Study: Mexican jumping beans use random walk strategy to find shade

Mexican jumping beans have been a curiosity for many curious children. Yes, they really “jump” thanks to the presence of tiny moth larvae inside the seed pods. Jumping can help internal moth larvae find shade to survive on hot days. And the jump movement seems to follow a random walk strategy to do so.

The concept of random walk is partly based on the physical concept of Brownian motion. Although this technically describes random collisions between particles, it is a convenient model that can be easily adapted to a wide variety of biological, physical, or other systems. The concept dates back to 1827, when a scientist named Robert Brown was studying pollen particles suspended in water under a microscope. He noticed a strange quivering motion and thought the pollen was probably alive. But when he repeated the experiment with dust particles he knew were “non-living,” he still saw jerky movements.

Brown did not pinpoint the cause of the motion, but Albert Einstein attempted to confirm the existence of atoms and molecules in his 1905 paper. , collided with other small particles such as pollen and dust suspended in the fluid, causing the “jittering” that Brown observed about 80 years ago.

Imagine walking along a straight line. Every time you take a step, you flip a coin. If it’s heads, you move forward. If it is backwards, it will retreat. The outcome of each coin toss is independent of all other coin tosses, so the odds of getting heads or tails are always equal for each toss. This means that the future final position is independent of the original starting position. Hence the term “random walk”. This concept has since been applied to modeling stock market volatility, population genetics (especially genetic drift), and neuronal firing in the brain. Also, during World War II, the Brown Random Walk was used to model the distance an escaped prisoner traveled in a given amount of time. This is because it can be an effective search strategy, especially in small densely populated areas.

That’s where Mexican jumping beans come in. Beans are actually the seed pods of a shrub native to Mexico, and despite being colloquially known as beans, are more closely related to the spurge than to the legume. Eggs are laid in hanging seed pods in the spring when they flower. Once the eggs hatch, new larvae burrow into the pods and begin feeding on the seeds. Meanwhile, the pods ripen and fall to the ground, splitting into three smaller parts. These are what we generally think of as Mexican jumping beans.

The larvae are still inside (they can survive for months there) and curl up and curl up periodically, causing the beans to “jump” when their heads hit the walls of the pod. As temperatures begin to rise, they jump more and the hot, direct sunlight can kill the larvae. Once a fully grown moth emerges from its seed pod, it usually only lives for a few days.

Previous studies have identified an ideal temperature range to initiate larval jumping behavior. Beans exposed to temperatures in the range of 20 to 30° Celsius (68 to 86° F) are the most active compared to beans that jump at higher or lower temperatures. Another study classified three basic types of movement. Flipping, rolling and jumping. Jumping is the most common (87%). Devon McKee and Pasha Tabatabai at Seattle University wanted to develop previous work to quantitatively explain the statistical behavior of jumps.