12 May 2018

Sexual Dimorphism - Part 2

Sexual Dimorphism in Butterflies : Part 2
Featuring Male-Female Differences

A mating pair of Common Mormon.  Female on top.

In Part 2 of our discussion on sexual dimorphism in butterflies, we take a look at the latter two categories of differences between male-female butterflies. We now evaluate examples of butterfly species that fall into the "Distinct" and "Exaggerated" categories of dimorphism. In Part 1 of our article, we considered the four broad categories as :

1) Negligible : Males and Females appear so similar that it takes a bit more effort to scrutinise secondary characteristics closely to ascertain and differentiate between the males and females.
2) Subtle : Males and Females appear similar but there are minor giveaways that can instantly distinguish between the males and females
3) Distinct : There are clear differences between males and females that a glance will be able to separate between the two sexes in these species.
4) Exaggerated : The males and females of these species of butterflies appear so different that one could even assume that they are two different species.

In many butterfly species that are distasteful to predators, the males and females look alike. The theory is that sexual dimorphism is unnecessary as the female does not need any protection via mimicry or camouflage to avoid predator attacks. However, there are also many exceptions to this.

Before we take a look at some butterfly species that display sexual dimorphism that can immediately distinguish between the male and female, let us first consider the evolutionary reasons why dimorphism happens. Not all species of butterflies share the same rate of evolution. And in many cases, do not even need to evolve to differentiate the male and female of the same species.

Whilst the female butterfly stops to lay eggs, it is temporarily vulnerable to predator attacks. This may be one of the reasons why the females of some species use mimicry or camouflage to mitigate risk of attacks whilst it goes about its business.

In natural selection, the evolution of the physical appearances of organisms is often related to adapting itself for survival and procreation. In the animal kingdom, quite unlike us humans, the males are usually more colourful and attractive, compared to the females. It is no different for the majority of species amongst butterflies. One may wonder why this is the case. Females have the role of laying eggs to ensure the continuity of the species. However, whilst it is carrying out this role, females of butterfly species are often vulnerable to predators. This may be one of the reasons why the female butterfly has evolved and adapted through strategies of mimicry and camouflage to mitigate the risk of being attacked and killed whilst she is ovipositing.

In many Lycaenidae species, the males are often brightly coloured on the uppersides, with iridescent blues, greens and other attractive colours. Females tend to be drably coloured - brown, greys and colours that do not call unnecessary attention to themselves.

Hence in butterflies, the females of many species tend to be more drab and 'unattractive' compared to the males. The colours of the wings of many female butterflies have evolved to avoid attracting unnecessary attention to themselves. This is more prevalent in species that are not distasteful and do not enjoy the protection of having the ability to sequester plant toxins from the leaves that their caterpillars eat.

The Common Mormon's female (right), according to Wallace, mimics the distasteful Common Rose for protection.

The evolutionary process of sexual dimorphism in some species is explained by A.R. Wallace who postulated that the phenomenon of "sex-limited mimicry" where the mimetic resemblance could be limited to the female sex of the species. He continued to explain, with particular reference to the female of the Common Mormon (Papilio polytes) that "the female needs to survive for longer for oviposition".

A mating pair of Wanderer, where the female mimics the distasteful Yellow Glassy Tiger and even flies like it. The male, on the other hand, has the erratic and swift flight of a typical Pieridae butterfly.

Mallet and Joron (1999) argue that, for example, females may need to engage in mimicry because selection of oviposition sites requires slow flight, and one way to be protected while flying slowly is to mimic the colour patterns of typically slow-flying and chemically defended aposematic species. An example of the dimorphism is the Wanderer (Pareronia valeria lutescens), where the male is brightly coloured, but the female resembles the distasteful Danainae, the Yellow Glassy Tiger (Parantica aspasia aspasia) and indeed, even copies the flight characteristics of the Yellow Glassy Tiger! The male on the other hand, flies rapidly like other Pierids.

