16 February 2020

Polymorphism in Butterflies

Polymorphism in Butterflies
Understanding "forms" in Butterflies

A newly-eclosed female Common Mormon f-cyrus perches on a vine

In earlier articles on sexual dimorphism (Part 1 and Part 2), we took a look at how different sexes of butterflies show differences in external wing patterns, colours, shapes and sizes. In some of the cases, the differences are so significant that, to the beginner, they would appear as two totally different species!

A two-formed species, the Common Tiger has both a white hindwinged and an orange hindwinged form.  Here, the form-genutia Common Tiger displays its orange hindwings

This article looks at the differences in which one or both sexes occurs in two or more very distinct and identifiable "forms". This phenomenon, known as polymorphism, highlights some species that have distinctly different physical appearances but occur regularly and consistently enough to be identified as a form of the species.

The Common Mime is polymorphic, but only form-dissimilis flies in Singapore

A Common Mime form-onpape, which resembles one of the Crows (Euploea spp) flies in northern Malaysia

In quite a few of these polymorphs or forms of the species, the probable underlying reason for having the forms are mimetic - primarily for protection against potential predators. Many of these examples of polymorphism involve females of the species concerned, and mimicry is likely to extend the survivability of the females of the species from attacks, whilst the female is ovipositing (and hence put in a vulnerable position for predation).

A Long Brand Bush Brown (Mycalesis visala phamis), probably a dry-season form which is rare in Singapore where there is a less distinct dry and wet seasons throughout the year

Then there are the seasonal forms, where differences in rainfall, temperature, humidity and other environmental factors trigger physical differences in some species of butterflies. Sharply differentiated seasonal forms display physical differences in which shape (e.g. more angular wings), colours and patterns (e.g. paler and less distinct markings or size of ocelli). However, this phenomenon is rather uncommon in Malaysia and Singapore, where the seasonal forms do not usually occur.

A female Blue Pansy (Junonia orithya wallacei) on its way to becoming a form?  The typical female Blue Pansy has very little or no blue colouration on its hindwings.

However, like many examples in evolutionary biology, there is rarely a single set rule or conclusion that applies universally. There are also examples of species where polymorphism cannot be attributed a purely mimetic resemblance for any form of protection. In these cases, why is there a need to display different forms of the species? Are there any other reasons why these species should have different forms? There are still many mysteries to uncover.

Some examples of polymorphism across the different families of butterflies found in Singapore are used here to illustrate polymorphism :

Common Mormon (Papilio polytes romulus)

A male Common Mormon puddling at a sandy streambank

The Common Mormon is a classic case study by researchers with regard to the polymorphic females. In particular, form-polytes that mimics the distasteful Common Rose (Pachliopta aristolochiae asteris) for protection. In other regions, there are other forms of the female, like f-romulus and f-theseus, that mimics other distasteful Papilionidae. Details of the polymorphic mimicry are outlined in this paper.

Top : Female Common Mormon form-cyrus and Bottom : Female Common Mormon form-polytes, which mimics the Common Rose

In this case, the polymorphism in the female is assumed to be for the species' protection against predation by mimicking a distasteful species. However, there is another form of the female, f-cyrus, which is almost indistinguishable from the male. If the mimetic protection is successful, then why is there a non-mimetic form? Or could it be case where the species is still evolving to a stage where the unprotected female form will be obsolete some time in the future?

Great Mormon (Papilio memnon agenor)

A male Great Mormon feeding at the flowers if Ixora

Another Papilionidae that features polymorphic females is the Great Mormon. The species continues to evolve with many forms of the female, some of which have been discovered only in recent years. In Singapore thus far, four different forms have been observed - f-esperi, f-butlerianus, form-distantianus and form-agenor.

A collage of a male Great Mormon (centre) and the four different female forms found in Singapore

The polymorphic mimicry of the first three forms is based on their resemblance to the distasteful species of Malayan Batwing (Atrophaneura nox erebus), Common Batwing (Atrophaneura varuna varuna) and Common Clubtail (Losaria coon doubledayi). The last form-agenor does not seem to have a model. However, it is important to note that the effects of protective mimicry in Singapore would have long been neutralised by the fact that all the distasteful models are no longer found here. So why does the Great Mormon still continue with its polymorphism?

