Image credits: Sockeye salmon: (c) William Rosmus, CC BY-SA 4.0 via wikipedia.org. “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.
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 (firstname.lastname@example.org) 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.