Growing up is tough for a spider

I mentioned a while back that we had this surplus of spiderlings and that we were going to do some measurements of survival under different population densities. Well, we’ve got two weeks of data now, so we can think a bit.

It was a simple experiment: we put different numbers of recently emerged spiderlings in two different sized containers. We had 5, 10, 15, or 20 spiders in containers that were either about 100ml in volume, or 5.7 liters, so spider density ranged from 0.0009 spiders/ml for the big, nearly empty containers to 0.17 spiders/ml in the small overcrowded ones. We’re basically asking how crowded to they need to be to start affecting each other’s survival, and what’s the greatest density we can get away with, anticipating that no matter what, some will die. And the answer is…

Density doesn’t matter. It didn’t matter how many spiders we started with, or whether it was a small or large container, we ended up with 1-3 spiders in a container at the end of two weeks. You start with 10 in a giant container, you end up with about 3; you start with 20 in a tiny box, you end up with about 3. They’re all spaced out, too; we found that individuals tended to occupy different corners, no matter how much room they had. There were no containers which had 100% mortality.

What does that look like? They seem to be murdering their siblings to set up exclusive territories. Ah, the life of an adolescent spider. What it means is that only about 20% of the spiderlings have survived this battle royale so far. Maybe eventually they’ll be reduced to one spider per container.

Dang. Next experiment is to set up containers for individual spiderlings to see if that increases the overall survival rate. If it does, then I’ve got to do some more thinking. I can’t possibly accommodate every spiderling produced by a parent, since that would mean I’d have 150n spiders in n generations, with a generation time of about a month, so in a year I’d have 1026 spiders, which would mean I’d have to pack about 1012 spiders per square meter of Earth’s surface area, and I’d have to take over the earth to provide housing for my brood. Oh, man, and all the flies I’d have to raise! Sorry, everyone, I’m going to have to draft everyone on the planet to help maintain my spider colony.

Alternatively, I have two more modest strategies. A) I handpick the small number of spider babies I need to repopulate my colony and maintain the population size, which would require raising their offspring in individual containers. Or B) I put a small number, say 10, spiderlings in small containers, expecting that most will die in a vicious battle royale, and only one can survive in each container. There can be only one! But that one will be the most savage, ruthless spider of the group. It’s mollycoddling vs. natural selection.

Maybe I can do both for a while and see which strategy leads to the healthiest next generation.

One worry is that (B) might lead to the total extinction of all males, since the females are bigger. In nature they can disperse far apart, so we don’t have as much fratricide/sororocide, other factors will cull them. Get males from population A, and females from B? This sounds like another experiment.


  1. inflection says

    If the spiders end up occupying corners, I wonder if the limiting factor is in fact the number of corners, with some effect from the separation distance. After all, all of your boxes have four bottom corners. The world has a lot more than three spiders in it, and in addition to being very big also has more than three or four corners for little spiders to snuggle up in.

    I wonder if an X-shaped box with more well-separated corners, possibly with visual barriers between them (or even a rectangular box with a few partial inner walls) would contribute to greater survival in a similar volume.

  2. says

    That starts getting elaborate and complicated. We also need to store multiple containers, so they need to stack and fit together efficiently.

  3. jrkrideau says

    This is starting to sound like a bad science fiction novel where PZ breeds a new master race.

  4. mikehuben says

    You might also consider that if there is variation in aggressiveness/space requirements, you might find a way to select for spiders that are more tolerant of crowding. It’s normal in the development of lab cultures to select for characteristics that make culture simpler.

  5. says

    Well, speaking as an electrical engineer, when I see a curve shaped like that I immediately think of the discharge of a capacitor, a falling exponential of the form e to the -t. Of course, also speaking as an electrical engineer, at this point I’d want to measure their capacitance…

  6. monad says

    I’m curious, what are the insides of these containers like? I vaguely remember seeing some experiments with competitive exclusion, where you put a few types of microbes in a dish and pretty soon only one is left, but then it turns out even some simple variation in landscape makes it possible to have more coexist. Would something like including a piece of egg carton to provide more individual spaces, the way they do with crickets in pet stores, be worth a try or would it destroy the whole point?

