The ultimate defence

As a defence mechanism, many butterflies are poisonous. Other butterflies mimic their poisonous cousins to warn off predators.

By Justin Bode

When a poisonous butterfly is bitten, the toxins released stimulate a vomiting reflex in the predator that results in immediate release. Predators quickly learn to avoid that particular pattern and colour in future.

Larvae of the subfamily Heliconiinae typically feed on poisonous plants, which they use to create their own protective toxins. For example, Garden Acraea feed on Kiggelaria (wild peach) that contain cyanogenic glycosides, a compound of sugar and cyanide. The Kiggelaria use this toxin to protect themselves from herbivores: when the plant is injured, the cyanogenic glycoside decomposes to create the toxic gas cyanogen. The butterflies’ larvae absorb the same compounds when feeding on the plant, and pass them on to the adults. The larvae and pupae are conspicuous and aposematic (denoting coloration or markings serving to warn or repel predators), and are usually avoided by predators (although some birds, such as cuckoos, are immune to the poisons).

Red-chested cuckoo with a Garden Acraea larva (Photograph: Justin Bode)

Similarly, the larvae of the African Monarch feed on poisonous plants, such as Milkweed, which contain a type of glycoside which can disrupt the heartbeat. The larvae and pupae are highly conspicuous, like those of the Garden Acraea. Until recently, it was thought that these cardiac glycosides were transferred to the adults but it seems like they simply serve to protect the larvae and pupae—recent research indicates that male Monarch butterflies can suck up chemicals called pyrrolizidine alkaloids from certain plants and use them to synthesise poisonous chemicals in their bodies. When the butterflies mate, the male transfers these poisonous alkaloids to the female with his sperm.

Other species of butterflies have found that it is easier to mimic a poisonous or distasteful species than to expend all that energy producing their own toxins. Many brightly coloured and unpalatable species have a mimic that is actually palatable but, because it looks like the poisonous species, tricks predators into striking it from the menu.

For example, the palatable female Common Diadem mimics the poisonous African Monarch. The species being mimicked is called the model. Mimicry of an unpalatable species by a palatable one is termed Batesian mimicry.

Female Common Diadem, left (Photograph: Justin Bode), which mimics the poisonous African Monarch, right (Photograph: A Coetzer)

In Muellerian mimicry, distasteful species resemble one another. Examples of this are White-barred Acraea, which mimics the African Monarch, and the Waterberg Copper, which mimics the Dancing Acraea.

The lines between Batesian and Muellerian mimicry have recently become blurred, with several species thought to be Batesian mimics actually proving to be unpalatable. The theory that all brightly marked butterflies are distasteful to at least some predators is gaining ground.

Sources

MC Williams, Afrotropical Butterflies, www.metamorphosis.org.za, 2016.

S Woodhall, Field Guide to Butterflies of South Africa. Struik, Cape Town, 2020

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