Monarch butterflies undertake one of the most extraordinary migrations of any animal, traveling thousands of miles annually. Their welfare and survival are threatened by habitat loss, pesticides, and climate change disrupting their migration phenology.
Invertebrate welfare is often excluded from welfare discussions, but monarchs provide a compelling case for consideration. Their migration requires navigating thousands of miles using a time-compensated sun compass and magnetic sensing - a feat of sustained behavioral complexity that implies significant neurological investment. Pesticide exposure has been shown to impair this navigation, causing butterflies to become lost or fail to locate milkweed for their larvae. The collapse of overwintering colony numbers means that millions of individual butterflies, having completed their extraordinary migration, arrive to find degraded or absent habitat and die of cold or starvation. Whether these failures constitute suffering in a morally significant sense is uncertain, but the scale and anthropogenic causation of population collapse warrants serious ethical consideration.