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Charles Darwin and Alfred Wallace are only right most of the time. A butterfly鈥檚 behaviour towards members of their own subfamily (Danainae) is among the latest discoveries that raises questions in insect evolutionary theory.

Scientists have that milkweed butterflies harass, subdue, and subsequently feed on live, dead, and dying caterpillars belonging to other milkweed butterflies 鈥� that is, their family鈥檚 young.

Caterpillars are the larval stage of butterflies and moths.

Caterpillars feed on toxic plants, sequestering chemicals for self-defence. The chemicals are noxious and unpalatable to predators, such as birds, and are later manifested in butterflies鈥� bright, warning colours.

These chemicals also help male butterflies produce mating pheromones, which function as nuptial 鈥榞ifts鈥� to females during courtship. To build upon this innate toxicity, male butterflies supplement their diet by seeking out additional sources of chemicals. Most frequently, these are obtained through plant sources, but in the forests of North Sulawesi, Indonesia, it appears that they have developed a taste for caterpillars 鈥� all in their quest to increase their supplies of this love drug, the researchers posit.

鈥淭his is the first time the behaviour has been reported,鈥� said lead author, PhD candidate Mr from the University of Sydney School of Life and Environmental Sciences.

鈥淭he behaviour does not fit neatly in the traditional modes of predation, parasitism, or mutualism, and so presents a new challenge to evolutionary theory. We have coined it 鈥榢leptopharmacophagy鈥� 鈥� chemical theft for consumption.鈥�

Ecology鈥檚 The Scientific Naturalist has published the research.

To supplement the chemical supplies they obtain as caterpillar juveniles, the butterflies engage in a behaviour known as 鈥渓eaf-scratching鈥�. 鈥淭hey damage [chemical containing] plants with their sharp tarsal claws, liberate plant juices and imbibe them using their long, curly tongues,鈥� said Mr Tea.

Scratching at live caterpillars, however, had never previously been reported. 鈥淐aterpillars are essentially bags of macerated leaves; the same leaves that contain these potent chemicals the milkweed butterflies seek out. To adult butterflies, they may simply be an alternative source of chemicals on which to feed,鈥� said Mr Tea.

Mr Tea observed this unusual behaviour with colleagues in 2019, where, in the coastal forests of North Sulawesi, he observed adult male milkweed butterflies scratching at both leaves and live and dead caterpillars and apparently imbibing their juices. 鈥淭he caterpillar larvae would contort their bodies rapidly in what appeared to be futile attempts to deter the scratching,鈥� he said.

While milkweed butterflies have been documented to feed on carcasses of other chemical-containing insects, this is the first time the behaviour has been reported extending onto living insects of the same order; in this case, their own larvae. Yet, it remains unclear whether the caterpillar fatalities were a direct result of the scratching.

鈥淣onetheless, these simple observations raise questions about the ecology of these well-known butterflies, providing numerous opportunities for future studies,鈥� said Mr Tea. 鈥淔or example, which exact compounds are these butterflies interested in? Does this behavior occur elsewhere in the world?鈥�

Declaration: This study was done in collaboration with Jonathan Soong, Ethan Beaver (South Australian Museum), and David Lohman (City University of New York). DL was supported by a National Geographic Society grant (WW-227R-17) and a National Science Foundation grant (DEB-1541557).

Hero image: Parantica cleona - a butterfly endemic to Indonesia. Credit: Tea et. al.

Milkweed butterflies originated more than 20 million years ago. They lay their聽eggs聽on various聽milkweeds; poisonous plants on which their聽larvae聽(caterpillars) feed. The family includes the iconic black, white and orange monarch butterfly (also found in Australia), which is threatened by pesticide use and deforestation in North and Central America.