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Baby-Drinking Milkweed Butterflies Scratch Open Their Young And Slurp Their Insides


Rachael Funnell

Social Editor and Staff Writer

clockSep 13 2021, 17:14 UTC
Milkweed Butterfly

The unfortunate caterpillars act like living milkshakes for the thirsty butterflies. Image courtesy of Yi-Kai Tea

An observational study of butterflies in Indonesia has discovered a grim feeding strategy among milkweed butterflies, who claw at caterpillars and slurp up their innards. While feeding on your own young might seem like an evolutionary backstep, the researchers on the new study suspect the action could be driven by heady, attractive scents of alkaloid-rich saps, which can be accessed by either scratching at the plant itself or, evidently, by tearing up baby butterflies.

The study, published in the journal Ecology, has termed the behavior “kleptopharmacophagy” and believes it to be the first time this has been formally recognized in any butterfly species. Its observations center around milkweed butterflies from the Danaine subfamily in the coastal forests of Tangkoko Batuangus Nature Reserve, North Sulawesi, Indonesia.


Kleptomania”, the urge to steal, is derived from the Greek “klepto” and is used in biology for organisms which “steal” nutrients or materials from other things, such as kleptotrichy birds who flagrantly steal fur from living mammals. Necrophagy describes the behavior of eating an animal after its death, one that has died at the hands of a carnivore other than the one that’s dining on its corpse. This is seen among scavenging animals such as vultures and hyenas.

Kleptopharmacophagy sits somewhere between these two feeding strategies, utilizing the body of another animal in order to benefit from the chemicals you harvest in the process. For the milkweed butterfly, this is the alkaloid-laden sap that the caterpillar feeds on.

The butterflies scratch at the caterpillars and then slurp up their innards. Image courtesy of Yi-Kai Tea

“Danaine butterflies use alkaloids for two main reasons,” explained PhD candidate Yi-Kai Tea to IFLScience. “The first, as a chemical deterrent. Milkweed sap is highly toxic to vertebrates, and many birds and lizards quickly learn to avoid danaine butterflies. Butterflies sequester these toxins during their caterpillar stages, which translate into the adults after metamorphosis. Adults also fortify their inherent toxicity by visiting alkaloid containing plants to scratch and drink from. They advertise their unpalatability with their bright colors and very slow nonchalant flight.


“The second reason is for pheromone production. Male danaine butterflies need alkaloids to produce mating pheromones, which they use to attract females for courtship. They also pass the toxins to females during mating in a sperm package.”

Before now, it was thought that milkweed butterflies stocked up on sap by “leaf scratching” the plant, but this new research discovered a group of several species of milkweed butterflies who instead of scratching the milkweed plant, were carving up living caterpillars. In total, the team on the study saw seven species of Danaine butterflies practicing kleptopharmacophagy.

Exactly why this caterpillar scratching behavior has emerged in this specific location isn’t yet known, and precisely what lures the adults to the ragged corpses of their young is similarly uncertain. It could be that they’re attracted by the larvae, the host plant, or a combination of the two, but it’s likely that the caterpillars feeding on the plant contain a much higher proportion of concentrated alkaloids compared to the plant itself.


The researchers hope next to explore how or if the toxins are directly transferred from the caterpillars to cannibalistic butterflies.

[H/T: Live Science]


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