An innocent-sounding plant called “sweet mignonette” lures hungry animals in with berries, but then unleashes a toxic “mustard oil bomb” that forces diners to spit, according to a new study.
The paper, published in the latest issue of Current Biology, documents the first seed spitting in rodents -- animals that, as most homeowners know, are otherwise not too picky about what they consume.
The “mustard oil bomb” unleashed in the seeds of sweet mignonette “has more of a punch than Grey Poupon,” according to co-author Denise Dearing, who is a professor of biology at the University of Utah. It’s a good thing that the spitting mechanism kicks in, because consumers -- including humans -- could experience health problems if they ate too many of the seeds.
“Toxicity is all about dose,” Dearing told Discovery News. “Even water becomes toxic at the right dose. The glucosinolates in plants (present in sweet mignonette) in high doses can cause goiters in humans.”
For the study, Dearing and her colleagues analyzed the fleshy-fruited shrub, Ochradenus baccatus, which grows 3 to 6-feet tall and lives in dry streambeds found in Ethiopia, Sudan, Egypt, Libya, Saudi Arabia and Israel east to Pakistan. The plant produces small berries that, when mature, turn black. Each berry holds up to 20 “mustard oil bomb” seeds. The toxic glucosinolate substance is only released when the seeds are chewed.
Videos of spiny mice eating the fruit showed that the rodents would deliberately wiggle the seed out of the pulp “like a person does when eating watermelon,” Dearing said. The mice would then spit out the seed. The spitting, however, appears to be at least somewhat controlled by the plant.
“We don’t know what the chemical cue is that causes the mouse to spit out the seed,” Dearing said. “(The mice) somehow know that crushing the seed activates the toxins.”
The plant ultimately benefits, since the spitting consumer helps to distribute the seeds in ideal locations. The mice tended to hang out, nibble and spit in cool spots that were not in direct sunlight. These areas are more suitable habitats for germination. Seeds spit out by mice germinate twice as fast as seeds left inside intact fruit.
In another experiment, the scientists “disarmed” the mustard oil bomb by deactivating the toxic enzyme. When the fruits were then presented to captive mice, the rodents left less than 20 percent of the seeds intact, compared with 73 percent of the seeds that still contained the active mustard bomb ingredient.
Yet another experiment determined that mice lost weight when they were fed ingredients necessary for an active mustard bomb, but not when they were fed them separately.
Camels, ibex, other rodents, lizards and many birds also regularly eat sweet mignonette berries. All may spit out the seeds, save for birds, which mysteriously “don’t feel the heat at all,” Dearing said. It could just be that birds tend to not crush seeds when feeding, allowing them to expel even the spiciest and most toxic of seeds whole in their waste.
The findings could help to explain why so many seeds are not tasty. Some seeds even practically fall out of the mouth. The aforementioned watermelon seeds, for example, are not only lousy tasting, but they also have a slippery coating that makes them easy to spit out. Dearing said that plants with nutritious, edible seeds, such as sunflowers, have a different growth strategy. They “make a lot of seeds to overcome seed predation. Some oak trees have this strategy.”
Doug Levey, program officer of Population and Community Ecology at the National Science Foundation, told Discovery News that the relationship between sweet mignonette and mice, in particular, is “a beautiful example of how a fruiting plant manipulates the behavior of rodents, turning them from seed predators to seed dispersers.”
“The series of field observations and lab experiments make a compelling story,” he added. “I especially like the diet study that demonstrated when the bomb was defused, the rodents completely switched their behavior.”