Butterflies are pretty, and poisonous too, here's why

Butterfly defence mechanism

In their myriad colours and forms, butterflies harbour nature’s secrets. So far, more than 18,000 species of butterflies have been identified. Many display colours that enable them to camouflage themselves wherever they sit and rest while others that have vibrant, flashy colours glide slowly during flight. Some have conspicuous circular patterns on their wings which resemble large eyes. Why do these organisms have such diversity in colour and design? One of the answers is that the vibrant colours provide butterflies protection from predators through a phenomenon called aposematism.

Aposematism is a biological defence strategy that prey employ to advertise their dangerous nature — they use their bright, vibrant colours to warn potential predators of danger. These prey either possess sharp spines, foul smells or tastes, or toxic chemicals, leaving predators with an unpleasant experience. While in the past butterflies were thought to gain their chemical defence through the food they consume or sequestration, researchers have recently discovered that they can often synthesise toxins on their own.

Sourced from plants

Butterflies often eat and store toxic chemicals while feeding on their host plant as caterpillars. Ironically, these are the plant’s own defensive chemicals to prevent herbivory. However, these insects have evolved to tolerate the plant's defensive chemicals and many have become often dependent on them.

Interestingly, researchers have found that Heliconius butterflies can sequester as well as synthesise cyanogenic glucosides, a defensive chemical, on their own.

This observation has led researcher Érika de Castro and her colleagues to pursue two Heliconius butterfly species – one with a preference for a particular hostplant (a specialist) and the other with a range of hostplants (a generalist).

They posed many questions. Do butterflies collect or synthesise chemicals depending on the host plants? Do they invest more energy in synthesising the chemicals themselves compared to sequestration, and is there associated energetic cost?

To look for the answers to these questions, researchers raised the specialist butterfly Heliconius melpomene or the common postman and the generalist Heliconius cydno or the cydno longwing in four different plant species of Passiflora or the passion vines.

The selected passion vines included the one preferred by the specialist H. melpomene and a variety that none of the two butterfly types fed on in nature. Through a simple experimental design, the researchers expected to find whether the butterflies would start to make their own defensive chemicals if their preferred plant was unavailable.

If the same chemical found on the plant was also found in the butterflies, it meant the butterflies sequestered chemicals. However, if there was a mismatch in the chemical compound found in the plants and butterflies, this could only mean that the butterflies did not sequester chemicals and that they synthesised the chemicals themselves.

Érika and her colleagues measured these chemicals in both the Passiflora host and the butterflies. They found that both butterflies were able to sequester the chemicals from only one species of Passiflora – the natural host plant preferred by the specialist common postman butterfly. Caterpillars that were reared on the other three passion vines when developed into adults contained defensive chemicals that differed from the compounds found in the plants, indicating that the butterflies had biosynthesised their own chemical defence.

Why did this happen?

It turns out that these butterflies can sequester cyanogenic glucosides when they are present in a simple form. Érika and her colleagues hypothesised that “many Passiflora species seem to have modified their CNglcs (cyanogenic glucosides) to prevent sequestration.”

The researchers also found that the adult butterflies’ chemical defence concentration was highest when reared on their natural host plant and lowest when the caterpillars were raised on the non-natural host plant. The case of the specialist butterfly H. melpomene that fed on the non-preferred host plant and synthesised its own chemical defences came at a cost.

The caterpillars that fed on the non-preferred host plant developed into adult butterflies with smaller wing sizes and body weight than those raised on the preferred host plant. Hence, the specialist butterflies trade off their growth to attain chemical defences — a cost they pay to ensure higher survival chances.

However, the ability to switch from sequestration to biosynthesis depending on the chemical composition of the host plants enables these butterflies “to widen their range of potential host within the Passiflora genus, while maintaining their chemical defences,” say the researchers. These fascinating animals are indeed gorgeous cyphers playing in nature’s garden!

The paper was published in the Biology Letters journal.