  7. Sean Boyd says

    Is the difference in size already present when they hatch, or does it manifest as they age and begin to nom on each other and the flies you give them? It seems like there would be less shuffling of spiders were it possible to separate the males from the general population (or at least enough males to help guarantee you’ve got a next generation) but that means being able to tell them apart. If they’ve already decimated each other by the time the differences become apparent, wow, that’s kind of a tough problem to solve short of each one getting it’s own crib. I certainly wouldn’t have expected the result you did get, though, that container size didn’t impact the number of survivors.

    Either way, it sounds like you and your students have got a lot of things to research over the coming year. Fun!

  8. charlesanthony says

    What is the male/female ratio at hatching? What is the male/female ratio of the surviviors?

  9. says

    #6: The big ones have a cardboard frame for climbing on. The small ones have cotton pad.

    #7: No detectable difference in size when they emerge, but they begin to diverge as they grow. First post-emergence molt is an important landmark; some die then.

    #8: Can’t distinguish sexes until the palps begin to grow, so I don’t know what sex they are even yet. Apparently, if you’re a skilled arachnologist, you can sort them out now, but I’m not that good.

  10. Artor says

    This experiment is playing out in my yard right now too. Earlier in summer, there were a gazillion pinhead-sized spiders to be found on every surface, and floating in the air. As the warm months progress, there are fewer and fewer, but larger spiders remaining. They are about pea-sized now, and I have to go to my car with an arm in front of my face to avoid getting webbed. By the end of summer, there will be very few spiders remaining, but they will be thumb-sized beauties with glorious orb webs. That’s when I get out the spray paint and paper, and make a few web-prints.

  11. says

    PZ Myers (#2) – Are the boxes recangular on all six sides? What would happen if the boxes were squares on two of the six sides?

    If there are no square sides and three spiders that prefer corners, two are much closer to each others. Maybe they’re the weaker two of the three, the isolated one is stronger.

    If the box has two square sides (e.g. top and bottom), there are four equidistant corners (two upper and two lower, diametrically opposed) providing the maximum and equal distance from one another. Would four survive in that situation?

    Square boxes can stack, too.

  12. =8)-DX says

    Only three surviving alongside the implication that geometry is to blame is rather annoying, since the logic of “spiders in the same or adjacent corners will fight”, seems to imply that you could fit 4 spiders in the 4 nonadjacent corners of any cube. Maybe this is just really unlikely due to some kind of game theory? Do the spiders fight on the walls? in the edges connecting one spider-corner to another or when a spider spots a competitor in an adjacent corner do they bum-rush and resettle the newly emptied corner leading to the topological impossibility (or statistical unlikeliness) of ending up with spiders in 4 nonadjacent corners?

    We may never know (off to write a spiders-in-box corner-topology deathmatch algorithm test program, no seriously).

  13. chrislawson says

    But the probability of any given 250^n set of spiders is so low that they cannot possibly arise in nature. /creationist logic

  14. =8)-DX says

    OK, so I wrote my Spider Deathmatch Box Topology Simulator and the results are as follows:

    For 500 spiderboxes and 100 deathmatch rounds, 5-50 initial spiders each box, the resulting average per box is:
    – if spiders prefer to run away to empty corners and prefer to eat flies (>95% preference eat a fly per round), which they have lots of, you can average slightly over 3 spiders per box (3.14) by the end.
    – if spiders agressively move to corners with other spiders and set up camp there, the spider fatality is much higher, leading to results of 1.8 average spiders per box.
    – if spiders prefer to run away, but get aggressive when they don’t snatch a fly (i.e. low fly feeding) it works out to about 2.5 per box.
    – to get close to averaging 4 spiders per box (high chance of the remaining spiders being in nonadjacent corners) you need: 95% preference for eating flies over other spiders, excessive fly supply, preference for running to empty corners, and Never Fight Spiders Bigger Than You, and no spiders dying of hunger or attrition.

    Still gotta mess around with it a bit more, but this was fun.

    C# script here: on Pastebin.