On second thought, don’t ask worms for directions

4 thoughts on “On second thought, don’t ask worms for directions”

  1. Maybe the worms don’t care about motion in a plane transverse to the vertical. They only care that they move generally up or down. It would be like us being able to distinguish between going up a flight of stairs versus going down them. We don’t care if the stairs take us east or west or north or south…we just want to go down!

    1. I thought this at first, but the “average angle” paragraph above argues against it. If the worm just wanted a rough measure of up v down it would be better off just following the field direction, and not some offset to it. It would be off the vertical by the angle (theta) corresponding to the field direction, but this is less than the average error from picking an orientation that’s along a cone of angle theta. What’s worse, if theta > 45 degrees, the cone crosses the horizontal, so there’s a non-zero chance of going “down” when it wants to go “up”!

  2. This is fascinating!

    Maybe traveling at an angle to the field direction is worth the gamble for the worms. On average they will do worse than just traveling along the direction of B, but suppose that the few times they get lucky offer enough of a reward to justify the cost of a lower mean displacement in the vertical. Are these worms racing each other for access to food? The paper does mention that “Rotting fruit on the surface represents an extremely rich, but transient, food supply. By contrast, rhizobacteria [deeper in the soil] represent a low-quality but stable source of food.” I’m not sure if this is plausible, maybe somebody who knows something about these worms?

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