slugs for their cuteness. She admires them for their remarkable reproductive behaviors.

Different species of banana slugs have distinctive sexual preferences. So even though two slugs might look similar, the only way to tell for sure is to watch how they act in bed … ahem … in piles of decaying forest matter. Leonard is driven by essential questions like, “How does sexual selection affect the process of evolution?”, “How do sexual behaviors drive speciation and diversity” and the jaw-dropping “Why do some banana slugs chew off their partner’s penis?”

Leonard speaks with the sort of slow but purposeful cadence that befits a slug researcher. She describes her work with dry humor, probably because she knows exactly how silly slug sex sounds. She’s dealt with snickering, blushing and giggling audiences her entire career.

And she knows how to make a sex joke.

Leonard’s hazel eyes sparkle and she can’t help but grin wryly when she delivers a punchline. Just like her slugs, sometimes it takes a minute to get there, but it’s always worth it.

This interview has been edited for length and clarity.


Lookout: Your work is unusual; how do you usually introduce what you study?

Janet Leonard: My background is in ethology, or the study of animal behavior.

When you say you work on animal behavior, that means you work on sex. A colleague once said that the advantage of working with sex is that you don’t have to explain the adaptive value of it to anybody.

Lookout: And you work specifically on banana slug sex. What do you and banana slugs have in common?

Leonard: Oh, you mean aside from a fascination with sex? I think maybe slowness, and laziness is a common thread.

The famous ethologist Konrad Lorenz said that in order to be an ethologist, you had to be a very lazy person because you have to sit there and let the animals do what they want to do on their own schedule. And it just takes a special breed of laziness to be a slug ethologist.

Lookout: What was your path to working with banana slugs?

Leonard: Well, as a child, I was interested in two things: animals and books. And in fifth grade I found out there was something called zoology that was the academic study of animals. And I said, “OK, that’s for me.” A combination of books and animals.

I did my dissertation on jellyfish, on a little animal called Sarsia tubulosa, a very small hydromedusa. It did occur to me at one point that I elected to do a video study of an animal that had evolved for hundreds of millions of years to be invisible.

But, starting from the time when I was in graduate school, sexual selection became a huge area of advancement and learning. And I became interested in sexual behavior and sexual selection in simultaneous hermaphrodites.

Lookout: Why simultaneous hermaphrodites?

Leonard: Well, they’re sort of a test case. Males and females in separate sex species differ in their sexual behavior. They also differ in any of a number of other characteristics. And the question is, which are cause and which are effect?

With simultaneous hermaphrodites, you’ve got each individual sort of as their own control. Each individual can act as a male or as a female or both at the same time. And so it’s a way of testing some of the assumptions about sexual selection.

UCSC student Deven Bustillos holds an adult banana slug.
(Kevin Painchaud / Lookout Santa Cruz)

Lookout: What do you think it’s like to be a banana slug?

Leonard: I think it’s slow. It’s not a rushed lifestyle.

Lookout: How did you get interested in this topic?

Leonard: I first got interested in banana slug mating behavior when I was already working on Navanax, these simultaneously hermaphroditic sea slugs, and I was living in this little house in Oregon. There were banana slugs all over. And one day I noticed a pair of them mating on the sliding glass door of the house. And I wanted to set up a camera but I was leaving for a meeting. But they were there for at least 24 hours.

Lookout: You’re currently working on documenting sexual selection and behavior in banana slug species from all over the West Coast. How did this project come about?

Leonard: I started this project and worked for many years with Dr. John Pearse, an intertidal ecologist and an invertebrate zoologist par excellence who was one of the founding members of UCSC’s marine science program. He passed away a couple of years ago. He was a very beloved person. But he had just retired at the time. I’d been working on sea slugs for quite a long time, and then I moved down here where we have so many different types of banana slugs.

When you say you work on animal behavior, that means you work on sex. A colleague once said that the advantage of working with sex is that you don’t have to explain the adaptive value of it to anybody.

— Janet Leonard

And John and I were talking one time, at a wine and cheese [party], about an old observation. First, in the 1890s, a Mr. Fred L. Button had noticed that all of the large slugs in a collection from Oakland lacked a penis completely. This was interesting because the penis was the basis of all the taxonomy of this group of slugs. And so they erected — it’s required to use that verb — erected a new genus called Aphallarion for that group.

Then in 1915, there was a professor at Stanford, Harold Heath, who taught invertebrate zoology. And, as one did in those days, he collected his own animals to use for dissection specimens. And he noticed over the years that about 5% of the Ariolimax slugs they collected from the Stanford golf course lacked a penis, just like Aphallarion. But it seemed unlikely there were two genera of banana slugs on the 18th hole of the Stanford golf course.

So he rounded up a bunch of students, he rounded up a bunch of banana slugs, and they spent a very moist evening out on the Stanford golf course watching sexual behavior. And they observed one pair — one individual reached over and started chewing and finally finished chewing off the penis of its partner.

Now, this seemed noteworthy and he wrote a little paper in a journal called The Veliger, a malacology [study of mollusks] journal. And it’s been one of those things that West Coast biologists talk about over beer ever since.

