Don’t get between a caterpillar and its milkweed

“Some were wandering around and eating,” said Elizabeth Brown, a biologist in Dr. Keene’s lab. But if an insect spotted a piece of food that was being monopolized by another, it would “stand up and, with its head, slit through the body of the other caterpillar,” she said.

Sometimes the strikes landed near the recipient’s head. In other cases, it was a bit more like “a punch in the gut,” Dr. Brown said. Either way, the beaten caterpillar usually shied away in defeat, freeing the milkweed for the voracious victor.

This is a “huge consequence” for the loser, said Dr Keene, because at this point in their life the larvae are “constantly eating.” Newly hatched caterpillars are born hungry and, when they swell in size, can strip entire plants of their leaves within days.

The older and bigger the caterpillars, the more their disdain for sharing grows, the researchers found. The greatest number of brawls occurred between insects in the final stage before metamorphosis, when the stakes of milkweed nibbling were likely particularly high.

Juxtaposed with the docile reputation of most butterflies, the study’s results can be a bit confusing. “We think monarchs are those beautiful, dazzling creatures that fly and pollinate flowers and lay eggs,” said Adriana Briscoe, butterfly researcher at the University of California at Irvine, who was not involved in the study. “We don’t generally think they have that kind of darker bellies.

But even adult monarchs, especially males, can get a bit quarrelsome when their territories are threatened, Dr Green said. Caught in tight quarters, their younger, flight-incapable counterparts might have all the more reason to indulge in the occasional kerfuffle.

Both Dr Green and Dr Briscoe stressed that the study results were limited to the lab, leaving open the possibility that the caterpillar carnage the researchers witnessed may differ in the wild, where there are more than place to move.