Scarce Food Turns Monarch Butterfly Caterpillars Into Monsters

Scarce Food Turns Monarch Butterfly Caterpillars Into Monsters
AP Photo/Herald Times Reporter, Sue Pischke
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North America's monarch butterflies are widely viewed as peaceful, fluttering icons of nature. Their squishy, cute, tiger-striped caterpillars are seen in a similarly positive light. But as biologists from Florida Atlantic University (FAU) recently reported in the journal iScience, these blubbery 'teddy bears' can turn into aggressive monsters when food is in short supply.

Monarch caterpillars feed almost exclusively on milkweed, voraciously devouring the plant's leaves over the few weeks the insects spend as caterpillars. Alex C. Keene, a Professor in Biological Sciences at FAU, and his colleagues wondered whether this vigor would turn into aggression in times of scarcity. So they bred monarch caterpillars in the lab and placed numerous individuals together while altering the amount of milkweed present.

"We noticed a stereotyped aggressive behavior sequence consisting of the aggressive caterpillar orienting towards the conspecific and performing a quick head-snap that made physical contact with the target," they described. "This tended to elicit cessation of feeding and a movement away from jointly occupied space by the target caterpillar.

Under the laboratory conditions, monarch caterpillars exhibited this behavior roughly twice as often when food availability was low compared to when it was intermediate and four times as often when food availability was low compared to when it was high.

"In nearly all cases, the attacked caterpillar was feeding at the time of the aggressive encounter, suggesting attacks may disrupt feeding and serve to protect a food source," the authors wrote.

The researchers noted that this behavior sharply contrasts with that seen in social insects like honeybees. In times of nutritional stress, bees tend to cooperate more than usual and make organized sacrifices to benefit the colony as a whole.

The same apparently can't be said for monarch butterfly caterpillars. It seems, at least in laboratory settings, when times get tough, they turn selfish.

Source: Collie, J., Granela, O., Brown, E.B., Keene, A.C., Aggression is induced by resource limitation in the monarch caterpillar, ISCIENCE (2020), doi: j.isci.2020.101791

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