It’s a phenomenon called apophallation, whereby individuals sometimes chew off their partner’s penis. It’s just “apo” — “cut,” “phallus” — “penis,” “ation” — “process.”

And so John and I decided, well, somebody ought to get to the bottom of this.

Lookout: So what did you do?

Leonard: In order to make plausible hypotheses about why apophallation would occur, we needed to know a lot of things about the reproductive biology of the animal — how often they mate, how often they lay eggs and so forth and so on. And more about the sexual behavior.

So we started this collection, this ranch of banana slugs. And that’s what we’ve been doing all these years is trying to understand the basic reproductive biology of the animals.

We know apophallation happens in at least two species. And we know about five out of 100 individuals will suffer an amputation in the course of its reproductive career.

But after 20-some years, we still don’t know why they do it.

Janet Leonard in her lab on UCSC's Coastal Campus.
(Kevin Painchaud / Lookout Santa Cruz)

Lookout: What have you learned after 20-some years of this work, and what’s your favorite finding?

Leonard: It turns out we’re not very clear on the taxonomy. And so we’ve had to do some work on that.

Also, in looking at this business about apophallation, we’ve found that in certain populations some individuals are born without a penis. It’s a phenomenon that’s been known in other groups of land snails, but was not known with banana slugs.

But I think my favorite finding is that there’s such diversity in sexual behavior among different species, and there’s a great diversity in species in a small area. Just here on the San Francisco Peninsula, we have four species. Between here and Los Angeles, we’ve got six, maybe seven species. And there’s been very rapid evolution of both the sexual behavior and the genitalia which makes it an interesting system to look at evolution.

Lookout: So do the slugs here in Santa Cruz have different sexual behaviors than the slugs in San Francisco?

Leonard: Definitely. If you looked at them externally, you wouldn’t really see any difference. But the mating behavior is quite different.

The slugs here in Santa Cruz, Ariolimax dolicophallus, do simultaneous reciprocal copulation. So each individual inserts a penis into the female aperture of the other individual and they copulate reciprocally.

Whereas if you go one county north to San Mateo County, the slugs, Ariolimax californicus, have unilateral fertilization. So one individual inserts a penis and the other just acts as female until some point at which time the female-acting individual reaches around and becomes male. And they go back and forth. I’ve seen a record of seven copulations with alternating sex roles.

Lookout: What questions do people ask you about your work?

Leonard: We typically don’t get past the whole concept of simultaneous hermaphrodite. We typically don’t get past, “They’re both male and female?!”

And the other thing I’ve noticed over the years is that if I’m giving a talk I have to save this business about apophallation for the extreme end. Otherwise, half my audience is just gone. There’s something about a Y chromosome that makes it hard to get past the amputation of the penis part.

Lookout: Is there any local lore about banana slugs?

Leonard: There’s a lot more lore than there is actual information about banana slugs, unfortunately.

One interesting thing is there is a local cookbook. It’s long out of print, but I did eventually get a copy that has recipes for banana slugs. The story is that back in pioneer days, especially farther north in Washington and Oregon, some of the French and German pioneers who were accustomed to eating snails at home would get banana slugs and eat them in times of hardship.

Some of the 40 or so banana slugs in Janet Leonard's lab on UC Santa Cruz's Coastal Campus.
(Kevin Painchaud / Lookout Santa Cruz)

I do have a colleague who has made linguini with banana slugs and claims it’s pretty good.

But my favorite recipe from the cookbook is the banana slug daiquiri. The story with the daiquiri is that you blend up the banana slugs — and then throw away the blender.

Lookout: Is it true that kissing a banana slug will make your lips tingle?

Leonard: Well, students are always talking about this. And so, eventually, I tried it. And it is sort of a funny feeling. But I don’t think it’s really anything special chemically. I think it’s just that they’ve got a very stiff mucus. As it dries on your lips it sort of feels funny.

But I would be cautious about kissing a wild banana slug because they’re famous for eating both poison oak and the death angel mushrooms. You pick one up from the field, you don’t know where it’s been. The one I kissed was one that had been in the lab for a long time, and I sort of knew what it had been eating.

A handful of banana slug eggs in Janet Leonard's UCSC research lab.
(Kevin Painchaud / Lookout Santa Cruz)

Lookout: What do people who are not scientists think of the work you do?

Leonard: Well, the polite way to express it is, “Who pays for that?!”

Lookout: Who does pay for that?

Leonard: Well, we get bits and pieces of money here and there. A lot of it is self-funded. That means I pay for it.

The sort of biology I do, it’s sort of like being a poet or a musician or whatever. There are people that make a living, or even a good living at it. But in general, you do it because you want to, because you’re fascinated with it.

Elise Overgaard is a June 2023 graduate of UC Santa Cruz’s science communication graduate program. Her goal is to bring unheralded science stories to life. With a Ph.D. in biomolecular science, she covers topics from particle physics and the molecules of life to marine mammals and deep sea creatures, always focusing on the brilliant humans behind the lab curtains. She produced this piece as part of UCSC professor and Lookout Community Voices editor Jody K. Biehl’s class.


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