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Why can monarch butterflies eat toxic swan plants?

LIVING WORLD: Ruby Smith aged 8 from Balclutha Primary School asked "Why do monarch butterflies only eat swan plants because we thought that swan plants were toxic - so why do they eat them?

The monarch butterfly's main food source is the swan plant. However the swan plant is toxic to many other living things and so it is interesting that monarch butterflies can tolerate eating it without any harmful effects.

The reason for this is that some plants and animals have evolved to protect themselves from being eaten by producing poison. However, in some cases other animals have evolved the ability to protect themselves from these same poisons, so they can eat the plant (or other animals) and use the poison to in turn protect themselves. Butterflies eat swan plants so that they taste bitter and bad to birds.

Our expert

Professor John Montgomery answered this question. He is a Fellow of Royal Society Te Apārangi, which means he’s one of the top experts in his field of life sciences and especially marine biology. John works at the University of Auckland. 

View his profileProfessor John Montgomery