Plain Tiger (Danaus chrysippus chrysippus)

Left : A female form-chrysippus Plain Tiger and Right : a male form-alcippoides Plain Tiger

The Plain Tiger, a common urban species that is often found in parks and gardens in Singapore features two different polymorphs - form-alcippoides (with white hindwings) and form-chrysippus (with orange hindwings). Of the two, the white hindwinged form is by far the commoner.

The orange-hindwinged male form-chrysippus Plain Tiger

The white-hindwinged male form-alcippoides Plain Tiger

It is not known for certain why this species displays this polymorphism. The Plain Tiger is already a distasteful species, hence the different forms do not need any protection through mimicry. Across the region, the orange-hindwinged form-chrysippus appears to be the commoner form in northern parts of Malaysia, but the opposite is true for Singapore and the southern regions of West Malaysia.

Lemon Emigrant (Catopsilia pomona pomona)

Two male Lemon Emigrants - Top : form-hilaria Bottom : form-alcmeone

Amongst the Pieridae, the Lemon Emigrant is the most well-known for its polymorphism. There are no fewer that seven different forms observed in Singapore - form-alcmeone, form-hilaria (for the males) and form-jugurtha, form-crocale, form-pomona, form-nivescens and form-catilla (for the females).

Three female Lemon Emigrants - Top : form-pomona Middle : form-crocale Bottom : form-catilla

That the species has both male and female polymorphs (or forms) is particularly interesting. This is especially so, when the basis of the polymorphism does not appear to be related to protective mimicry. Then why does a species needs to have so many forms in both the males and females? It is a question that evolutionary biology researchers may well find out in the future.

Malay Baron (Euthalia monina monina)

A female Malay Baron

The Malay Baron is another curious species that is polymorphic, but only in the males! Whilst it is clear that females are polymorphic for predative protection by mimicry, why then would males need to be polymorphic? In the case of the Malay Baron, the different forms of the males - form-monina, form-gardineri and form-decorata do not seem to have any functional purpose. Furthermore, there are many intermediates and variations to the different forms that suggest that perhaps there could be more forms in the making.

A collage of Malay Barons with the polymorphic males and variations shown

The female is monomorphic and much larger than the males. It resembles several other related species in the sub-family - all of which have no known distastefulness to predators. Again, the question of why a species is polymorphic whilst other related species are not, becomes a mystery that remains to be researched further, where there are more questions than answers.

Courtesan (Euripus nyctelius euploeoides)

A male Courtesan perches on a leaf.  It is about 2/3 the size of the female

The Courtesan is a rare species in Singapore. The polymorphic females, form-isina and form-euploeoides mimic the male and female Magpie Crow (Euploea radamanthus radamanthus) respectively . It is obvious that the polymorphism is based on protective mimicry where the females mimic the distasteful Danainae.

Top : Female form-isina Courtesan Bottom : Female form-euploeoides Courtesan

Of interest is the very subtle difference between the two forms, unlike some of the other species highlighted in this article. The male and female Magpie Crow is only different in the more extensive white markings on the hindwing. The female Courtesan mirrors this subtle difference in the two forms. Essentially, either form would have been able to satisfy the protective resemblance.

Great/Jacintha Eggfly (Hypolimnas bolina bolina/jacintha)

A Male Great Eggfly foraging on a forest trail

Another species of interest is the Great Eggfly and Jacintha Eggfly, where both subspecies fly in Singapore. It is likely that eventually, one subspecies will prevail and the other fade out. However, at the moment, there is an element of doubt separating the two subspecies and requires further research.

Polymorphic females of the Great and Jacintha Eggfly found in Singapore

The polymorphism is only applicable to the females of the Great Eggfly and Jacintha Eggfly. Although there were earlier attempts to name the forms of the females, there appears to be so many variants and new forms appearing over recent years that may cause confusion as to the forms that are in existence. Also, in Singapore, there appears to be more female forms of the subspecies jacintha than of subspecies bolina. A useful paper on polymorphism in this species can be found here.

Text by Khew SK : Photos by Rey Aguila, Bob Cheong, Foo JL, Federick Ho, Khew SK, Loh MY, Loke PF, Bobby Mun, Horace Tan and Anthony Wong