During the first observation, the scientists led by Yi-Kai Tea from the University of Sydney found many adults milkweed butterflies scratching caterpillars and then drinking "from the wounded and oozing caterpillars" for hours.
The scientists discovered that adult milkweed butterflies found in North Sulawesi, Indonesia "harass, subdue, and subsequently feed on live, dead, and dying caterpillars belonging to other milkweed butterflies".
Caterpillars consume toxic plants and secrete chemicals to protect themselves. These chemicals, which are harmful to predators like birds, later are displayed as bright colours on butterflies' wings. Interestingly, these chemicals also help male butterflies in producing mating pheromones that are presented to female counterparts as 'courtship gifts'.
Generally, male butterflies supplement their diets with this chemical from various plant sources. However, the scientists found that these Indonesian butterflies have developed a taste for another alternative--their own caterpillars.
To supplement the chemical supplies they obtain as caterpillar juveniles, the butterflies engage in a behaviour known as “leaf-scratching”.
“They damage [chemical containing] plants with their sharp tarsal claws, liberate plant juices and imbibe them using their long, curly tongues,” said Tea. Scratching at live caterpillars, however, was never heard of.
Although there have been reports of milkweed butterflies feeding on carcasses of other chemical-containing insects, this is the first time the bizarre behaviour of consuming living insects of the same order; in this case, their own larvae have been reported.
The scientists who discovered this grisly phenomenon for the first time in 2019 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.
The scientists who wrote their findings in Ecology’s The Scientific Naturalist reported that “the behaviour does not fit neatly in the traditional modes of predation, parasitism, or mutualism, and so presents a new challenge to evolutionary theory."
"Many questions remain about this unusual (and ghastly) behaviour, 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. These simple observations raise questions about the ecology of these well-known butterflies, providing numerous opportunities for future studies," Tea said.