The one thing I teach that nobody ever forgets

Image credits: Sockeye salmon: (c) William Rosmus, CC BY-SA 4.0 via “Semele consumed by Jupiter’s [=Zeus’s] thunder”, 1733, Bernard Picart. In Tafereel, of Beschryving van den prachtigen Tempel der Zang-Godinnen, H. Chatelain, Amsterdam.

NOTE: the etymology in the story I tell below may not be right!  Or it may… it’s not entirely clear, and I think the story is interesting.  Blog post on it here (but read the story below first).

I’ve been teaching undergraduate courses in ecology and population biology for 23 years*. I’ve seen a lot of students from behind my (metaphorical) lectern: brilliant ones and less brilliant ones; motivated ones and passive ones; a few with their whole future careers mapped out and a few hopelessly drifting and lost.

Like every teacher, I’d love to think that my students retain most of what I teach them – ideally, for their entire lives; but at least until the final exam. But of course they don’t, and every year I find grading the exams a bit depressing because (and here’s a shocker) some students get some questions wrong. But there’s one thing I teach that nobody ever forgets – and it came up in my Population Biology course just last Friday. Perhaps surprisingly, it’s the definition of a particular piece of technical terminology (or jargon): the definition of semelparity.

A semelparous organism is one that reproduces just once, and then dies – like century plants, mayflies, annual plants, and Pacific salmon. (Nearly all birds and mammals, among others, instead survive for multiple bouts of reproduction: iteroparity). I use Pacific salmon as an example because they’re ecologically important, economically valuable, and fascinating – but that’s not why my students remember semelparity.

The life-history distinction between semelparity and iteroparity is an interesting one. Perhaps because our own species is iteroparous, most people’s intuition is that it’s foolish to put so much energy into reproduction that you can’t survive it. Why don’t semelparous organisms hold back just a bit, and survive to reproduce again? And yet: some very simple modeling suggests that it’s actually iteroparity that’s surprising. That’s because, for an iteroparous organism, selection will favour abandoning the effort to survive (and instead putting every scrap of resource into reproduction), as long as doing so produces just one extra offspring (this is Cole’s Paradox)**. So there are two possible strategies, and it’s not hard to convince yourself that either is bizarre, which is pretty weird – but that’s not why my students remember semelparity either.

Here’s why my students remember semelparity. When it comes up, I don’t just define the term; I explain its etymology. OK, I could hear you rolling your eyes from here – how can my students get excited about etymology? Well, semelparity may have the best etymology of any term in biology. The parity part is from the Latin parere, to bring forth young (hence parent). The semel part comes from Greek mythology, and the story of Semele, a mortal woman who slept with a god. If you already know this story, well, you probably stopped reading a while ago. If not, read on (but don’t be shocked when things turn NSFW). There are different versions of the Semele myth, but here’s the one I tell my students.

Semele was a beautiful, mortal woman, and a virgin priestess in Zeus’s temple. Zeus – or in the Roman versions, Jupiter – was a god (hence the temple), and head honcho among the gods. One day Semele slaughtered a bull at Zeus’s altar, and that’s a pretty messy business, so afterwards she stripped down to wash off the blood in the river. Zeus, flying by in the shape of an eagle, saw her and fell in love, or perhaps in lust (Zeus, like many of the Greek gods, was pretty randy).

So Zeus did what any (male) Greek god would have done: disguised himself as a mortal man, and seduced Semele, eventually impregnating her. This annoyed Hera, Zeus’s goddess wife, which is certainly understandable (although it’s less understandable that she was more annoyed with Semele than with Zeus). Hera disguised herself as an old woman, pretended to befriend Semele, and told her that her lover was actually Zeus, and that she (Semele) should ask to see him in his true, divine, form.

Semele consumed by Jupiters thunder

So at Zeus’s next visit, Semele asked her lover to do her a favour. Eager to please his beloved, or perhaps eager to do whatever it took to move past the favours and into the bedroom, Zeus promised to do whatever Semele desired. When she announced that she wanted to see him in his divine form, Zeus protested, but she bound him to his promise. Unfortunately, Zeus’s divine glory involves a lot of lightning bolts, and no mortal can look upon him and live. Semele was instantly incinerated.

Remember that Semele was pregnant with his child, though.  Zeus tore the fetus from her dead body, ripped open his own thigh, and sewed the fetus there until he could “deliver” the baby. That baby was Dionysus, who grew up to be the god of wine, ritual madness, religious ecstasy, and theatre***.

TL;DR: Semele reproduced once, and doing so killed her. And that’s why “semelparity” is the one thing I teach that nobody ever forgets.

© Stephen Heard ( September 21, 2015

Oh – and my friend and colleague Andrew Hendry also teaches one thing that nobody ever forgets.  Here’s his.

*Which I just now calculated. Apparently I am much older than I thought.

**This assumes equal adult and juvenile survivorship, among other things, but even with assumptions relaxed it’s surprisingly easy to select for semelparity.

***Which goes to show that those making fun of the theatre club in high school really didn’t understand; and that whoever hands out portfolios to gods is very, very important.


17 thoughts on “The one thing I teach that nobody ever forgets

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  2. Jeremy Fox

    I just wanted to say that I like how you put the tl;dr at the end, where it’ll only be seen by the people who’ve already read it. 🙂 I presume you were having flashbacks to how Am Nat used to put summaries of their papers at the end, rather than abstracts at the beginning. 🙂

    Just giving you a hard time. Great post.


      1. Jeremy Fox

        Bill Cosby (yes, I know…) has an old bit where he complains about how “dip in road” signs are always placed too close to the dip to do drivers any good. He jokingly suggests that the signs might as well be placed after the dips, to tell drivers who’ve gone through a dip what just happened to them.

        Placing the “tl;dr” at the end of the post strikes me as much like putting the “dip in road” sign after the dip. 🙂


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  4. Jeremy Fox

    Question Stephen: do the students remember what semelparity is, why it’s important, how it evolved, etc.? Or do they just remember the story about the Greek gods? I’m asking because I can imagine contexts in which colorful “asides” like this serve as a mnemonic, or help the students learn the material in other ways (e.g., encouraging them to come to lecture and pay more attention than they otherwise would, because the lectures are sprinkled with entertaining vignettes). But I can also imagine that these sorts of colorful “asides” could just be a distraction, or maybe have no effect on learning one way or the other. For instance, some students have trouble following the thread in lectures with too many asides. Or they might have trouble focusing on the substantive parts of the lecture if there are too many “entertaining” bits.

    I ask this as someone who sometimes gives in to the temptation to throw in colorful asides in his lectures. And who has a lecture in which he throws a textbook against a wall in an attempt to create a “teachable moment”. 🙂


    1. ScientistSeesSquirrel Post author

      They do remember what it is, the definition at least. Do all of them remember why it’s evolutionarily important, or the arguments for why it evolves? I’m less sure about that. My feeling is that an awake and engaged student is a good thing, and I’ll take some risk that for some students there may be distraction.

      And you have GOT to tell us more about throwing the textbook at the wall!

      Liked by 1 person

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