Here's an interesting video from the "Smarter Every Day" youtube channel, a channel dedicated to showing some pretty cool, easy to understand science aimed at the layperson. The hypothesis here is that caterpillars have evolved a group locomotion technique in order to move at greater speed. While I think that the idea is pretty cool, and the explanation wonderful, I've got a bone to pick. But first, the video.
Catch the idea? Caterpillars, by working together, may have been able to create a way in which a group of caterpillars can move faster than any individual caterpillar (a point very eloquently made using legos).
Here's the problem. Caterpillars, like most billion-year-old species, have evolved musculoskeletal systems in response to certain selective evolutionary pressures. One of these relates to their flight response, and the muscles they have evolved to propel themselves forward as rapidly as possible. In simple terms, a caterpillar on its own is capable of moving forward at one caterpillar-speed (1 cats), moving a mass of one caterpillar-mass (1 catm). This requires the caterpillar to impart unit momentum (1 catm-cats) to itself by rubbing off of the ground.
If, now, the caterpillar is required to support the weight of a second caterpillar atop its back, it is still only capable of producing this same 1 catm-cats of momentum in a small amount of time. The mounted caterpillar, assuming it can still move unencumbered, also imparts the same momentum, and so moves at speed one with respect to the poor caterpillar it lies atop.
Therefore since the mass of the two caterpillars is 2 catm, the speed at which the bottom caterpillar can move is just 1/2 cats! Causing the upper caterpillar to move at speed 1.5 cats, and the average of the two to move only at the same unit speed.
Aside: Why wouldn't is be possible that caterpillars could actually exert more energy, keeping themselves, even when laden down at unit speed? Why would it! If they could exert more energy, this would be of benefit to them in running away themselves, increasing the value of one caterpillar-speed, but not changing the group locomotion. Also, this extra energy would need to be expended, and the caterpillars could tire out soon after joining the group.
This seems to be related to the phenomenon of group and phase velocity. The group velocity of a packet of waves is the speed at which energy is transferred. The phase velocity is the speed at which individual crests in the packet of waves move. In the video below, you can see as ripples are created on the surface of a lake. Small wave crests move fast inside the growing outer ring of the ripple. These small waves have a greater phase velocity than the group velocity!
In the case of caterpillars, the "energy" is the location of the center of mass of the pile and the individual crests are the caterpillars. While an individual caterpillar may find themselves handily brought forth to the front of the pile, they will eventually be returned to the slow, plodding bottom, supporting the weight of the whole group.
Obviously, my concerns need to be falsifiable. So here are two tests of the main assumptions I make:
1: Do caterpillars on the bottom of the pile move slower than caterpillars on the top? And by how much?
2: Would a caterpillar on its own beat the big group in a race?
3: Do these large configurations persist for a long time, or are they short lived? How does that timescale relate to the periods for which a caterpillar can move on its own?
The second part of this will address a few considerations as to the reason this cannot happen, as well as an explanation for exactly why it is this behavior (because it does exist, after all) is useful.
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Oceanographer, Mathemagician, and Interested Party