Mexican jumping beans have been a curiosity to many curious kids, and yes, they do “jump” thanks to the presence of tiny moth larvae in the seed pods. according to a newer paper These jumps, published by Seattle University physicists in the journal Physical Review E, may help the moth larvae inside find shade to survive on hot days. And the jumping movements seem to follow a random walk strategy.
The concept of a aimless walk based in part on the physical concept of Brownian motion. Although technically this describes random collisions between particles, it is a useful model that can be easily adapted to many different systems, biological, physical or otherwise. The concept dates back to 1827 when a scientist named Robert Brown studied pollen particles floating in water under a microscope. He noticed a strange, shaky movement and thought the pollen might be alive. But when he repeated the experiment with dust particles he knew weren’t “alive,” he still saw the quivering movement.
Brown never determined what caused the motion, but Albert Einstein did in a 1905 paper attempting to confirm the existence of atoms and molecules. Einstein’s pertinent finding was that molecules in a liquid, such as water, would move randomly and collide with other small particles suspended in the liquid, such as pollen or dust, resulting in the “tremors” that Brown observed some 80 years earlier had.
Imagine you are walking in a straight line. At each step you toss a coin. If it’s head, step forward; if it’s numbers, you go back. Since the outcome of each coin toss is independent of all others, there is always an equal chance that it will land heads or tails on any toss. This means that your future ending position is independent of your original starting position – hence the term “random walk”. The concept has since been adapted to model stock market fluctuations, population genetics (particularly genetic drift), and the firing of neurons in the brain, among other things. And during World War II, Brownian random walks were used to model the distance an escaped prisoner would travel in a given time, as it can be an effective search strategy, especially in a small, densely populated area.
This is where the Mexican jumping beans come into play. The beans are actually the seed pods of a shrub native to Mexico, to which they are more closely related spurge as legumes, although they are colloquially known as beans. In spring, when the shrub is in flower, moths lay their eggs on the hanging seed pods. When the eggs hatch, the new larvae burrow into the pods and begin eating the seeds. Meanwhile, the pods ripen and fall to the ground, splitting into three smaller segments. These are what we commonly refer to as Mexican jumping beans.
The larvae are still inside (they can survive there for months), periodically coiling and uncoiling, and when their heads touch the pod walls, the beans “jump”. They jump more when temperatures start to rise, and hot direct sunlight can kill the larvae. The prevailing hypothesis is that the beans jump to get somewhere cooler, shady so they can survive long enough to reach the pupal stage. Once the adult moth hatches from the seed coat, it generally lives for only a few days, as such is the life cycle.
Previous research identified the ideal temperature range that triggers the larvae’s jumping behavior. Beans exposed to temperatures in the 20-30° Celsius (68-86° F) range are most active at higher or lower temperatures compared to jumping beans. Another study classified the three basic movement types: flip, roll and jump, with jumping being by far the most common (87 percent). Seattle University’s Devon McKee and Pasha Tabatabai wanted to build on this earlier work to quantitatively describe the statistical behavior of the jumps.
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