Too many forewing eyespots is bad for butterflies

NUS researchers found that butterflies which carry more eyespots on their forewings are more prone to attacks by predators.

Most nymphalid butterflies have half as many eyespots on their forewings compared to their hindwings. It is a paradox that these small eyespots protect prey animals such as the butterflies by attracting attacks from predators. Such eyespots direct predator attacks toward less important parts of the prey’s body, namely the hindwings, thus helping them to escape and live another day.

Prof Antónia MONTEIRO and her team from the Department of Biological Sciences, National University of Singapore showed that the location of these eyespots is key to their protection (see Figure). Bush brown butterflies have many more eyespots on their hindwings than on their forewings. Variants with more eyespots on their forewings suffered more attacks by predators on these wings, which ultimately led to increased rates of successful predation.

In one set of experiments, the research team observed the behaviour of mantid predators attacking two different types of bush brown butterflies from the same species. The first type had two forewing eyespots while the second type had four forewing eyespots. It was found that the two extra eyespots caused the predators to switch from attacking the hindwing alone to attacking both wings at the same time.

Dr Ian CHAN, a research fellow on the team said, “These results are telling because they show that the forewing becomes a more important target for predators in butterflies with more forewing eyespots. Butterflies can cope with damaged hindwings but their forewings are critical for all stages of flight—from take-off to landing, and particularly when evading and escaping predators. With more forewing damage, these butterflies are less likely to escape an attack and even if they do, would struggle to survive future attacks.”

To demonstrate the specific effects of the altered predator behaviour on butterfly health, measurements were made for three specific aspects of the butterflies’ fitness: how much wing damage they suffered, how many eggs they laid, and how long they lived. The research team found that butterflies with more forewing eyespots received more forewing damage, laid fewer eggs, and had shorter lifespans.

“Our findings demonstrate how the location of the eyespots on butterfly wings influences and is ultimately influenced by the behaviour of their predators. This reveals a little more of the complexity behind how animals communicate with one another. Such a discovery raises further questions about why some butterflies have the opposite pattern, carrying more forewing than hindwing eyespots instead,” added Prof Monteiro.

Moving forward, the researchers are looking to investigate whole predator-prey communities to uncover further drivers of eyespot number diversity in butterflies.

The two variants of Bicyclus anynana used in the study. (Left) Wildtype and (right) Spotty, a natural variant found at low numbers in the wild. [Credit: William PIEL and Zohara RAFI]

Reference

Chan IZW*; Ngan ZC; Naing L; Lee Y; Gowri V; Monteiro A*, “Predation favours Bicyclus anynana butterflies with fewer forewing eyespots” PROCEEDINGS OF THE ROYAL SOCIETY B 20202840 DOI: 10.1098/rspb.2020.2840 Published: 2021.