Female (left) and Male (right) of the Purple Duke (Eulaceura osteria kumana) displaying sexual dimorphism

In other species, sexual dimorphism takes on a different strategy where the females evolve to camouflage themselves to avoid predation. The female butterflies then adapt their colours and patterns on their wings to blend in with the habitats which it prefers. Generally, the evolutionary mechanisms in sexual dimorphism tends to drive adaptations in females of butterfly species towards making the females less obvious to predators.

The Malay Baron (Euthalia monina monina) - One female (centre), many male 'morphs' including variations in colours and patterns

And then there are species that have dimorphism and even polymorphism between the sexes that have apparently no rationale. However, nature always holds mysteries that we humans may not always have an answer for. Further research and field observations will certainly be necessary to unveil answers to some of these mysteries that are currently not so obvious. Let us now take a look at some of the sexual dimorphism of various butterfly species that fall in the categories of "distinct" or "exaggerated" either dorsally, ventrally or even both.

Sexual Dimorphism (Distinct differences)

The large black spots on the hindwing of the female Common Birdwing is distinctive in separating the males and females of this species.

Amongst the Papilionidae, there are a few species that display distinct differences between the males and females of the species. A glance at these obvious differences will immediately establish whether it is a male or female. An example is the Common Birdwing (Troides helena cerberus). The female possesses a series of large black submarginal spots on the hindwing that makes it distinct from the male.

The Common Mormon (Papilio polytes romulus) is unique in that the more obvious female form-polytes, features red spots on the hindwing that makes it look immediately different from the male. The dimorphism in this female form of the Common Mormon is believed to mimic the distasteful Common Rose (Pachliopta aristolochiae asteris) for protection. However, it is still a mystery why the other female form-cyrus, which resembles the male, still exists.

Great Mormon (Papilio memnon agenor) - One male, many female 'morphs'. The various female forms mimic other distasteful Papilionidae species. However, in Singapore, all the other distasteful 'models' have since gone extinct. Hence the protection that the female of this species hopes to get from mimicry is therefore irrelevant. Further evolution in the works?

In terms of sexual dimorphism, another Papilionidae species of interest is the Great Mormon (Papilio memnon agenor). In this species the differences between the male and female are even more interesting in that the female is polymorphic - having no fewer than 5 different female forms recorded in Singapore. In the case of the Great Mormon, the female forms mimic various distasteful species like the Malayan Batwing and Common Clubtail. In at least one female form-distantianus, the dimorphism can be considered exaggerated as the differences between male and female are so extreme as to suggest a different species.

Leopard Lacewing - male (left) female (right) show distinct differences between the sexes

An example of Nymphalidae that displays distinct differences between the sexes would be the Leopard Lacewing (Cethosia cyane). The colour difference on the upperside of the wings of the female distinguishes it from the male quite easily. As the Leopard Lacewing is known to be distasteful to predators, the sexual dimorphism is probably unnecessary. Hence there may be other reasons why the female has evolved to look different from the male.

One of the uncommon examples of distinct sexual dimorphism in Hesperiidae. The family shows the least amount of dimorphism between the sexes. Is it because they are crepuscular? Or they use their tremendous speed to escape predation even whilst the female is ovipositing? Food for thought!

There is very little sexual dimorphism displayed amongst the majority of species in the Hesperiidae family. One example that comes to mind is Quedara monteithii monteithii, where the female's striking forewing white band sets her apart from the drab unmarked dark brown male. When seen individually, the female may be mistaken for another species - the Banded Demon.

Sexual Dimorphism (Exaggerated differences)

A rare gynandromorph (half male, half female) example of the Archduke that was found in the Academy of Natural Sciences in Philadelphia.  It was reported that the butterfly aviary imported pupae from the Penang Butterfly Farm (now called Entopia).

In this category of dimorphism, we investigate species in which the males and females are so significantly different in appearance that it would be understandable if a casual observer would assume that they are two different species. Whilst not many species fall into this category, those that do, exhibit this spectacular and interesting dimorphism between the sexes.

