What causes Mexican jumping beans to leap? Recent findings from Binghamton University, State University of New York, indicate that the larvae inside Mexican jumping beans are sensitive to different wavelengths of light, resulting in increased jumping activity under various colors. This behavior may serve as a survival mechanism against extreme temperatures. However, if their host ‘beans’ are harmed, the larvae struggle to escape stressors.
Mexican jumping beans, often bought as novelties at tourist spots, are actually seeds that have been taken over by the larvae of small white moths. The jumping motion occurs when the larvae hit the inner walls of the seeds, prompting movement.
“When a seed falls to the ground from a bush, the moth larva inside has to deal with whatever temperature it encounters,” explained Lindsey Swerk, an assistant research professor in biological sciences at Binghamton University. “The ground can be extremely hot under direct sunlight, and a small larva can only tolerate so much heat — thus, they jump away.”
The seeds that host these moth larvae are part of a concept called ‘extended architecture’, which refers to a structure that an organism utilizes as part of its anatomy but does not create itself (similar to a hermit crab’s shell). For Mexican jumping beans, the seed serves as this extended architecture for the larva.
Research on Mexican jumping beans and how they react to environmental stressors is limited. To gain more insights into these larvae, Swierk and her team created two specific experiments.
In a study published in Behavioral Processes, the researchers examined how various colors of light affect the larvae’s movement. Swierk’s students (Faith Summers, Amber Tuske, Cassandra Puglisi, Annie Wong, and Andrés Rojo) proposed that variations in color might act as an “early warning system” for larvae to steer clear of extreme heat. The team tested the larvae’s mobility in response to different light conditions (red, purple, and green, with white light as a baseline) and measured how much light could penetrate the seed walls to reach the larvae. Despite less than 1% of the light actually getting through, the larvae exhibited more movement under red light and less under purple light, displaying contrasts in the visual spectrum.
“Somehow, larvae are sensing these differences. We aren’t certain if it’s due to tiny temperature variations or exceedingly sensitive photoreceptors,” noted Swierk. “But they appear to be using light cues to adjust their behavior, likely because these specific lighting conditions correlate with different environments. Red and white lights are typical of daylight, while green and purple lights are more common under forest canopies or at dawn and dusk.”
Building on the idea of environmental effects on jumping behavior, Swierk and her students investigated an intriguing balance. The larvae can mend damage to their host seeds using silk threads, but this ability might impact their capacity to jump and evade heat. In a follow-up study published in the Journal of Insect Behavior, her students (Anna Purtell, Jesse Anderson, Rebecca Ferguson, Konrad Juskiewicz, Michael Lee, and Megan Lee) hypothesized that repairing with silk would impose a cost, potentially diminishing the larvae’s jumping response to heat.
The researchers conducted an experiment with two groups of larvae – one with damaged seed walls they could repair, and another with damaged walls but no opportunity for repairs. A control group faced no damage at all. Both damaged groups showed diminished jumping in response to high temperatures, while the control group reacted appropriately and moved away.
According to Swierk, this suggests that the damage itself hinders the larvae’s movement, rather than the silk production costs. It’s possible that external damage, mimicking predation, can disconnect the silk strands that hold the larva inside the seed, restricting its ability to initiate jumping or rocking motions.
“These larvae are extremely attuned to temperature changes. Interestingly, we observe that they utilize subtle cues to modify their behavior regarding heat, but additional stressors, like predator attempts, can further hinder their responses to temperature,” stated Swierk.
This research could have wide-ranging implications for insects globally, according to Swierk.
“Temperature response is critical. As the climate shifts, understanding how animals detect imminent thermal stress and what restricts their adaptive measures is essential. Insights from Mexican jumping bean larvae may aid in understanding how other insects with limited mobility deal with heat stress in their habitats.”
