A long-standing theory is that they serve as anti-predator defences – small eyespots along the wing margin can protect butterflies by directing predators to attack less important parts of the body, such as the hindwings, enabling them to escape.
Most nymphalid family butterflies have half as many eyespots on their forewings compared to their hindwings. In particular, this has been observed in the bush brown butterfly Bicyclus anynana.
Recent research by biologists from the National University of Singapore (NUS) sought to understand the impact of uneven distribution of eyespots. The team, led by Professor Antónia Monteiro from the NUS Department of Biological Sciences, found that the location of these eyespots is key to their protective function.
In their studies, the researchers found that variants of the bush brown butterfly which have more forewing eyespots suffered higher levels of predator attacks on these wings which are crucial for flight. This ultimately led to increased rates of successful predation, causing a decline in their population. The team also discovered that this variant laid fewer eggs due to its faster demise, and thereby had lower fitness, in evolutionary terms, than those with less forewing eyespots.
The findings were first published in the journal Proceedings of the Royal Society B on 26 May 2021.
Observing predator behaviour
In a set of predation experiments, the research team observed the behaviour of mantids in attacking two variants of bush brown butterflies with differing forewing eyespot numbers. Butterflies in the first group each had two forewing eyespots, while those in the other group had four. The results showed that the butterflies with two additional eyespots in the second group experienced more intense attacks to their forewings, in addition to the typical attacks on the hindwings.
