As with many species, Monarch butterflies’ bright coloring warns predators of the insects’ potential toxicity, which in many cases is true. Biologists have now discovered that female Monarchs infected with a particularly noxious parasite will choose to lay their eggs on a more toxic version of milkweed, their basic food foliage, which works to reduce pass-along parasite infection in their offspring and is harmless to the larvae.

“These experiments provide the best evidence to date that animals use medication,” says Jaap de Roode, the biologist who led the Emory University study. Some scientists theorize that animals’ practice of self-doctoring by using nature’s medicine cabinet may be more widespread than we realize.