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Inspired by his butterfly garden at home, a neuroscientist at Florida Atlantic University had a unique look at how caterpillars of monarch butterflies (Danaus plexippus) behave when food is scarce. The results resemble a combination of boxing and “bumper” cars.
With less access to their favorite food – milkweed – they go from docile to domineering, aggressively hitting their heads, hurling and throwing other caterpillars to the side to ensure their own survival. And they are very aggressive just before the final stages of their metamorphosis. A lack of nutrition during larval stages has been shown to retard larval development and reduce adult body size, reproductive performance and lifespan.
“Aggression is common in insects, including fruit flies, where single pheromone receptors or single genes have been shown to trigger their aggression,” said Alex Keene, Ph.D., lead author and professor of science. Biologicals, Charles E. Schmidt College of Science. “I decided to investigate monarch caterpillars because I was intrigued by their fighting behavior, which I observed first hand in my garden. They are large and easily recognizable compared to many other insects. These are charismatic animals that everyone loves, and there is a growing appreciation for their potential in telling us about how the brain controls behavior. “
For the study, published in the journal iScience, the researchers faced a number of challenges in maintaining a population of monarchs as they sought to model resource constraints. To address these challenges, Keene and her team built an open euphorbia garden behind their Boca Raton lab and let nature take care of harvesting the caterpillars. Back in the lab, the researchers placed the caterpillars in groups with varying amounts of milkweed. The results were clear: the less food, the more likely the caterpillars are to try to give each other their heads to satiate themselves.
The process of getting there was also challenging. Researchers have had a hard time raising monarchs in the lab and found that nearly all nurseries sell their milkweed with pesticides. That’s why they ended up growing their own euphorbia.
To examine whether the caterpillars exhibit aggressive behavior, Keene and co-workers quantified the presence of aggressive lunges in a range of conditions, as well as the effect of attacks on target conspecifics. Monarch caterpillars feed predominantly on milkweed and often strip entire plants of leaves over a two-week period. In many locations, euphorbia is only available for part of the year, placing a significant constraint on the development of the monarch. Monarchs also influence the milkweed plants they consume: in their largest and most hungry phase, a single caterpillar can devour an entire milkweed leaf in less than five minutes.
“If you compare a monarch caterpillar to a fruit fly where there are many larvae on a rotting piece of fruit, you will find that they feed socially with little evidence of territoriality,” Keene said. “But each of these caterpillars at some point in their development cycle will encounter resource constraints because they can strip an entire euphorbia of leaves.”
While observing the caterpillars, the researchers noted that the monarch’s tentacles, large mechanosensory appendages, were not used during combat. This finding suggests that alternative sensory modalities, such as pheromonal, olfactory, or tactile signals that are independent of the tentacles, initiate aggression. Researchers believe that the aggression induced by the limited availability of food in monarch caterpillars is likely present in many different species throughout the animal kingdom.
“Although our research has shown that caterpillars respond aggressively to limited food, we still hope to learn more about what drives this response in their brains, which is important to learn more about how these responses work beyond. out of the lab, ”Keene said. “One of the fundamental problems with a job like this is that we’re testing animals in a very derivative environment. And that’s not what the brain evolved to do. So now that we have this model of invertebrates in a relatively controlled environment, but making an ecological approach relevant to behavior, which becomes important in terms of observing the mechanism and function of this behavior in more complex organisms “.
In addition to the study of aggression in caterpillars, the monarchs present an emerging model for studying the molecular mechanisms underlying behavior and pave the way for future investigations into the neuroethology of aggression in this system.
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