Female (left) Male (right) of the Chocolate Albatross (Appias lyncida vasava)

The Striped Albatross displays exaggerated sexual dimorphism in that the males and females look like different species

To the uninitiated, the male (left) and female (right) of the Wanderer may be mistaken for two different species of butterflies. Even the flight characteristics are different, besides the colours, patterns and wing shapes between the sexes

There are some examples amongst the Singapore Pieridae that display this level of dimorphism. Of note are the Striped Albatross, Chocolate Albatross and Wanderer, each of which shows significant differences between males and females of the species.

Male (left) and Female (right) Cruiser (Vindula dejone erotella) showing exaggerated differences in sexual dimorphism.

This exaggerated dimorphism occurs more often amongst the species in the subfamily Nymphalidae. Amongst the more interesting examples are the Cruiser, Horsfield's Baron, Green Baron and Archduke just to feature a few.

Another example of the sexes that look very different from each other - Horsfield's Baron. Male (left) Female (right)

In the Horsfield's Baron, the female has evolved to appear similar to several related species like the Malay Baron and Malay Viscount. Whilst it has no apparent benefit in mimicking these other species, it is believed that the drab colouration may help to reduce attention to itself, compared to the male's more contrasting and bright blue wing margins.

Male (top) Female (bottom) of the Green Baron. The dimorphism extends to size as well, where the males are obviously smaller than the females.

Similarly, the Green Baron's female appears different in wing shape, pattern, colour and even size - both dorsally and ventrally. Again, it is not quite understood as to how the evolution of this dimorphism may be beneficial to both sexes. Studies have not yet been conclusive as to whether the Green Baron is distasteful to predators or not, since it shares its caterpillar host plant with the Painted Jezebel (which is known to be distasteful).

The Archduke is a classic example of exaggerated sexual dimorphism where the male and female appear very different from each other. Male (left) Female (right)

The Archduke's sexual dimorphism can be considered an exaggerated case where the male looks totally different from the female. It is likely that the female's spotted appearance helps it to camouflage itself on the forest floor amidst the dappled sunlight amongst forest litter, where it usually forages for food. Whilst the underside of both sexes is quite similar, the dorsal patterns and colours of the Archduke makes it a striking example of exaggerated dimorphism.

Lycaenids tend to display dorsal (upperside) differences in colours and patterns on their wings. On the ventral (underside), however, the sexual dimorphism differences may be very subtle

Moving on to the Lycaenidae, the evolutionary path of sexual dimorphism appears to direct the differences towards the dorsal appearance between the sexes. Many species are almost identical ventrally but the main difference occurs on the uppersides of the butterflies. Many of the females amongst the sub-family Theclinae are drab, brown and unmarked, whilst the more showy males display iridescent greens and blues on the uppersides.

The degree to which each species displays sexual dimorphism and the reasons for this evolutionary process does not always fit within specific rules. Whilst in some cases, they fall into the postulation that AR Wallace describes - sexually-selected mimicry to avoid predation, and in other cases, the drab colours of the females avoid calling attention to themselves, there are examples of sexual dimorphism in some species that defy simple explanations.

The examples of sexual dimorphism featured in this article is by no means exhaustive nor is the article intended to be scientifically conclusive by any measure.  If anything, it probably leaves more questions than answers, but it serves to elicit further thought and discussions. It is left to further research and continued field observations to unravel some of these mysteries and for both professional and citizen scientists to record these observations for discussion. Until then, it is unlikely that there is a single answer to explain all aspects of sexual dimorphism (and polymorphism) in butterflies.

Text by Khew SK : Photos by Rey Aguila, Isa Betancourt, James Chia, Bob Cheong, Antonio Giudici, Khew SK, Loke PF, Bobby Mun, Terry Ong, Horace Tan, Lemon Tea, Anthony Wong

Sexual Dimorphism in Butterflies Part 1

No comments: