Blog written by Eunice J. Tan, Bodo D. Wilts and Antónia Monteiro. Read the full article here.
Animals have a bewildering variety of colour patterns, many of which provide protection from potential predators. However, to identify how these colour patterns, or “signals”, serve to protect the animals can be often challenging.
Research in the last decades have focused on the mechanistic origin of colour in animals (e.g.Srinivasarao (1999)) and on understanding how various signals function to protect animals (e.g. Stevens and Merilaita (2009)). An important strategy, crypsis, prevents the initial detection of the animal. There are two main ways how animals become cryptic: one way is to have colours that allow them to blend in with their background, thus making the animal’s shape difficult to detect or recognise; another way is to have continuous patterns, such as bands, traverse different but adjacent body parts, thus making it difficult to detect or recognise individual body parts.
Butterflies are well-known for their dazzling colour patterns, but the functions of these patterns are still poorly understood. The Banded Swallowtail butterfly, Papilio demolion demolion, is a mostly black butterfly with a greenish-blue band that crosses the wings (Image of butterfly below), but the origin and the function of the greenish-blue band was unknown. These butterflies occur across Southeast Asia to Australia, can be found in forests and forest edges, and are active and fast fliers, feeding on flowers of shrubs and trees.
To better understand how the colour pattern of the Banded Swallowtail protects the butterfly, we examined the butterfly wing scales that make up this pattern closely with a scanning electron microscope. This allowed us to compare the colours of the green-blue band with the surrounding black wing. We found that this blue-green colour is caused by pigments immersed in the scales, resulting in a matt appearance. Potential predators therefore see an identical green-blue colour pattern from any observation angle.
We hypothesized that the greenish-blue band of the Banded Swallowtail protects it from predators through two possible means: i) its shape could help disguise the butterfly outline, and ii) its greenish-blue colour could help blend it with the surrounding green vegetation, thus preventing recognition by predators.
To test our hypotheses, we created four types of paper butterfly model, imitating the Banded Swallowtail butterfly at rest. The first paper model was like that of the natural butterfly (Model A). In order to test the protective function of the green-blue colour, we created a greyscale version (Model B). Next, to test the protective function of the shape of the band, we distorted the band so that the band is discontinuous (Model C). Finally, to test the protective functions of both the band shape and its green-blue colour simultaneously, we created a greyscale version with a distorted band (Model D). We placed these paper models, with mealworms attached as live baits, in the natural habitats of the Banded Swallowtail, in Singapore. To monitor the predation on the paper models, we checked the models daily over three days, to see if the mealworms had been eaten.
We found that the natural-looking models (Model A) suffered the least predation, while the grey model with distorted band (Model D) suffered the highest predation. Both models that had only the colour or shape of the band changed (Models B and C) suffered similar, moderate predation.
Our results indicate that both the colour and the shape of the band are important to protect the butterfly. We suggest that the shape of the band helps disguise the butterfly outline, and its greenish-blue colour, by matching the surrounding green vegetation, helps further in preventing recognition by predators.
The presence of bands on animals is an intriguing feature. Bands were shown to reduce predation in other invertebrates such as in another species of butterfly, and in spiders (Hoese, Law, Rao, & Herberstein, 2006; Seymoure & Aiello, 2015). In fish, bands are more frequently found on the bodies of fast-moving fish (Barlow, 1972). We speculate that bands are effective in disguising fast-moving species across a range of animal species, including the fast-flying Banded Swallowtail.
We also considered whether the green-blue colour could be a warning colour, i.e., a colour that warns predators about a prey’s unpalatability. The Banded Swallowtail is probably palatable because the Banded Swallowtail larvae feed on the leaves of non-toxic plants. However, not all warning colours signal unpalatability, some of these colours could be used to signal unprofitability. Pinheiro, Freitas, Campos, DeVries, and Penz (2016) showed that warning colouration in butterflies can also function as a signal to indicate difficulty of capture by insectivorous birds. As the Banded Swallowtail is a strong flier, its blue-green band may serve as a warning colour to signal unprofitability to potential predators.
Both the colour and the band of the Banded Swallowtail may help it form a mimicry ring with other similar-looking local species. Animals in a mimicry ring look similar and advertise their common unprofitability to predators. In fact, the Common Bluebottle butterfly, Graphium sarpedon luctatius, may be involved in a mimicry ring with the Banded Swallowtail. We have seen both butterfly species in the same forests, and both species possess green-blue bands across black wings.
While our experiments cannot distinguish whether the natural-looking models were least attacked because of crypsis or warning colouration, future experiments could test this. Following previous studies (e.g. Wüster et al. (2004)), the predation rates of models in a background with vegetation versus in an artificial grey background could help distinguish among these two possibilities.