Toxic masculinity
Male milkweed butterflies eat their young to get the chemical weaponry they need to defend themselves
Butterflies are the embodiment of all that is delicate, graceful and charming. And yet new research reveals behaviour among south-east Asian milkweed butterflies that’s anything but – they tear open their own larvae to consume the tissues within.
Many caterpillars protect themselves from predators by accumulating toxins from their food-plants and advertising their unpalatability with bright warning colours. The chemical weapons of choice among the milkweed butterfly family are pyrrolizidine meerkat matriarchs alkaloids. But the adults use the toxins for more than just defence. They’re turned into sexual pheromones by the males, who also pass on alkaloids to their mates in their sperm as a nuptial gift.
This means that males need a lot of alkaloids. They can top up their supplies by scratching the leaves of their food-plants with their feet and sucking up the sap. But biologists, led by Yi-Kai Tea ( inset) at Australia’s University of Sydney, have discovered that they perform a similar behaviour on the milkweed caterpillars.
In Sulawesi, Indonesia, the team observed male butterflies clawing the skin of living caterpillars and imbibing the fluids that oozed from the wounds. The findings are published in the journal Ecology. Tea describes the fate of one caterpillar observed over three days, which “slowly shrivelled and died from the repeated harassing by the butterflies.” He says it’s not yet known whether the victims are already in poor health and therefore unable to defend themselves.
Neither is it clear whether the butterflies are specifically targeting the caterpillars or whether they’re simply drawn to anything containing alkaloids.
“To a butterfly, a caterpillar might as well be a leaf,” says lepidopterist Mathieu Joron of the French National Centre for Scientific Research, who wasn’t involved in this study. “Perhaps the most remarkable part of this behaviour is the active use of their legs. You don’t really see butterflies using their legs for anything other than standing on.”
The biologists have coined the name kleptopharmacophagy, meaning the theft of chemicals for consumption, for this highly unusual predator-prey relationship.