The dark cost of being toxic

An international research team including scientists from the Max Planck Institute for Chemical Ecology in Jena has discovered that the striking orange and black wings of monarch butterflies not only send the message to predators that these butterflies are highly toxic, but that the storage of toxins and development of the colourful wings come at a cost to the butterfly's body. The team reared monarch caterpillars on their milkweed food plants that had different levels of toxins. Monarchs that had ingested high levels of toxins from their food plants as caterpillars, experienced high levels of oxidative damage after storing these toxins in their bodies, and were less conspicuous in their coloration. The study demonstrated experimentally that the storing of toxins is costly for insects that are highly specialized on their food plants.

Monarch butterflies (Danaus plexippus) feed on milkweeds of the genus Asclepias when they are caterpillars, storing the plants' cardenolide heart poisons in their bodies for their own defence. The combination of the toxins with the striking orange and black wings of the monarch is called aposematism (derived from the Greek terms apo = away and sema = signal). Hannah Rowland head of the Max Planck Research Group on Predators and Toxic Prey at the Max Planck Institute for Chemical Ecology explains: "aposematism works because predators learn that eye-catching prey are best avoided. Predators learn faster when the visual signal is always the same. Bright orange means "`'don't eat me'. But other scientists and I have repeatedly found that aposematic animals can have varying degrees of warning signal strength, and we wondered what about pale orange, or deep orange? What does this mean, and what causes the difference?"

Rowland, together with her colleague Jonathan Blount from the University of Exeter, along with their international team of scientists, tested whether the storage of the plant's toxins is costly to the butterfly's body condition. Specifically, whether the storage of toxins causes oxidative stress, whichhappens when antioxidant levels are low. Because antioxidants can be used to make colourful pigments, they tested if the amount of toxins in the monarch is related to their conspicuousness and their oxidative state.

The researchers reared monarch caterpillars on four different milkweeds of the genus Asclepias that have different toxin levels. With this, they were able to manipulate the amount of toxins ingested to subsequently measure concentrations of cardenolides, determine oxidative state, and compare the resulting wing coloration.

"Monarch butterflies that sequestered higher concentrations of cardenolides experienced higher levels of oxidative damage than those that sequestered lower concentrations. Our results are among the first to show a potential physiological mechanism of oxidative damage as a cost of sequestration for these insects," says Hannah Rowland. The scientists also found that the colour of the wings of male monarchs depended on how much cardenolides they sequestered, and how much oxidative damage this had resulted in. Males with the highest levels of oxidative damage showed decreasing colour intensity with increased toxin uptake, while males with the least oxidative damage were the most toxic and colour intense.

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