One sunny day in late summer, I sat on a beach log to feed myself and an attentive raven. A little piece of my sandwich dropped to the sand. As I reached down to retrieve it, I saw a small amphipod (a crustacean; some are called sand hoppers or beach hoppers) take a flying leap, covering a distance of about seven inches. This little beast was only about five millimeters long, so it traveled about thirty-six times its body length—and it would have gone farther if my boot hadn’t been in the way. Some amphipods have been recorded to jump more than one meter (roughly 39 inches), equivalent to more than fifty times their body length. They propel themselves by flexing the body, tucking the rear end underneath and quickly extending it again. Small amphipods are said to be more agile than large ones.
I made a very quick, non-exhaustive, and totally unscientific search for some comparisons with other critters that are well-known jumpers. Fleas are notorious for their jumping ability. Using their hind legs, a three-millimeter flea might jump a hundred and fifty millimeters, or fifty times its body length. Springtails jump by using a special forked appendage that is held below the body under tension until the “spring” is released. A five-millimeter springtail is reputed to be able to jump about twenty times its body length. Grasshoppers, using their hind legs, are said to do about the same. And froghoppers (spittlebugs) are reported to leap a hundred times their body length, using the hind legs. Frogs are famous jumpers, able to jump thirty to fifty times their body lengths, smaller ones going farther than larger ones, relative to body length.
Even taking those numbers with the proverbial grain of salt, they are remarkable. For comparison: A big red kangaroo, standing four or five feet tall, hops about six and a half feet when relaxed but almost thirty feet when escaping danger — maybe about six times its body length. If Olympic jumping records were recorded as relative to body length of the jumpers, all those invertebrates (and the frogs) would win easily. Olympic records for the men’s long jump are held by four guys who jumped more than twenty-nine feet. Mind you, that’s a running jump, not from a standing start. These champs were generally a bit more than six feet tall, so—as an approximation—they were jumping about four and a half times their body length. If they had jumped from a standing start, as the frogs, fleas, springtails, and the other invertebrates do, their jumps would have been much shorter.
In general, those invertebrate jumpers make rather erratic, undirected leaps. They take off and land wherever — even the general direction is not well-controlled in many instances. Amphipods and springtails often land sideways or upside down, bouncing a bit before righting themselves. If an amphipod happens to land in the midst of others, the whole crowd may erupt with wild jumps. Frogs also land ungracefully, with a thump and sometimes tipping over, if they are in a big hurry.
Even this casual survey reveals a simple, fundamental relationship: in general, smaller critters can jump farther than bigger ones (given similar body forms), as measured in relation to body length. The reason is simple: As body length increases, body volume and weight increase much more. Imagine two fictitious critters, one that is two inches long and another that is three inches long. If they were cube-shaped and solid (for simplicity), the volume (and weight) of the smaller one would be proportional to two times two times two, or eight cubic inches. The other one, longer by only one inch, would have a volume (and weight) proportional to twenty-seven cubic inches, and therefore be proportionately heavier. Real critters are neither cubical nor solid, of course, but the same principles apply.
So the bottom line is that bigger animals are just harder to get off the ground because they weigh so much more in relation to their body length. Given the limitations of that basic principle, there can be many modifications of a basic body design that improve jumping performance to some degree. For example, legs might be somewhat longer, increasing leverage. Some muscles might be better developed, increasing the power of takeoff. But there comes a point at which bigger legs just can’t overcome the total weight of the animal and lift it off the ground. Thus, the big ones simply can’t jump as far as the smaller ones, relative to their body length, even if their legs are bigger.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Friday. Her essays can be found online at onthetrailsjuneau.wordpress.com.
