14 June 2012
Researchers from the Technion Faculty of Biology have discovered how fruit chemistry alters animal behavior. The researchers found in the fruit of the desert plant called sweet mignonette or taily weed a chemical mechanism that encourages seed dispersal. This mechanism contains stable, non-toxic substances called glucosinolates, which are found only in the fruit pulp and break down into toxic products when the seed, which contains the enzyme myrosinase, is damaged mechanically. Apparently, the compartmentalization of glucosinolates and myrosinase in the fruits of sweet mignonette affects the interaction between the plant and rodents that are known as predominantly seed predators. One of the rodents examined, the common spiny mouse, was even found to be a quality dispenser of the sweet mignonette seeds. This is, in fact, the first documentation of a chemical mechanism in fruits that encourages seed dispersal by mammals.
According to the directed deterrence hypothesis, defensive chemicals (secondary metabolites) in ripe fruits deter seed predators, but have no or little effect on seed dispersers. Indeed, there is some evidence that birds (seed dispersers) and mammals (seed predators) differ in their responses to defensive chemicals. However, this mechanism was only demonstrated based on differences at the class level, namely differences in vanilloid receptors found in mammals but not in birds.
"Here we present the findings of physiological and behavioral experiments demonstrating the use of defensive chemicals of the mustard oil bomb to encourage broad-range, class-independent (e.g. mammals vs. birds) seed dispersal in sweet mignonette fruits, in order to force a behavioral change at an ecological timescale, converting rodents from seed predators to seed dispersers". Says researcher Michal Samuni-Blank, who has researched the subject under the guidance of Profs. Zeev Arad of the Technion and Ido Izhaki of Haifa University. "This change is achieved through the unique compartmentalization of the mustard oil bomb, which causes activation of the system only upon seed and pulp co-consumption. This 'motivates' seed dispersal which has led to the first ever documentation of a rodent dispersing seeds via seed spitting". The research findings demonstrate the power of fruits defensive chemicals to shift the animal-plant relationships from predation to mutualism, and provide support for the directed deterrence hypothesis at the intraspecific level, in addition to the interspecific level.