“Some just went out and ate,” said Elizabeth Brown, a biologist at Dr. Keene’s laboratory. But if one insect discovered a piece of food that was monopolized by another, it would “sit up and jump its head onto the other caterpillar’s body,” she said.
Sometimes the strikes landed near the recipient’s head. In other cases, it was more of a punch to the gut, said Dr. Brown. Either way, the battered caterpillar would usually sneak away defeated and free the milkweed for the voracious victor.
That is a “big consequence” for the loser, said Dr. Keene, because at this point in their life the larvae “basically eat all the time”. Newly hatched caterpillars are born starved and, due to their balloon size, can remove leaves from entire plants within a few days.
The older and bigger the caterpillars got, the greater their disdain for sharing grew, the researchers found. Most of the brawls occurred in the terminal stages before metamorphosis among insects, when the use of milkweed was likely to be particularly high.
Given the docile reputation of most butterflies, the study’s results can be a bit disoriented. “We think of monarchs as these beautiful, dazzling creatures that fly around pollinating flowers and laying eggs,” said Adriana Briscoe, a butterfly researcher at the University of California at Irvine who was not involved in the study. “We don’t usually think they have that kind of darker underbelly.”
But even adult monarchs, especially men, can argue a bit when their territory is threatened, said Dr. Green. In a confined space, their younger, flightless colleagues may have all the more reason to get involved with Kerfuffle occasionally.
Dr. Green and Dr. Briscoe both pointed out that the results of the study were limited to the laboratory, leaving the possibility open that the caterpillar slaughter the researchers observed might be different in the wild, where there is more room to roam.