Plastic butterflies

The squinting bush butterfly (Bicyclus anynana) is found in tropical woodland in east Africa. Like many butterfly species, they have spots on their wings. Some of the spots look like eyes, and are likely to be a defensive feature. Others, on less visible parts of the wings, are used in signalling between sexes during courtship. The spots aren’t a permanent feature, they exhibit seasonal change. The tropical regions where these insects live have distinct dry and wet seasons. In the latter the exposed eye spots are large and very visible, whereas in the dry they are much smaller and less distinct. These alternate phenotypes (morphs) probably correspond to the degree of threat from different predators in the two seasons. In the wet season, the threat comes predominantly from invertebrates such as mantids, and large eye spots are likely to threaten or distract this type of predator. This changes during the dry season when birds become the main predator; shrinkage and fading of the spots reduces the threat from birds by making the butterflies less visible.

Individual butterflies do not alternate between the two morphs; their adult lives take place within one or other of the two seasons, and they are committed to the one, corresponding morph. This one species therefore has two adult phenotypes, depending on the environmental conditions into which the adult phase is hatched, a phenomenon known as plasticity. Plasticity may be defined as the ability of a single genome to generate different phenotypes in response to varying environments. In other words, an organism’s genes may not necessarily code for a single overall phenotype, instead, they may enable organisms’ phenotypes to be flexible and environment-responsive.

Plasticity extends the range of environmental conditions that the squinting bush butterfly can tolerate as adults (not all butterfly species can tolerate different seasons and they often spend harsh ones as pupae, or migrate thousands of miles as in monarch butterflies). Similar seasonal plasticity is seen in other organisms, such as changes between summer and winter plumage in birds or fur in mammals. In all these cases the environmental changes to which the plasticities are linked are repeated and quite predictable, thus the plastic responses to them are consequently distinct and very specific. In other words, these plasticities are responses to normal environmental changes anticipated within the animals’ biology. But plasticity also enables phenotypic adjustments to unexpected environmental circumstances.

Scroll to bottom of page to comment