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Milkweed Butterflies Crack Open Caterpillars And Drink Them Alive To Attract Females, Research Suggests

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Scientists have come across new evidence that suggests that Milkweed butterflies tear open caterpillars and drink them alive.

They observed adult male milkweed butterflies in North Sulawesi, Indonesia, using tiny claws on their feet to scratch wounds in caterpillars' bodies so they could lap the liquid that oozed out.

Male butterflies seek certain compounds produced by milkweed (flowering plants in the family Apocynaceae), which repel predators and help the butterflies produce pheromones that attract females. Since caterpillars are stuffed with juices from chewed-up plants, they make an easy target for butterflies looking to chemically boost their attractiveness to females.

"The caterpillar larvae would contort their bodies rapidly in what appeared to be futile attempts to deter the persistent scratching of adults," said the researchers who observed the butterfly baby-drinking. They described their observations in a study published Sept. 8 in the journal Ecology.

"Multiple adults were observed scratching many caterpillars along a stretch of coastal vegetation" that spanned more than 1,600 feet (500 meters), the researchers reported. They then saw the butterflies actively drink "from the wounded and oozing caterpillars" for hours, with the butterflies sometimes gathering in mixed-species groups. So intent were the butterflies on drinking from the caterpillars that not even the touch of a human observer could distract them, the study authors wrote. To describe the behavior, the scientists coined the term "kleptopharmacophagy," which means "consuming stolen chemicals."

"The alternative neologisms 'kairopharmacophagy' (feeding on defensive chemicals from wounded caterpillars detected via 'eavesdropping') or 'necropharmacophagy' (feeding on defensive chemicals from dead caterpillars) might also be appropriate," the researchers reported.

Many questions remain about this unusual (and ghastly) behavior, such as which specific plant compounds attract the butterflies to the caterpillars and do butterflies in other parts of the world also practice baby-drinking, Tea said in a statement.

"These simple observations raise questions about the ecology of these well-known butterflies, providing numerous opportunities for future studies," he said.